Conductor Ampacity Calculation

Conductor Ampacity Calculation – Part One In part one of this subject, I will list all the terms definitions that we can subjected to. These terms will include the following sections: 1. Conduc Conductor tor/Ca /Cable ble Terms, Terms, 2. Conductor Conductor/Cab /Cable le Insulati Insulation on Materia Materials ls Terms, Terms, 3. Conduc Conductor tor/Ca /Cable ble Tests Tests Terms Terms,, 4. Conductor Conductor/Cab /Cable le Manufac Manufacturing turing Process Process Terms. Terms. Definitions for All above terms are included in the below table, please review it very well before we proceed to explain the steps of conductor Ampacity Calculations.

Term

Definition 1- Conductor/Cable Terms

Bare Conductor

Any all-encompassing temperature within a given area. The maximum current an insulated wire or cable can safely carry without exceeding either the insulation or jacket material limitations. (Same as Current Carrying Ampacity) Appliance wiring material is a classification of  Underwriters Laboratories, Inc., covering insulated wire and cable intended for internal wiring of  appliances and equipment. The size of a conductor cross-section, measured in circular mils, square inches, etc. Abbreviation for American Wire Gauge. A standard system used in the United States for designing the size of an electrical conductor based on geometric progression between two conductor sizes. Based on a circular mil. System 1 mil. Equals .001 inch. A conductor having no covering. A conductor with no coating or cladding on the copper.

Building Wire

Wire used for light and power, 600 volts or less, usually exposed to outdoor environment.

Bunch Stranding

A group of wire of the same diameter twisted together without a predetermined pattern.

Ambient Temperature

Ampacity

Appliance Wire and Cable

Area of Conductor

AWG

Buried Cable Bus Butt Butt-Splice

Cable Cable Assembly

Cable Filler

Cabling Circular Mil

Coating Coaxial Cable

A cable installed directly into the earth without use of underground conduit. Also called “direct burial cable”. Wire used to connect two terminals inside of an electrical unit. Joining of two conductors end to end, with no overlap and with the axes in line. A splice where in two wires from opposite ends butt against each other, or against a stop, in the center of  a splice. A group of individually insulated conductors in twisted or parallel configuration, with or without an overall coating. A completed cable and its associated hardware ready to install. The material used in multiple conductor cables to occupy the spaces formed by the assembly of  components, thus forming a core of the desired shape (normally cylindrical). The twisting together of two or more insulated conductors to form a cable. The area of a circle one mil. (.001”) is diameter, 7,845 x 10-7 sq. inches. Used in expressing wire cross sectional area. A material applied to the surface of a conductor to prevent environmental deterioration, facilitate soldering, or improve electrical performance. A cable consisting of two t wo cylindrical conductors with a common axis, separated by a dielectric.

Bare Conductor

Any all-encompassing temperature within a given area. The maximum current an insulated wire or cable can safely carry without exceeding either the insulation or jacket material limitations. (Same as Current Carrying Ampacity) Appliance wiring material is a classification of  Underwriters Laboratories, Inc., covering insulated wire and cable intended for internal wiring of  appliances and equipment. The size of a conductor cross-section, measured in circular mils, square inches, etc. Abbreviation for American Wire Gauge. A standard system used in the United States for designing the size of an electrical conductor based on geometric progression between two conductor sizes. Based on a circular mil. System 1 mil. Equals .001 inch. A conductor having no covering. A conductor with no coating or cladding on the copper.

Building Wire

Wire used for light and power, 600 volts or less, usually exposed to outdoor environment.

Bunch Stranding

A group of wire of the same diameter twisted together without a predetermined pattern.

Ambient Temperature

Ampacity

Appliance Wire and Cable

Area of Conductor

AWG

Buried Cable Bus Butt Butt-Splice

Cable Cable Assembly

Cable Filler

Cabling Circular Mil

Coating Coaxial Cable

A cable installed directly into the earth without use of underground conduit. Also called “direct burial cable”. Wire used to connect two terminals inside of an electrical unit. Joining of two conductors end to end, with no overlap and with the axes in line. A splice where in two wires from opposite ends butt against each other, or against a stop, in the center of  a splice. A group of individually insulated conductors in twisted or parallel configuration, with or without an overall coating. A completed cable and its associated hardware ready to install. The material used in multiple conductor cables to occupy the spaces formed by the assembly of  components, thus forming a core of the desired shape (normally cylindrical). The twisting together of two or more insulated conductors to form a cable. The area of a circle one mil. (.001”) is diameter, 7,845 x 10-7 sq. inches. Used in expressing wire cross sectional area. A material applied to the surface of a conductor to prevent environmental deterioration, facilitate soldering, or improve electrical performance. A cable consisting of two t wo cylindrical conductors with a common axis, separated by a dielectric.

Color Code Combination Unilay

Compact Stranded Conductor

Compressed Stranding

Compound Concentric Stranding

Concentricity

Conductivity Conductor Control Cable Cord

A system for a circuit identification through use of  solid colors and contrasting tracers. A stranding configuration that uses two strand sizes to achieve a 3% reduction in the conductor diameter without compression. A unidirectional or conventional conductor manufactured to a specified diameter, approximately 8 to 10% below the nominal diameter of a noncompact conductor of the same sectional area. A stranding configuration with concentric strands, in which either all layers or the outer layer only is passed through a die to reduce the conductor diameter by 3%. An insulating or jacketing material made by mixing two or more ingredients. A central wire surrounded by one or more layers of  helically wound strands in a fixed, round, geometric arrangement. In a wire or cable, the measurement of the location of the center of the conductor with respect to the geometric center of the surrounding insulation. The capability of a material to carry electrical current, usually expressed as a percentage of copper conductivity (copper being 100%). An uninsulated wire suitable for carrying electrical current. A multi-conductor cable made for operation in control or signal circuits. A small, flexible insulated cable.

Core

Cross-Sectional Area

Current-Carrying Capacity

Derating Factor

Dielectric Direct Burial Cable Duct Feeder

Fixture Wire

Flat Cable Flat Conductor Flat Conductor Cable Flex Life

Flexible Flexibility Gauge Hard Drawn Copper Wire Hook-Up Wire

Insulation

Insulation Level-100% Level-100%

Insulation Level-133% Level-133%

In cables, a component or assembly of components over which additional components (shields, sheath, etc.) are applied. The area of a conductor exposed by cutting the conductor perpendicular to its longitudinal plane, expressed in circular mils, square inches, or square millimeters. The maximum current an insulated conductor or cable can continuously carry without exceeding its temperature rating. It is also called ampacity. A factor used to reduce the current carrying capacity of a wire when used in environments other than that for which the value was established. Any insulating material between two conductors that permits electrostatic attraction and repulsion to take place across it. A cable installed directly in the earth. An underground or overhead tube for carrying electrical conductors. The circuit conductor between the service equipment and the final branch circuit over current device. A conductor used in lighting or similar equipment or used to connect a lighting fixture to branch circuit conductors. Common types include TF, TFN, and TFFN. A cable with two smooth or corrugated, but essentially flat surfaces. A wire having a rectangular cross section, as opposed to round or square conductors. A cable with a plurality of flat conductors. The measurement of the ability of conductor or cable to withstand repeated bending. The quality of a cable or cable component that allows for bending under the influence of outside force, as opposed to limpness which is bending due to the cable’s own weight. The ease with which a cable may be bent. A term used to denote the physical size of a wire. Copper wire that has not been annealed after drawing. A single insulated conductor used for low current, low voltage (usually under 600 volts) applications within enclosed electronic equipment. A material having high resistance to the flow of  electric current. Often called a dielectric in radio frequency cable. Cable for use on grounded systems or where the system is provided with relay protection such that ground faults will be cleared as rapidly as possible but in any case within one minute. Cable for use on grounded systems or where the faulted section will be de-energized in a time not exceeding one hour.

Insulation Resistance (I.R) Insulation Thickness Jacket Kcmil Leakage Current Listed

MC Metal-Clad Cable MCM Member

Messenger

Metal-Clad Cable

Mil Multi-Conductor Rated Temperature

Rated Voltage Rope Strand

Semi-Conducting Tape

Separator

Sheath Shield

The resistance offered by insulation to an impressed DC voltage, tending to produce a leakage current through the insulation. The wall thickness of the applied insulation. An outer covering, usually nonmetallic, mainly used for protection against the environment. 1,000 circular mils. The undesirable flow of current through or over the surface of insulation. Conductors or other equipment included in a list published by a nationally recognized testing laboratory. MEC type designation for power and control cables enclosed in a smooth metallic sheath, or interlocking tape armor. One thousand circular mils. A group of insulated wires to be cabled with other stranded groups into multiple membered cable. The linear supporting member, usually a high strength steel wire, used as the supporting element of a suspended aerial cable. The messenger may be an integral part of the cable or exterior to it. Type MC; a multi-conductor cable, similar to type AC, in which the conductors are twisted together under aluminum or steel armor. With or without an overall PVC covering.

A unit used in measured diameter of a wire or thickness of insulation over a conductor. One onethousandth of an inch (.001”). More than one conductor within a single cable complex. The maximum temperature at which an electric component can operate for extended periods without undue degradation or safety hazard. The maximum voltage at which an electric component can operate for extended periods without loss of its basic properties. A conductor whose cross-section is substantially circular. A tape of such resistance that when applied between two elements of a cable, the adjacent surfaces of the two elements will maintain substantially the same potential. A layer of insulating material such as textile, paper, polyester, etc. Used to improve stripping qualities, flexibility, mechanical or electrical protection to the components. The outer covering or jacket of a multi-conductor cable. A metallic layer placed around a conductor or group of conductors to prevent electrostatic interference

between the enclosed wires and external fields.

Skin Effect Solid Conductor Strand Stranded Conductor Temperature Rating Tensile Strength Test Lead

Thermal Rating Tinned Copper

Tray Cable

UF USE Voltage Drop Voltage Levels High Voltage Voltage Rating

VW-1

Water Absorption Wire Wire Gauge Yield Strength

The tendency of alternating current, as its frequency increases, to travel only on the surface of a conductor. A single unit not divided into parts. A single uninsulated wire. A conductor composed of individual groups of wires twisted together to form an entire unit. The maximum temperature at which an insulating material may be used in continuous operation without loss of its basic properties. The pull stress required to break a given specimen. A flexible insulated lead wire used for making tests, connecting instruments to a circuit temporarily or for making temporary electrical connections. The maximum and/or minimum temperature at which a material will perform its function without undue degradation. Tin coating added to copper to aid in soldering and inhibit corrosion. A factory assembled multi-conductor or multi-pair control, signal or power cable specifically approved under the National Electrical Code for installation trays. Thermoplastic underground feeder and branch circuit cable. Underground Service Entrance cable, rubberinsulated, neoprene or XLP jacketed. The amount of voltage loss from original input in a conductor of given size and length or over a connection such as a termination. Power-limited 0-300 volts. Low voltage 600-2000 volts. Medium voltage 5000-69000 volts. Generally, a wire or cable with an operating voltage of over 35,000 volts. The highest voltage that may be continuously applied to a wire in conformance with standards or specifications. A flammability rating established by Underwriters Laboratories for wires and cables that pass a specially designated vertical flame test, formerly designated FR-1. Multi-conductor flat or round portable power cables without grounding conductor. Water by percent weight absorbed by a material after a given immersion period. A single conductor, typically with a covering of  insulation. A measure of the diameter or size of wires. The sizes are expressed by numbers. The minimum stress at which a material will start to physically deform without further increase in load.

2- Insulation Materials Terms

RHW

Jacketing compound based on chlorinated polyethylene. An ozone-resistant rubber consisting primarily of  ethylene propylene copolymer (EPM) or ethylene diene terpolymer (EPDM). A chemical added in insulation materials to make them less combustible, such as antimony oxide (to PVC) or alumna trihydrate. The ability of a material to restrict the spread of  combustion to a low rate of travel, so that the flame will not be conveyed. Thermoplastic-insulated machine tool wire. 90°C to 105°C, 600V. A group of polyimide polymers that are used for wire and cable jacket. A polyvinyl chloride formulation that does not produce electrical contamination through plasticizer migration. A chemical agent added to plastics to make them softer and more pliable. Polyethylene terephtalate that is used extensively in the production of a high strength, moisture resistant film used as a cable core wrap. A thermoplastic material having the chemical identity of polymerized ethylene. A substance made of many repeating chemical units or molecules. The term polymer is often used in place or plastic, rubber or elastomer. A thermoplastic polymer of propylene. A thermoplastic material composed of polymers of  vinyl chloride, which may be rigid or elastomeric, depending on specific formulation. Portable Power Elastomer. Same as Type W, except that it is a thermoplastic elastomer insulation and jacket, whereas Type W is all thermostat. The first layer of non-conductive material applied over a conductor, whose prime function is to act as electrical insulation. Polyvinyl chloride, a common thermoplastic insulation and jacketing material for building wire and cable. Type RH, a rubber or XLP-insulated conductor for use at 75°C in dry locations. Type RHH, a rubber or XLP- insulated conductor for use at 90°C in dry locations. Type RHW, a rubber or XLP-insulated conductor for use at 75°C in dry and wet locations.

RHW-2

Type RHW-2, a rubber or XLP-insulated conductor for use at 90°C in dry and wet locations.

CPE Ethylene Propylene Rubber (EPR) Flame Retardant

Flame Resistance MTW Nylon Non-Contaminating PVC Plasticizer Polyester Polyethylene Polymer Polypropylene: Polyvinyl Chloride (PVC)

PPE

Primary Insulation

PVC RH RHH

Secondary Insulation

A high-resistance dielectric material whose flame is placed over primary insulation to protect it from

abrasion.

Self-Extinguishing

SIS

SJ-SJE SJO-SJEO SJOOW-SJEOOW SJT SJTO SO-SEO

SOOW ST Stabilizer STO STOW/STOW

STW/STW

Sunlight Resistance TEW TFFN THHN THHN-2

The characteristic of a material whose flame is extinguished after the igniting flame is removed. Indicates single conductor having synthetic thermosetting insulation of heat resistant, moisture resistant, flame retarding grade. Also made with chemically cross-linked polyethylene insulation. Used for switchboard wiring only, 90°C. Junior hard service, rubber insulated pendant or portable cord. Same construction as type S, but 300B. Jacket thickness different. Same as SJ, but neoprene, oil resistant compound outer jacket, 300V, 60°C Same as type SJO, except oil resistant insulation and oil and weather resistant jacket. Junior hard service, thermoplastic or rubber insulated conductors with overall thermoplastic jacket. 300V, 60C to 105C. Same as SJT, but oil resistant thermoplastic outer jacket. 60C Hard service cord, same construction as type S, except oil resistant neoprene jacket, 600V, 60°C to 90C Service cord with oil resistant jacket, oil resistant and insulation and weather resistant. Also is water resistant. 600V. Hard service, jacketed, same as type S, except all plastic construction. 600V, 60°C to 105°C. A metallic compound added to PVC to maintain the integrity of the insulation compound during processing and use. Same as ST, but with oil resistant, thermoplastic outer jacket. 600V, 60°C. Service cord with oil resistant, thermoplastic jacket and weather resistant. STOW meets CSA approval for outdoor use. Can be water resistant. UL 600V. Service cord with thermoplastic and weather resistant jacket, but not oil resistant. Can be UL water resistant. STW meets CSA approval for outdoor use. 600V. The ability of a conductor or cable insulation to resist degradation caused by exposure to ultraviolet rays. Canadian Standards Association type appliance wires. Solid or stranded single conductor, plastic insulated. 600V, 105C. Fixture wire; thermoplastic covered, stranded with a nylon sheath. 90C. 90C, 600V, nylon-jacketed building wire for dry and damp locations. Incorrect reference, commonly misapplied when THWN-2 is called out.

XHHW-2

Thermoplastic, vinyl insulated building wire. Flame retardant, moisture and heat resistant. 75°C. Dry and wet locations. 75°C, 600V, nylon jacketed building wire for dry or wet locations. 90°C, 600V, nylon-jacketed building wire for dry or wet locations. High temperature (90°C), chemically cross-linked, polyethylene jacketed, small diameter building wire.

XLP

Cross-linked polyethylene.

THW THWN THWN-2

3- Conductor/Cable Tests Terms Continuity Check

Dielectric Test

Flammability Test

A test to determine whether electrical current flows continuously throughout the length of a single wire or individual wires in a cable. A test in which a voltage higher than the rated voltage is applied for a specified time to determine the adequacy of the insulation under normal conditions. A test to determine the ability of a cable to resist ignition when placed near a source of heat or flame and to self extinguish when removed from this source.

Heat Shock

Hi Pot Life Cycle

Megohommeter

A test to determine stability of a material by sudden exposure to a high temperature for a short period of  time. A test designed to determine the highest voltage that can be applied to a conductor without electrically breaking down the insulation. A test to determine the length of time before failure in a controlled, usually, accelerated environment. A testing device that applied a DC voltage to a conductor and measures the resistance (in millions of  ohms) offered by the conductors insulation.

Spark Test

Tank Test

A test designed to locate imperfections (usually pinholes) in the insulation of a wire or cable by application of a voltage for a very short period of  time while the wire is being drawn through the electrode field. A voltage dielectric test in which the test sample is submerged in water and voltage is applied between the conductor and water as ground.

4- Manufacturing Process Terms

Anneal

Drawing

The process of controlled heating and cooling of a metal to achieve predetermined characteristics as to tensile strength and elongation. . Annealing copper renders it less brittle. In wire manufacturing, pulling the metal through a die or series of dies to reduce diameter to a specified size.

Extrusion

Oxidation

The process of continuously forcing both a plastic or elastomer and a conductor core through a die, thereby applying a continuous coating of insulation or jacket to the core or conductor. The process of uniting a compound with oxygen, usually resulting in an unwanted surface degradation of the material or compound.

In the next Article, I will explain the following points: 1. Measurement Units of Conductors Cross Section Area, 2. Conversions between different Measurement Units of Conductors Cross Section Area, 3. Standard Sizes of Conductors, 4. Conductor Ampacity Tables. Please, keep following.

Conductor Ampacity Calculation – Part Two In Article " Conductor Ampacity Calculation – Part One ", I listed the definitions for related terms which were included under the following sections: 1. Conductor/Cable Terms, 2. Conductor/Cable Insulation Materials Terms, 3. Conductor/Cable Tests Terms, 4. Conductor/Cable Manufacturing Process Terms. Today, I will explain the following points: 1. Measurement Units of Conductors Cross Section Area, 2. Conversions between different Measurement Units of Conductors Cross Section Area, 3. Standard Sizes of Conductors,

1- Measurement Units of Conductors Cross Section Area

1.1 Traditional Measurement Units of Conductors Cross Section Area

As we know that the traditional Measurement Units of Conductors Cross Section Area are: •

Square millimeters (mm2),

•

Square inches.

1.2 The measurement unit “SQUARE MIL”:

Definition: The Mil: is a convenient unit of measurement of the diameter of a conductor Noting that Mil = 0.001 inch = 0.0254 mm Or, Inch = 1000 mils

Definition:

The square Mil: is a unit of measurement used to determine the crosssectional area of a square or rectangular conductor (views A and B of Fig.1). Also, A square mil : is defined as the area of a square, the sides of which are each 1 mil. , as shown in view A of Fig.2.

Rule#1: Cross-Sectional Area Of A Square Conductor For Square Conductors, as in View A in fig.1, to obtain the cross-sectional area in circular mils of a square conductor, multiply the dimension of any side of the square by itself. So, Cross-sectional area (A ) in mils = square of Side Length Noting that Side Length must be in mils.

Fig (1)

Fig (2)

Example#1:

Assume that you have a square conductor with a side dimension of 3 mils. What its crosssectional area in circular mils?

Solution:

Cross-sectional area (A ) in mils = square of Side Length = 3 x 3 = 9 square mils.

Rule#2: Cross-Sectional Area Of A Rectangular Conductor For Square Conductors, as in View B in fig.1, to obtain the cross-sectional area in circular mils of a rectangular conductor, multiply the length times the width of the end face of the conductor (side is expressed in mils). So, Cross-sectional area (A ) in mils = Length x Width Noting that Side Length and Width must be in mils.

Example#2:

Assume that one side of the rectangular cross-sectional area is 6 mils and the other side is 3 mils. What its cross-sectional area in circular mils?

Solution:

Cross-sectional area (A) = Length x Width = 6 mils × 3 mils= 18 square mils.

Example#3:

Assume that a conductor is 3/8 inch thick and 4 inches wide. What its cross-sectional area in circular mils?

Solution:

The 3/8 inch can be expressed in decimal form as 0.375 inch. Since 1 mil equals 0.001 inch, the thickness of the conductor will be 0.001 × 0.375, or 375 mils. Since the width is 4 inches and there are 1,000 mils per inch, the width will be 4 × 1,000, or 4,000 mils. Cross-sectional area (A) = Length x Width = 375 mils × 4,000 mils= 1,500,000 square mils.

1.3 The measurement unit “CIRCULAR MIL”

Definition:

The circular mil: is the standard unit of measurement of a round wire crosssectional area (view C of Fig.1). This unit of measurement is found in American and English wire tables. Also, A circular mil: is the area of a circle having a diameter of 1 mil, as shown in view B of Fig.2.

Important!!! The diameter of a round conductor (wire) used to conduct electricity may be only a fraction of an inch. Therefore, it is convenient to express this diameter in mils to avoid using decimals. For example, the diameter of a wire is expressed as 25 mils instead of 0.025 inch.

Rule#3: Cross-Sectional Area Of A Round Conductor For Round Conductors, as in View C in fig.1, to obtain the cross-sectional area in circular mils of a round conductor is obtained by squaring the diameter, measured in mils. So, Cross-sectional area (A) in mils = D2 , D in mils Noting that Diameter D must be in mils.

Example#4:

Assume that a wire having a diameter of 25 mils. What its cross-sectional area in circular mils?

Solution:

Cross-sectional area (A) = D2 = 25 x 25 = 625 circular mils

Comparing Square Mils with Circular Mils for the same Conductor:

Example#5:

A wire having a diameter of 25 mils, compare between its area calculated in Square Mils and in Circular Mils?

Solution:

a- Calculation of conductor area in square mils: To determine the number of square mils in the same conductor, apply the conventional formula for determining the area of a circle (A = 3.14r2). In this formula, A (area) is the unknown and is equal to the cross-sectional area in square mils, and r is the radius of the circle, or half the diameter (D). Through substitution, A = 3.14 x (12.5) 2 = 490.625 square mils. b- Calculation of conductor area in circular mils: A wire having a diameter of 25 mils has an area of 252, or 625 circular mils.

The cross-sectional area of the wire has 625 circular mils but only 490.625 square mils.

Important!!! For one conductor (wire or cable), a circular mil represents a smaller unit of  area than the square mil.

Definition: A wire in its usual form is a single slender rod or filament of drawn metal. In large sizes, wire becomes difficult to handle. To increase its flexibility, it is stranded. Strands are usually single wires twisted together in sufficient numbers to make up the necessary cross-sectional area of the cable. So, Stranded Conductor: A conductor composed of individual groups of  wires twisted together to form an entire unit.

Rule#5: Cross-Sectional Area Of A Stranded wire/cable The total area of stranded wire in circular mils is determined by multiplying the area in circular mils of one strand by the number of strands in the cable. So, Cross-sectional area (A) in mils = N x Cross-sectional area (A) in mils for one stand Where N = number of strands in the cable

1.4 The measurement unit “American Wire Gauge (AWG)”

Definition: American Wire Gauge (AWG): A standard system used in the United States for expressing wire diameter and designing the size of an electrical conductor based on geometric progression between two conductor sizes, Based on a circular mil system. The American Wire Gage (AWG) is the same as the Brown and Sharpe (BS) Gage.

Important!!! As the AWG number gets smaller, the wire diameter (gauge) gets larger.

Rule#6: Expressing wire/cable Size in AWG 1- Wire sizes up to size 4/0 AWG: They are expressed as XX AWG, with XX being the size wire. For example, a wire size expressed as 12 AWG. 2- Conductors larger than 4/0 AWG: They are sized in circular mils, beginning with 250,000 circular mils. For example, a wire size expressed as 250,000-circular-mils. Noting that: 1000 circular-mils = MCM = Kcmil For example, 250,000-circular-mils conductor was labeled 250 MCM or 250 Kcmil

Example#6:

What is the circular mil area of an 8 AWG solid conductor that has a 0.1285-in. diameter?

Solution:

Diameter (D) in mils = 0.1285 in. x 1000 = 128.5 mils Cross-sectional area (A) of Round conductor in mils = D2 = 128.5 x 128.5 = 16,512.25 circular mils Or 16,510 circular mils (rounded off).

2- Conversions between different Measurement Units of Conductors Cross Section Area

Rule#4: Unit Conversions Mil = 0.001 inch = 0.0254 mm Inch = 1000 mil = 25.4 mm Square inch = 645.16 mm2 = 1,000,000 square mils Circular mil = 0. 7854 square mil 1000 circular-mils = MCM = Kcmil

Example#7:

A 12-gauge wire has a diameter of 80.81 mils. What is (1) Its area in circular mils and (2) Its area in square mils?

Solution:

(1) A = D2 = 80.81 X 80.81 = 6,530 circular mils (2) A = 0.7854 x 6,530 = 5,128.7 square mils

Example#8: A rectangular conductor is 1.5 inches wide and 0.25 inch thick. What is (1) Its area in square mils and (2) In circular mils? What size of round conductor is necessary to carry the same current as the rectangular bar?

Solution:

(1) 1.5 inches = 1.4 x 1000 = 1,500 mils 0.25 inch = 0.25 x 1000 = 250 mils A = 1,500 x 250 = 375,000 square mils (2) To carry the same current, the cross-sectional area of the round conductor must be equal. There are more circular mils than square mils in this area. Therefore: A = 375,000 / 0.7854 = 477,000 circular mils.

3- Standard Sizes of Conductors

An AWG table for copper wire (solid only) is shown at table #1.

Table#1

Notes to table#1:

•

•

•

•

•

The largest wire size shown in the table is 0000 (read "4 naught"), and the smallest is number 40. The following sizes can be expressed in other way as follows: 0000 =4/0, 000 = 3/0, 00 = 2/0, 0 = 1/0. It shows the diameter in mils, circular mil area, and area in square inches of AWG wire sizes. It shows the resistance (ohms) per thousand feet and per mile of wire sizes at specific temperatures. It shows the weight of the wire per thousand feet in the last column.

Example#9: (Using table #1) You are required to run 2,000 feet of AWG 20 solid copper wire for a new piece of equipment. The temperature where the wire is to be run is 25º C (77º F). How much resistance will the wire offer to current flow?

Solution:

In table#1, under the gauge number column, find size AWG 20. Now read across the columns until you reach the "ohms per 1,000 feet for 25º C (77º F)" column. You will find that the wire will offer 10.4 ohms of resistance to current flow. Since we are using 2,000 feet of wire, multiply by 2. Resistance of 1,000 feet for 25º C (77º F) = 10.4 ohms So, Resistance of 2,000 feet = 10.4 ohms x 2 = 20.8 ohms

Important!!! An American Standard Wire Gauge (fig.3) is used to measure wires ranging in size from number 0 to number 36. To use this gauge, insert the wire to be measured into the smallest slot that will just accommodate the bare wire. The gauge number on that slot indicates the wire size. The front part of the slot has parallel sides, and this is where the wire measurement is taken. It should not be confused with the larger semicircular opening at the rear of the slot. The rear opening simply permits the free movement of the wire all the way through the slot.

Fig (3)

Table#2 introduces copper wire AWG sizes (solid and stranded) as follows:

AWG

MM2

AWG

MM2

30

0.05

6

16

28

0.08

4

25

26

0.14

2

35

24

0.25

1

50

22

0.34

1/0

55

21

0.38

2/0

70

20

0.50

3/0

95

18

0.75

4/0

120

17

1.0

300MCM

150

16

1.5

350MCM

185

14

2.5

500MCM

240

12

4.0

600MCM

300

10

6.0

750MCM

400

8

10.0

1000MCM

500

Table#2

Or you can use the following excel sheet converter from AWG to MM2 or vice versa.

For downloading your copy of AWG to MM2 Converter, please click on the link.

In the next Article, I will continue explaining the Conductor Ampacity calculations. Please, keep following.

Conductor Ampacity Calculation – Part Three

Today, I will explain the methods for Conductor Ampacity Calculations as follows.

The need for Ampacity calculation When the ampacity of conductors is not accurate, the conductors may carry more currents than their rating and become overloaded, so they will heat up and short out.

Note: As the conductor heats up the current carrying capacity goes down.

When do we need Ampacity calculation? 1. When modifications are done to existing circuits, 2. On all new designs, the conductors/feeders ampacity should be checked.

Rule#1: Methods for Conductors Ampacity Calculations as per NEC code As per NEC Article 310, Methods for Conductors Ampacity Calculations will depend on the voltage rating of these conductors, it is divided to: 1. Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts, 2. Methods for Ampacity Calculations of Conductors Rated 2001 to 35,000 Volts.

Part one: Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts

Rule#2: Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts

As per 310.15(A)(1), The allowable Ampacities for conductors rated 0-2000 Volts shall be permitted to be determined by two methods: 1. Tables as provided in 310.15(B) or 2. Under engineering supervision, as provided in 310.15(C).

First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B)

In this method, I will explain the following points: •

General Overview of Allowable Ampacity Tables for conductors rated 0 to 2000 volts,

•

General rules controlling the conductor ampacity calculations,

•

Factors affecting conductor ampacity.

1- General Overview of Allowable Ampacity Tables for conductors rated 0 to 2000 volts

The Allowable Ampacities for conductors rated 0 to 2000 volts are specified in the following tables: •

Table 310.15(B)(16)

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Table 310.15(B)(17)

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Table 310.15(B)(18)

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Table 310.15(B)(19)

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Table 310.15(B)(20)

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Table 310.15(B)(21)

To download a PDF file that includes The Allowable Ampacities for conductors rated 0 to 2000 volts, click on the link.

The above tables need to be modified to meet existing installation conditions as per the following tables: •

TABLE 310.15(B)(2)(A)

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TABLE 310.15(B)(2)(b)

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TABLE 310.15(B)(3)(a)

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TABLE 310.15(B)(3)(c)

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TABLE 310.15(B)(7)

These tables will be explained later.

1.1 Allowable Ampacities Tables Construction: For example, Table 310.15(B)(16) which provides Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F).

Table 310.15(B)(16) will consists of:

1.1.A Conditions for table application These conditions are existing in the table description at the top of the table (see below image), and in table 310.15(B)(16), these conditions are: 1. Conductors Rated Up to and Including 2000 Volts 2. Actual conductor temperature ranges from 60°C Through 90°C (140°F Through 194°F) 3. Not more than three current-carrying conductors in a raceway, cable or earth (directly buried). 4. An ambient temperature of 30°C (86°F).

1.1.B Conductor Material Sections Two main Sections for conductor material type are existing (see above image): 1. One for copper, 2. One for aluminum or copper-clad aluminum conductors.

1.1.C Temperature Categories Each of The two main sections for conductor material types is divided into three temperature categories (see below image): 1. 60°C (140°F), 2. 75°C (167°F) and 3. 90°C (194°F).

Noting that each temperature category is applicable with certain insulation properties which are listed in the row under each temperature category.

1.1.D Conductor Sizes Columns Two Columns for conductor sizes range from 18 AWG to 2,000 kcmil (see below image): 1. The right left one beside copper conductors column is for copper conductors sizes, 2. The right one beside aluminum or copper-clad aluminum conductors column is for aluminum or copper-clad aluminum conductors sizes.

1.2 Notes for Allowable Ampacities Tables:

Note#1 The asterisk (*) next to the ambient temperature of 30°C (86°F) included in table description at the top of the table, Refer to table 310.15(B)(2) for application of the ampacity correction factors where the ambient temperature is other than 30°C (86°F).

Note#2 The asterisk (**) next to wire sizes 10, 12, and 14 AWG refers to article 240.4(D) and is letting you know that conductor sizes #14, #12, and #10 copper and aluminum are limited to certain size overcurrent protection devices (OCPD) even if the ampacity of the #14, #12, and #10 conductor is higher. The limitations are: A- For Copper Conductors: •

#14 copper limited to 15 amp OCPD

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#12 copper limited to 20 amp OCPD

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#10 copper limited to 30 amp OCPD

B- For Aluminum Conductors:

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#14 aluminum not allowed

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#12 aluminum limited to 15 amp OCPD

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#10 aluminum limited to 25 amp OCPD

Note#3 Ampacity tables, particularly Table 310.15(B)(16), do not take into account all the many factors affecting ampacity. If loads are not calculated in accordance with the requirements of Article 220, the table ampacities, even when corrected in accordance with ambient correction factors and the notes to the tables, might be too high. Especially where many cables or raceways are routed close to one another underground.

Note#4 Copper conductors of the same size have three different allowable ampacities. The same is true for aluminum (and copper-clad aluminum) conductors. This is because The maximum allowable ampacities depend on the conductor’s temperature rating. For example, a 3 AWG copper conductor with a temperature rating of 60°C has a maximum allowable ampacity of 85 amperes (A). The maximum allowable ampacity of the same 3 AWG copper conductor with a temperature rating of 75°C is 100A. If the temperature rating of the 3 AWG copper conductor is 90°C, the allowable ampacity is 115A .

Note#5 All The Allowable Ampacities Tables refer to table 310.104(A) (see below image) which contains information about conductors rated 600V. Conductor information in this table includes trade name, type letter, maximum operating temperature, application provisions, insulation, thickness of  insulation, and outer covering (if any). To download a PDF file for Table 310.104(A) , click on the link.

Note#6: Conductor Lettering You can note in allowable ampacity tables that each temperature category is applicable with certain wiring properties which is listed in the row under each temperature category. When looking at these wiring properties, you will notice that the wires contain some type of letter identifier such as THHN or THW. These letters serve to identify specific properties of the conductor and/or its insulation. Listed below are some of the letters commonly used in allowable ampacity tables:

Letter Identifier T W R  F FF -2 H HH N

Conductor and/or Insulation Property Thermoplastic Wet or Damp Rubber Fixture Wires Fixture wire, flexible stranding Conductor is permitted 90ºC operating temperature 75ºC insulation rating 90ºC insulation rating Nylon outer cover

The combination of letters will tell you most of what you need to know about the conductor.

NOTE: This is a general rule and there are exceptions such as THW (see Table 310.13). The one H indicates 75° C rating but it can be used at a 90° C conductor with ballasts in dry locations.

Examples for Conductor Lettering: •

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TW - Thermoplastic (T) insulation, (W) suitable for wet and dry locations (When suitable for wet generally means it can be used for dry also), and is rated 60° C (no H in group). RHW - Rubber (R) insulation, (H) rated 75° C and (W) suitable for wet and dry locations.

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TF - Thermoplastic (T) insulation, (F) fixture wire

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RFH – Rubber (R) insulation, (F) fixture wire, (H) rated 75° C

1.3 How to use these Allowable Ampacities Tables:

Step#1: Determine the proper table to use based on the existing wiring method conditions

For example, if we have a 6 AWG Type THWN copper wire in free air based on an ambient air temperature of 30° C (86° F), which table we will use? Yes, it is table 310.15(B)(17), because tables Table 310.15(B)(16) and table 310.15(B)(18) don’t be used with single conductor in free air. Don’t use table TABLE 310.15(B)(19) because

the ambient air temperature is 30° C, and THWN insulation is not listed in it.

Step#2: Determine the proper section in the selected table based on the conductor material type (Copper – Aluminum or Copper Clad Aluminum)

In same example above, because this is a copper conductor, we will use the section in the left side of table 310.15(B)(17).

Step#3: Locating the Ampacity Value

In the table section selected above, move down to the given conductor size, which is 6 AWG in our example, the amapcity value are existing in the 6 AWG Row. Now, move to the right in the 6AWG row until you reach the column for THWN insulation for a copper conductor. This will be the 75° C (167° F) column.

Step#4: Reading the Ampacity Value Now read the amapcity of the conductor, you should read 95.

Another example:

What is the allowable ampacity of a non-metallic-sheathed cable containing three 12 AWG Type THW-2 Copper wires based on an ambient air temperature of 30° C(86° F)?

Solution:

Step#1: Determine the proper table to use

Because this cable containing three conductors, use table Table 310.15(B)(16). Don’t use table 310.15(B)(17) or table TABLE 310.15(B)(19) because there are three conductor in a cable. Don’t use table 310.15(B)(18) because the ambient air temperature is 30° C, and THW-2 insulation is not listed in it.

Step#2: Determine the proper section in the selected table based on the conductor material type (Copper – Aluminum or Copper Clad Aluminum)

Because these are copper conductors, we will use the section in the left side of table Table 310.15(B)(16).

Step#3: Locating the Ampacity Value In the table section selected above, move down to the given conductor size, which is 12 AWG , the amapcity value are existing in the 12 AWG Row. Now, move to the right in the 6AWG row until you reach the column for THW-2 insulation for a copper conductor. This will be the 90° C (194° F) column.

Step#4: Reading the Ampacity Value Now read the amapcity of the conductor, you should read 30.

In the next Article, I will continue explaining the Conductor Ampacity calculations from Tables as provided in 310.15(B) . Please, keep following.

Conductor Ampacity Calculation – Part Four

In Article " Conductor Ampacity Calculation – Part Three ", I explained the following points: 1. The need for ampacity calculation, 2. When do we need ampacity calculation?, 3. Methods for Conductors Ampacity Calculations as per NEC code, 4. Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts, 5. First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B),

6. General Overview of Allowable Ampacity Tables for conductors rated 0 to 2000 volts.

Today, I will continue explaining the First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B) as follows.

For more information and good following, please review the following articles: •

Conductor Ampacity Calculation – Part One

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Conductor Ampacity Calculation – Part Two

2- General rules controlling the conductor ampacity calculations In article " Conductor Ampacity Calculation – Part Three ", I explained the first point: how to use the allowable ampacity tables, this was as a general method, but this general method is controlled and refined by the other rules as follows.

Rule#1: Temperature Limitation of Conductors. As per 310.15(A)(3), No conductor shall be used in such a manner that its operating temperature will exceed that designated for the type of insulated conductor involved.

The above rule said that: •

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Conductor with 60°C temperature rating must have ampacity from the 60°C column. Conductor with 75°C temperature rating must have ampacity from the 60°C column or the 75°C column and according to Rule#2 in below. Conductor with 90°C temperature rating must have ampacity from the 60°C column or the 75°C column and according to Rule#2 in below or from the 90°C column after applying correction factors.

Using The 90°C Column Be careful when using the 90°C column because no equipment is listed and identified for use with 90°C conductors other than individual lugs, terminal bars and equipment listed for use on circuits over 600V. The 90 ° C ampacity can be used for corrections only for factors:

Ambient Temperature Number of Conductors

Rule#2: Selection Of Conductor Ampacity Based On Its Terminations’ Temperature Ratings As per 110.14(C), Conductors must be sized in accordance with the lowest temperature rating of any terminal, device, or conductor insulation of the circuit.

Example#1:

A THHN conductor will have a 60°C termination on one end and a 75°C termination on the other, which ampacity column will be used?

Solution:

The lowest temperature rating of conductor’s two terminals is 60°C. So, the conductor’s ampacity must not exceed the rating listed in the 60°C column Conductors.

How to Select Conductor Ampacity Based On Its Terminations’ Temperature Ratings

Selection of the appropriate conductor amapcity column depends on the temperature rating of  the termination (or connection) points as follows:

First: For Conductor/equipment Rated 100A or Less or marked for 14 AWG through 1 AWG

The selected ampacity column will be as follows:

1- Use the 60°C column. Example#2:

What is the Maximum allowed ampacity for a single #2 THHN Conductor installed in a circuit.

Solution:

Step#1: 110-14(c) (#14 thru #1), so Use the 60°C column Step#2: use Table 310.15(B)(16), (60°C Column) and Read Ampacity The conductor ampacity is 95 Amps

2- If any termination is either 60°C or unknown, Use the 60°C column regardless of the insulation rating of the conductor. Example#3:

A THHN conductor will have a 60°C termination on one end and a 75°C termination on the other, which ampacity column will be used?

Solution: Because one of the connection points has a 60°C rating, the conductor’s ampacity must not exceed the rating listed in the 60°C column Conductors.

3- If the equipment is listed and identified for use with certain conductor temperature rating, Use the column with temperature rating = the lowest temperature rating of  conductor terminations.

If Temp. rating of listed conductor is 75°C 75°C 90°C 90°C

Termination#1 Termination#2 75°C 75°C 75°C 75°C

75°C 60°C 75°C 60°C

Use column 75°C 60°C 75°C 60°C

Example#4:

A THHN conductor will have 75°C termination on one end and a 60/75°C termination on the other, which ampacity column will be used?

Solution:

A temperature rating of 60/75°C means the equipment has been listed for both 60°C and 75°C conductors; therefore, it is permissible to use the 75°C rating if the installed conductor is rated at least 75°C. Because all of the connection points in this example have at least a 75°C rating, the conductor’s ampacity can be based on the 75°C column

4- Use the 75°C column for conductors supplying power to motor marked with a design letter B, C or D and the conductor temperature rating is not less than 75°C.

If a motor marked with a design letter B, C or D use conductor with temp. rating 75°C 90°C

Use column 75°C 75°C

Example#5:

THHN conductors will have 75°C terminations on one end and a motor marked with a design letter D on the other end. After complying with the applicable requirements in Article 430, the conductors supplying power to this motor must have an ampacity of at least 55 amperes (A). What is the minimum size of THHN conductors required to supply power to this motor?

Solution:

Motor with a design letter D and the THHN conductors will have 75°C terminations on the other hand. So, we must use the 75°C column even if THHN is not listed in the 75°C column and listed in the 90°C column. Using table 310.15(B)(16), for a 55 A motor, the smallest conductor with ampacity higher than 55 A is 6 AWG (with 65A ampacity). So, Conductors supplying power to this motor must be at least 6 AWG.

Notes for example#5:

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In table 310.15(B)(16), The allowable ampacity of an 8 AWG THHN conductor (in the 90°C column) is 55A. Although conductors with a temperature rating of 90°C can be installed, the ampacity must not exceed the 75°C ampacity. The ampacity of an 8 AWG conductor in the 75°C column is 50A. Because this motor requires a minimum ampacity of 55A, installing 8 AWG conductors is not permitted.

Second: For Conductor/equipment Rated over 100A or marked for larger than 1 AWG

The selected ampacity column will be as follows:

1- Use the 75°C column. Example#6:

THWN conductors (larger than 1 AWG) will supply power to a circuit rated greater than 100A, the conductors will have 75°C terminations on both ends. Which ampacity column will be used?

Solution:

The ampacity can be based on the 75°C column because the conductors are larger than 1 AWG, and the circuit it is supplying is rated greater than 100A.

2- If the equipment is listed and identified for use with certain conductor temperature rating, Use the column with temperature rating = the lowest temperature rating of  conductor terminations.

If Temp. rating of listed conductor is 75°C 75°C 90°C 90°C

Termination#1 Termination#2 75°C 75°C 75°C 75°C

75°C 60°C 75°C 60°C

Use column 75°C 60°C 75°C 60°C

Example#7:

What is the maximum ampacity for a 1/0 AWG THHN copper conductor fed from a 150A breaker? Assume an ambient temperature of 30°C and no more than three current-carrying conductors in the raceway. The conductors will have 75°C terminations on one end, but the temperature rating of the terminations on the other end is unknown.

Solution:

We must not exceed the 75°C ampacity for this conductor. Although the temperature rating on one end is unknown, the ampacity of this 90°C conductor (since it is THHN) can be based on the 75°C column Using table 310.15(B)(16), and Because the conductors are larger than 1 AWG and the circuit it is supplying is rated greater than 100A. This 1/0 AWG THHN copper conductor has a maximum ampacity of 150A.

Rule#3: Selection Of Ampicity For Multi-Ampacity Conductors As per 310.15(A)(2), Where more than one ampacity applies for a given circuit length, the lowest value shall be used.

For example, in below image, The ampacity for No. 3 THHN (90°C) is 110A, but the correction factors of Table 310 reduce the conductor ampacity to only 96A because the lowest ampacity value = 110A x 0.87 = 95.7A and You round up to get 96A.

Exception to Rule#2 A higher ampacity is permitted, if the length of the reduced ampacity doesn’t exceed 10 ft and is not longer than 10% of the total length of the circuit.

For example, in below image,, The ampacity for each No. 12 THHN is 30A, but the correction factors in Table 310-15(b)(2)(a) reduce the conductor ampacity by 50% to be 15A inside the panelboard but it will be 30 amps outside the panelboard.

Rule#4: Minimum Size of Conductors As per 310.106(A), the minimum size of conductors shall be as shown in Table 310.106(A) see below image, except as permitted in other sections, such the following: •

Small conductor sizes 18 and 16 AWG as permitted by 240.4(D)(1) and (2)

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Flexible cords as permitted by Table 400.4

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Fixture wire as permitted by 402.6

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Motors rated 1 hp or less as permitted by 430.22(F)

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Cranes and hoists as permitted by 610.14

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Elevator control and signaling circuits as permitted by 620.12

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Class 1, Class 2, and Class 3 circuits as permitted by 725.49(A) and 725.127, Exception Fire alarm circuits as permitted by 760.49(A); 760.127, Exception; and 760.179(B) Motor-control circuits as permitted by 430.72

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Control and instrumentation circuits as permitted by 727.6 Electric signs and outline lighting as permitted in 600.31(B) and 600.32(B)

Notes for Rule#4:

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The smallest size conductor permitted by the NEC for branch circuits, feeders, or services is 14 AWG copper or 12 AWG aluminum. Some local codes require a minimum 12 AWG for commercial and industrial installations. Conductors smaller than 14 AWG are permitted for:

1. Class 1 remote-control circuits, 2. Fixture wire, 3. Flexible cords, 4. Motor control circuits, 5. Non-power-limited fire alarm circuits, 6. Power-limited fire alarm circuits.