SIZING BASIC CIRCUITS
BRANCH CIRCUIT WIRING WI RING FUNDAMENTALS 1. What is a branc branch h circu circuit? it? (PEC 2009 ART 2.10) The circuit conductors between the final overcurrent device protecting the circuit and the outlet. a. Single Single phase phase bran branch ch circ circuit uit two wires + ground (commonly called single phase 3 wires) b. Three phase branch circuit – three wires + neutral wire +ground wire ( commonly called 3phase 5 wires)
Four Categories Branch Circuit, Appliance – A branch circuit that supplies energy energy to one or more outlets to which appliances are to be connected and that has no permanently connected luminaires (lighting fixtures) that are not part of an appliance
BRANCH CIRCUIT WIRING WI RING FUNDAMENTALS 1. What is a branc branch h circu circuit? it? (PEC 2009 ART 2.10) The circuit conductors between the final overcurrent device protecting the circuit and the outlet. a. Single Single phase phase bran branch ch circ circuit uit two wires + ground (commonly called single phase 3 wires) b. Three phase branch circuit – three wires + neutral wire +ground wire ( commonly called 3phase 5 wires)
Four Categories Branch Circuit, Appliance – A branch circuit that supplies energy energy to one or more outlets to which appliances are to be connected and that has no permanently connected luminaires (lighting fixtures) that are not part of an appliance
Branch Circuit, General Purpose – A branch circuit that supplies two or more receptacles or outlets for lighting and appliances.
Branch Circuit, Individual- A Individual- A branch circuit that supplies only one utilization equipment Branch Circuit, Multiwire – A branch circuit that consists of two or more ungrounded conductors that have voltage between them, and a grounded conductor that has equal voltage between between it and and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system
2. Relevant Code Requirements a. The ampacity of b r a n c h c i r c u i t c o n d u c t o r s must not be less than the maximum load to be served (PEC ART 2.10.2.1) b. The m ax i m u m l o ad to be served by the branch circuit conductors when supplying two or more receptacles must not be more than 80% of the ampacity of the conductors. (PEC ART2.10.2.3 (a), (b) ,(c) ) Table 2.10.2.3(b)2 c. The rating of a branch circuit is established or defined by the rating or setting of its protective device. (PEC ART2.10.2.2 (a) thru (d) d. The ampacity of the branch circuit conductors must not be less than the rating of the branch circuit. e. The total load of on any overcurrent device in a panelboard must not exceed 80% of the rating of the overcurrent device.
Relevant Code Requirements (continuation) f. Circuit conductors shall be protected against over-current in accordance to their ampacities, but where the ampacity of the conductor does not correspond with the standard ampere rating of a fuse or circuit breaker, the next higher rating shall be permitted only if this rating does not exceed 800 amperes (PEC ART 2.40.1.4) g. The normal ampacities of conductors in cable or raceways are given in PEC Table 3.10.1.16 copper and aluminum based on a 30 oC ambient temperature. For ambient temperature over or under 30 oC, correction factors must be considered . (to be found just below Table 3.10.1.16) h. These normal ampacities may have to be reduced or derated where there are more than three conductors in a cable or raceway . This means a change in ampacities of circuit conductors (Table3.10.1.15(b)(2)a) i. The current permitted to be carried by the branch circuit may have to be reduced if the load is continuous. This does not mean a change in the ampacities of the conductor but the rule refers to a limit of the load to be carried by the conductors.
j. Continuous load refers to a load that operates for three hours or more such as store lighting, office lighting and similar lighting loads. k. Overcurrent protection for any single non-motor operated appliance with ratings of 10 amperes or more must not be more than 150% of its ampere rating. l. General purpose receptacle outlet other than dwelling shall be taken as a load of 180va for each single or for each multiple receptacle on one yoke. (ART 2.20(k))
Case 1: 20Amps Circuit Breaker
3.5mm2 THW, ampacity is 20A
16 amps maximum continuous load
Lighting outlets
The circuit is in accordance with the code requirement. The rating of this circuit is 20 Amperes. The load therefore must be only 16A maximum allowed by the code
Case 2: 20Amps Circuit Breaker
2.0mm2 THW ampacity is 15A
12 amps maximum continuous load
Lighting outlets
The circuit is in violation of the code requirement. The rating of this circuit is greater than the ampacity of the circuit conductors. The conductors will be overloaded even before the overcurrent device works.
Case 3: 20Amps Circuit Breaker
8mm2 THW, ampacity is 40A
16 amps maximum continuous load
Lighting outlets
The circuit is in accordance with the code requirement. The rating of this circuit is 20 amperes not 40 amperes. The circuit should be loaded continously at 16 amperes maximum
TABLE 3.10.1.15(B)(2)A Adjustment Factors for More than Three current Carrying Conductors in a Raceway or Cable
Number of Current carrying Conductors
% Values in Tables 3.10.1.16-19 as adjusted for ambient temperature if Necessary
4 to 6
80%
7 to 9
70%
10-20
50%
21-30
45%
31-40
40%
41 and above
35%
3. DERATING CONDUCTOR AMPACITIES a. The preceding illustrations are true if the wires are placed in conduits not exceeding 3 current-carrying conductors in ambient temperature of 30oC. Typical Circuit Distribution in a Panelboard In this example the ampacity of 8 of # 3.5MM2THW conductors (20 amps) has to be derated to 70% or 14 amps. Correspondingly the circuit protection shall be resized to 15 amps based on the new ampacity of 14 amps
Notes: Limiting the load to 80% for being continous load (office lighting loads that operate for three hours or more Must not be construed as a change in ampacity of the Circuit conductors
Eight #3.5MM2THW Cu circuit conductors serving continuous loads
Derated ampacity = 20 x 0.70 x 0.80 =11.2Amps
4. CORRECTION FACTORS IN TEMPERATURES OVER 30oC a. On top of the derating factors due to the large number of conductors in a conduit or cable, there is a correction to the conductor ampacities when installed or operated at temperatures over or under 30oC. b. In tropical countries like Philippine setting, especially circuits installed in between ceiling and the roofing, the factor corresponding 31-40 deg C has to be applied. Consideration must likewise to be observed in hot rooms in industrial plants as boiler rooms, furnace areas and the like. 5. CONDUCTOR AMPACITY REQUIREMENTS Example 1 What’s the circuit ampacity for three 30mm2 THHN conductors run in a raceway through the ambient temperatures zones as shown in the figure a) b) c) d)
115 110 120 100
amps amps amps amps
Temp =35oC 0.96 correction
Temp =28oC 1.00 correction
115A
A
Temp =22oC 1.04A correction
110A
B
Temp =45oC 0.87correction
120A
100A
C
D
90oC conductor
Ambient temperature ampacity adjustment for 30mm2 THHN (rated 115A)
The answer is d) 100. The ampacity for 30mm2 THHN (90oC is 115amps, but the correction factors of table reduce the conductor ampacity to only 100A ( 115amps x 0.87= 100) Note: Where more than one calculated or adjusted ampacities could apply to a given circuit length, the lowest ampacity must be used for the entire circuit in calculations.
6. CIRCUIT SIZING EXERCISES Example 1: A single phase circuit is to be wired in 75 oC THW copper wire dedicatedly installed inside a conduit in an area with an ambient temperature of 36oC . If the circuit is supposed to carry 135 amps noncontinuous load current, what is the minimum size of wire and the circuit overcurrent protection? Answer: a) Derating factor for 2 wires in a conduit = 100% b) Correction factor, 36oC ambient temperature =88% c) Non-continuous loading factor = 100% Conductor size = 135A / (1.0 x 0.88 x 1.0) Conductor size = 153.4 amps Use 60mm2 THW, Ampacity = 160 amps (Table 3.10.1.16 p. 350) Derated ampacity = 160A x 0.88 = 141Amps Use 150Amps standard rated MCCB
Example 2: There are 2 single phase circuit to be wired in 75oC THW copper wire installed inside a conduit in an area with an ambient temperature of 36 oC . If each circuit is designed to carry 135 amps continuous load current, what is the minimum size of wire and the circuit overcurrent protection for each circuit? The branch circuit OCPD (or conductor) must have a rating of not less than the noncontinupus load plus 125% of the continuous load before applying an ampacity adjustment factor. {Articles 2.10.1.1(a) and 2.10.2.2(a)} Answer: a) Derating factor for 4 wires in a conduit = 80% (Table 3.10.1.15) b) Correction factor, 36oC ambient temperature =88% c) Continuous load limitation = 80% Size of each conductor shall be: Conductor size = 135A / (0.8 x 0.88 x 0.80) Conductor size = 239.7 amps Use 125mm2 THW, Ampacity = 255 amps (Table 3.10.1.16 p. 350) Derated ampacity = 255A x 0.8 x0.88 = 179.5Amps (Note: 80% load limitation for continuous loads is not a derating factor) Use 175Amps standard rated MCCB
Example 3: Four single phase circuits are to be wired in 75oCTHW copper wire inside a conduit passing through an area with an ambient temperature of 45 oC . If each circuit is designed to carry 12 amps continuous load current, what is the minimum size of wire and the circuit overcurrent protection for each circuit? Answer: a) Derating factor for 8 wires in a conduit = 70% (Table 3.10.1.15) b) Correction factor, 45oC ambient temperature =82% c) Continuous load limitation = 80% Size of each conductor shall be: Conductor size = 12A / (0.7 x 0.82 x 0.80) Conductor size = 26.13 amps Use 5.5mm2 THW, Ampacity = 35 amps (Table 3.10.1.16 p. 350) Derated ampacity = 35A x 0.7 x0.82 = 20.09Amps (Note: 80% load limitation for continuous loads is not a derating factor) Use 20 Amps standard rated MCCB
WIRES AND CABLES
7. SIZING CONDUCTORS BASED ON TEMPERATURE RATING Question : If the engineer wants to use a THW wire , then we refer to the 75oC column YES If the engineer wants to use a THHN wire , then we refer to the 90 oC column NOT QUITE Has anyone heard of operational problems with THHN wires & cables? The problem is in the difference in temperature ratings from device terminals to wire terminals when connected together.
POINTS TO REMEMBER: 1. For device or equipment terminals rated 100A or less, or for 2.0mm238mm2conductors, terminal sizes shall be based on the 60 oC temperature rating ( Art 1.10.1.14(c) ) Ex. What size THHN conductor does the PEC require for a 50A circuit if the terminals are not marked with a temperature rating? a) 5.5mm2 b) 8mm2 c) 14mm2 d) any of these Ans –c) 14mm2 (70 amps) and not 8mm2 (55amps) shall be used because conductor size must be selected on the 60oC terminal rating of the device not the 90oC rating of the insulation
Ex. What size THHN conductor does the PEC require for a 50A circuit if the terminals are marked for use on 75oC temperature rating? a) 5.5mm2 b) 8mm2 c) 14mm2 d) any of these Ans. b) 8mm2 (55amps) .Size the conductors to the 75oC terminal rating of the device and not 90oC. 2. Circuits over 100A -unless the terminals are marked otherwise, or for conductors larger than 38mm2 equipment /device terminals rated over100A shall be sized according to 75oC temperature rating (Art. 1.10.1.14(c) ) Ex. What size THHN conductor is required to supply a 225amps feeder a) 50mm2 b) 60mm2 c)80mm2 d)125mm2 Ans. 125mm2 (265Amps). Size the conductors to the 75oC terminal rating 3. What then is the purpose of 90 oC wire if we can not use its higher ampacity? Its rating comes into play when adjusting conductor ampacity for elevated ambient temperature or when bundling more than three current carrying conductors together (derating). The advantage is that it can keep the designer from using a larger wire which requires larger raceways, greater labor and increased material cost. • •
•
To Review: Important
in the electrical & thermal relationship for circuit components are the conductor size, rated ampacity, the insulation temperature rating and the permissible connector device temperature limits For
instance, the ampacity for a conductor with 90degC insulation (THHN) is generally greater than of a conductor of the same size but with 60degC (TW) insulation. However,
the greater ampacity of a THHN conductor with 90degC insulation is not always permitted to be used due to limitations of the terminal temperature rating and /or the requirements of the PEC.
EXAMPLE: FEEDER CONTINUOUS LOAD
The feeder circuit OCPD (or conductor) must have a rating of not less than the noncontinupus load plus 125% of the continuous load before applying an ampacity adjustment factor. {Articles 2.15.1.2(1) and 2.15.1.3} 1. What size of OCPD/feeder THHN conductor does the PEC require for a 104A continuous load? a. 150A -50mm2 c. 125A – 30mm2 b. 150A- 38mm2 d. 125A – 38mm2 Step 1 : The overcurrent protective device (OCPD) must have a rating of not less than 125% of the continuous load. Thus , 104 A x 1.25=130A The nearest OCPD rating is 150A protection Step 2: The feeder conductor must have an ampacity of not less than 125% of the continuous load before any ampacity adjustment. Thus the feeder must be : 104A x 1.25= 130A 38mm2THHN is rated 130A at 75 deg C and the 150A overcurrent protection device can protect it. The answer is (b), 150A OCPD with 38mm2 THHN
SIZING AND INSTALLING PARALLEL CONDUCTOR SETS In the case of high ampacity feeders, one conductor per phase may not be practical. Under these circumstances , two or more conductors per phase may be desirable or even necessary. The objective is to keep the resistance of each parallel path equal so the load current will divide equally on each wire of the set. The wires and terminations heat up when they carry current; therefore the resistance changes when the temperature of the wire and terminations changes. For these reasons, special requirements are necessary when installing parallel sets of wires. The following is a summary of some special installation rules for sets of multiple conductors for each ungrounded conductor and grounded conductor. 1. Power conductors are permitted to be paralleled only for size 50mm2 (#1/0 AWG) or larger, except for special applications. 2. All parallel conductors of the phase or neutral set shall be of the same length. 3. All conductors of the phase and neutral set shall be of the same material, cross sectional area, and the same insulation type. 4. All conductors of a phase or neutral set shall be terminated in the same manner. 5. If run in more than one raceway, the raceways shall have the same physical properties, and the same length, and shall be installed in the same manner 6. If more than one raceway is used, make sure each phase wire and neutral, if present, is placed in each of the raceways to prevent eddy currents. 7. If an equipment grounding wire is present and there is more than one raceway, an equipment grounding wire shall be in each raceway.
