Sizing Circuits

II: II MYTHS & FACTS IN WIRES & CABLES APPLICATION

WIRES & CABLES: APPLICATIONS ON LOW VOLTAGE FEEDERS & BRANCH CIRCUITS

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MYTHS & FACTS IN WIRES & CABLES APPLICATION

A TYPICAL LARGE CORPORATE OFFICE BUILDING

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14 12 10 8 6 4 2 1 1/0 2/0 3/0 4/0

MM Size

2.0 mm2

3.5 mm2

5.5 mm2

8.0 mm2

14 mm2

22 mm2

30 mm2

38 mm2

50 mm2

60 mm2

80 mm2

100 mm2

Ampacity THW 75 deg C 15 20 30 45 65 85 110 125 145 160 195 220

Ampacity TW 60 deg C 15 20 30 40 55 70 90 100 120 135 160 185

255

205

170

150

130

115

90

70

50

40

30

25

Ampacity THHN 90 deg C

AMPACITY TABLE (Based on Table on AVESCO pocket book) Not more than 3 conductors in a raceway or cable

Approximate AWG Size

THERMOPLASTIC WIRE & CABLES (TW, THW, THHN) 600 VOLTS, Stranded

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540

455

580

515

535

470

400

355

295

265


AMPACITY TABLE (Based on Table on AVESCO pocket book) Not more than 3 conductors in a raceway or cable

1000 MCM

500 mm2

485

405

800 MCM

400 mm2

475

400

750 MCM

435

370

650 MCM

325 mm2

375

315

500 MCM

250 mm2

330

280

400 MCM

200 mm2

280

240

300 MCM

150 mm2

255

250 MCM

125 mm2

210

Approximate AWG Size

MM Size

Ampacity THW 75 deg C

Ampacity TW 60 deg C

THERMOPLASTIC WIRE & CABLES (TW, THW, THHN) 600 VOLTS, Stranded

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Note: This Table is not quite identical with NEC Table 310.16

(Lifted from AVESCO pocket book)

AMPACITY TABLE

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See the differences on the next slide… 6

BECAUSE THEY ARE NOT ACTUALLY IDENTICAL…

BE CAREFUL IN REFERENCING TABLES ON WIRE & CABLE AMPACITIES.

COMPARISON: NORTH AMERICAN, PHILIPPINE & EUROPEAN MANUFACTURED CABLES

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TW..? THW..? THHN..?

Just curious… WHICH IS THE MORE EXPENSIVE WIRE..? WIRE..?

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Why…?

TW..? THW..? THHN..? THHN ?

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WHICH IS THE SUPERIOR WIRE?

WHY…?

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THE MOST POPULAR BUILDING WIRES ARE THE THHN?

DO YOU AGREE THAT TODAY, IN THE PHILIPPINES:

How do we size a circuit…?

It’s worthwhile revisiting the basics…

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Let’s Let s Test Ourselves with the following exercise…

It’s worthwhile revisiting the basics…

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Note: NEC TABLE 3.16 doesn’t have mm sizes.

a) # 10 b) # 8 c) # 6 d) Any of these?

Q: What size (TW, THW, THHN) conductor does the NEC require for a 50A circuit?

NEC TABLE 3.16

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THW: Use, # 8 (8.0 mm2), Ampacity: 50A

THHN: Use, # 8 (8.0 mm2), Ampacity: 55A

TW: Use, # 6 (14 mm2), Ampacity: 55A

PROBABLE ANSWERS:

2) Referring to the Table NEC 310.16:

1) Rating of Circuit: 50 A (Given)

SOLUTION: SOLUTION:

TEST CASE

NEC TABLE 3.16

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WHAT IS THE BEST CHOICE..? 15

#8 THW or #8 THHN…

If your answer is:

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THAT IS A MYTH…!

Sorry Guys,

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THHN: Use, # 6 (14 mm2), Ampacity: Ampacity: 55A @ 60 deg C Column

THW: Use, # 6 (14 mm2), Ampacity: Ampacity: 55A @ 60 deg C Column

TW: Use, # 6 (14 mm2), Ampacity: Ampacity: 55A

THE CORRECT ANSWERS ARE:

Rating of Circuit: 50 A (Given)

SOLUTION: SOLUTION:

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LET US FIRST GO BACK TO BASICS…

HOW COME..?

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HOW DO WE DESIGN A NON NON-MOTOR BRANCH CIRCUIT…?

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A branch circuit is that part of a wiring system extending beyond the last or final protective device to the load it specifically serves.

In general terms, a branch circuit could either be non non--motor (as in lighting, receptacle outlets, & computers)) or motor loaded l d d circuit. i i

A safe electrical system starts from the very basic wiring fundamentals – the branch circuit. circuit.

A branch circuit will qualify as such when it has a protective device from the point of tapping.

A branch circuit may have several lighting or receptacle outlets connected to it as a circuit or may serve a single load as in motor or heavy appliance.

The branch circuit represents the last step in the transfer of power from the service or source of energy to utilization devices.

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1))

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The OverOver-Current Protective Device ((OCPD)) 2) The Conductor (Wire or Cable) 3) The Load (Motor or NonNon-Motor Loads)

Therefore, the branch circuit is made up of :

From where shall we base our loads?

From where shall we base our OCPD?

From where shall we base our sizing of conductors?

QUESTIONS:

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Th The ampacity it off b branch h circuit i it conductors must not be less than the maximum load to be served. (NEC Section 210210-19a).

RELEVANT CODE REQUIREMENTS

BRANCH CIRCUITS – WIRING FUNDAMENTALS

The rating of a branch circuit is established or defined by the rating or setting of its protective device. (NEC Section 210210-3).



The ampacity of branch circuit conductors must not be less than the rating of the branch circuit. (NEC Section 210210-19a).



THIS MEANS A MANDATORY MATCH BETWEEN THE CONDUCTOR AMPACITY WITH ITS OCPD (OVER(OVERCURRENT PROTECTIVE DEVICE).

Circuit conductors shall be protected against overovercurrent in accordance to their ampacities, ampacities, but where the ampacity of the conductor does not correspond with the standard ampere rating of a fuse or a circuit breaker,, the next higher g rating g shall be p permitted only if this rating does not exceed 800 amperes. amperes. (NEC Section 240240-3).


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The maximum continuous load that can be served by the branch circuit conductors must not be more than 80% of the ampacity of the conductors. (NEC Section 210210-19a).



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The current permitted to be carried by the branch circuit conductors has to be 80% if the load is continuous. This rule refers to a limit of the load to be carried by the conductors. (NEC 21021022c).



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This rule limits the load on the circuit conductors; it does not change the ampacity of the circuit conductors or the rating or setting of the circuit overover-current protective device. (NEC 210210-22c).

Continuous load refers to a load that operates for three (3) hours or more, such as store lighting, office lighting and similar loads.



384--16c). 384

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The total load on any overover-current device in a panelboard must not exceed 80% of the rating of the overover-current device. device. (NEC Section



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SOME EXERCISES…



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ARE THE CIRCUITS DISCUSSED IN PRECEDING EXAMPLES ALREADY SAFE FROM FIRE HAZARDS…?

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because…

NOT QUITE…!

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There is the soso-called DERATING & CORRECTION FACTORS

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Note that the Derating & Correction Factors Cause to Change the CURRENT CURRENT-CARRYING CAPACITY OF THE CONDUCTORS!

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30 deg C, Correction Factors must be considered.

For ambient temperatures under or over

conductors in cables or raceways are given in NEC Tables 310 310--16 (copper) on a 30 deg C ambient temperature.

The normal maximum ampacities of

BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS

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circuit conductors. 48

This means a change in ampacities of

(Note 8 to Tables 310310-16 through 31031019).

be derated where there are more than th three conductors d t iin a cable bl or raceway

These normal ampacities may have to



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Correction to the conductor ampacities when installed or operated at temperatures over or under 30 deg C ambient. ambient

AMPACITY DERATING DUE TO “MORE THAN THREE (3) CURRENT CARRYING CONDUCTORS IN A CONDUIT OR CABLE”, CABLE”, and…

THESE ARE THE FOLLOWING:

THUS THERE ARE TWO THINGS TO CONSIDER THAT REDUCE CONDUCTOR AMPACITIES…

Approximate AWG Size # 14 # 12 # 10 #8 #6 #4 #2 #1 1/0 2/0 3/0 4/0

MM Size

2.0 mm2

3.5 mm2

5.5 mm2

8.0 mm2

14 mm2

22 mm2

30 mm2

38 mm2

50 mm2

60 mm2

80 mm2

100 mm2

Ampacity THW 75 deg C 20 25 35 50 65 85 115 130 150 175 200 230

Ampacity TW 60 deg C 20 25 30 40 55 70 95 110 125 145 165 195

260

225

195

170

150

130

95

75

55

40

30

25


THERMOPLASTIC WIRE & CABLES (TW, THW, THHN) 600 VOLTS, Stranded (NEC Table 310.16)

50

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Q3: What is the maximum permissible nonnon-continuous load for the circuit?

Q2: What is the maximum permissible continuous load for the circuit?

Q1: What is the effective ampacity of the conductor?

Given: 6 # 12 THW in a conduit at ambient temperature of 40 deg C

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Max permissible load (Non(Non-Cont. Load) = 14.08 x 1 or 14.08 A

Max permissible load (Cont. Load) = 14.08 x 0.80 or 11.264 A

a) Derating Factor of 0.80 for six conductors in conduit b) Correction Factor for of 0.88 for ambient temp of 40 deg C

N Notes: t

Effective Ampacity: 20 A x 0.80 x 0.88 = 14.08 A

Ampacity: # 12 THW = 20 A (NEC Table 310310-16)

Q5: WHAT WILL BE THE SIZE OF THE BRANCH CIRCUIT OCPD? 54

c) The Maximum Permissible Load which is11.264 A?

b) The ‘Effective’ Ampacity of #12 AWG which is14.08 A?

a) Published Ampacity of #12 AWG which is 20 A?

Q4: WHERE SHALL WE BASE THE OCPD?

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Note that in this case, the effective ampacity of the # 12 conductor happens to be derated at 14.08 A only.

Because of the mandatory match of the OCPD with respect to the ampacity of the conductor!

Answer is: 15 A

WHAT WILL BE THE SIZE OF THE BRANCH CIRCUIT BREAKER OR FUSE?

THE DERATING & CORRECTION FACTORS WILL MAKE THE CIRCUIT CONDUCTORS BIGGER…!

IF THE 20 A OCPD FOR A #12 AWG CONDUCTOR IN PREVIOUS EXERCISE IS TO BE RETAINED…

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Not # 12 THW!

# 10 THW

If you decide 20 A OCPD, then the size of the conductors shall be:

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Q: IS SIZING OF CIRCUITS FINISHED…?

ASSUMING THAT THE DERATING & CORRECTION FACTORS HAVE ALREADY BEEN CONSIDERED,

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NOT QUITE…!

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CONDUCTORS MUST MATCH THE TEMPERATURE RATINGS OF CIRCUIT DEVICES…

BECAUSE,

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Properly sized conductors allow the circuit breaker thermal-sensing elements to match the conductor thermal protection requirements. 61

Molded-case circuit breakers are marked with both tthe bot es size ea and d insulation su at o temperature te pe atu e rating at g (e.g., #2 Cu, 60/75°C) of the conductors approved for use with the circuit breaker.

Wiring Conductor Ampacity to Temperature Rating

& other devices the conductor terminates inside an enclosure.

When selecting a conductor for a circuit, one has to be selected to accommodate the temperature termination rating rules outlined in NEC 110-14(c). This includes the compatibility of a conductor type as to the OCPD’s 62

SIZING CONDUCTORS BASED ON TEMPERATURE RATING

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The wire temperature rating is determined by testing the circuit breaker under full-load current with conductors sized for the appropriate temperature rating, 60°C or 75°C.

Underwriter Laboratories Inc. (UL) standards require that molded-case circuit breakers rated at 125 amperes or less be marked with the conductor insulation-temperature rating.


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If th the 90°C (THHN) wire i size i were to t be b selected based on the ampacity allowed in the 90°C column of the Ampacity Table, the smaller resulting wire size would generate additional heat at the circuit breaker terminals and possibly cause nuisance tripping.

Conductors with 90°C rated insulation (THHN) can be used on circuit breakers rated for 60°C or 75°C wiring only if their size is based on the ampacity of the lower temperature-rated wire.



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NEC Table 310.16

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NEC Table 310.16

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listed in NEC Ampacity Table 310-16.

temperature rating’

POINT 1: For device or equipment terminals rated 100 A or less, wire sizes shall be based on the ‘60 60 deg C

IMPORTANT!!!

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NEC Table 310.16

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Unless the terminals are marked otherwise, equipment/ i t/ device d i terminals rated over 100A shall be sized according to the ‘75 deg C temperature rating’ listed in NEC Table 310-16.

Sec. 110-14(c)(2)]:

Circuits over 100A [NEC

POINT 2:


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The advantage of 90 deg C wire (THHN) is that it can keep the designer from using a larger wire (when ampacity adjustments are needed), which require larger conduits & raceways, greater labor & increased material costs.

when adjusting conductor ampacity for elevated ambient temperature (correction due to temperature) or when bundling more than three current-carrying conductors together (derating).

THHN’s can not be typically used for sizing circuit conductors. However, its rating only comes into play

Ninety deg C rated conductor ampacities like the

This is now the catch!

What then is the purpose of 90 deg C wire if we cannot use its higher ampacity?

POINT 3:

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However, the greater ampacity of a THHN conductor with 90°C insulation is not always permitted to be used due to limitations of the terminal temperature rating and/or the requirements of the NEC®. (Reference 110.14 in the NEC® for specific requirements.)

For instance, the ampacity for a conductor with 90°C insulation (THHN) is generally greater than of a conductor of the same size but with 60°C (TW) insulation.

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.

TO REVIEW:

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These simple rules generally should be followed because these are the norms for the device component p product p standards and performance evaluation to these standards for fuses, blocks, disconnects, holders, circuit breakers, etc.

However, there are some simple rules to follow for circuits of 100A and less.

In other words, even if a 90° 90°C THHN conductor is used, it has to be rated for ampacity as if it were a 60° 60°C conductor [110.14(C)(1)(a)(2)]..

2. Higher temperature rated conductors can be used, but the ampacity of these conductors must be as if they are 60° 60°C rated conductors. conductors.

1. Use 60° 60°C rated conductors [110.14(C)(1)(a)(1)]. This assumes all terminations are rated for 60° 60°C rated conductors.

Simple rules for 100 amps and less: less:

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For this circuit, if a 90°C, 6 AWG conductor is evaluated, the ampacity of this conductor must be according to the 60°C conductor ampacity, which is 55A. (Ampacities are from NEC® Table 310.16.)

The answer is 4 AWG, 90°C conductor. A 6 AWG, 90°C conductor has an ampacity of 75 amps per (NEC® Table 310.16); but this ampacity can not be used for a 60°C termination.

For instance, assume an ampacity of 60A is needed in a circuit that has terminations that are rated for 60°C conductors. If a 90°C conductor is to be used, what is the minimum conductor size required?

However, the industry norm is that most devices rated 100A or less, such as blocks, disconnects and circuit breakers, have 60°C or 75°C rated terminations.

3. Conductors with higher temperature ratings can be used at their rated ampacities if the terminations of the circuit devices are rated for the higher temperature rated conductor [110.14(C)(1)(a)(3)].

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5. If a conductor is run between two devices that have terminals rated at two different temperatures, the rules above must be observed that correlate to the terminal with the lowest temperature rating.

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4. For motors with design letters B, C or D, conductors with insulation rating of 75°C of higher are permitted as long as the ampacity of the conductors is not greater than the 75°C rating [110.14(C)(1)(a)(4)].

For circuits greater than 100A, 100A, use conductors with at least a 75° 75°C insulation rating at their 75° 75°C ampacity rating. rating.

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The answer lies in the fact that those higher g ampacity p y ratings g can be utilized when derating due to ambient conditions or due to exceeding more than 3 current carrying conductors in a raceway.

So why would anyone ever want to use a conductor with a 90° 90°C or a 105° 105°C rating if they can’t be applied at their ampacity ratings for those temperatures?

First, since one termination temperature rating is higher than the other, the lowest one must be used, which is 60° 60°C. The first choice might be a 4 AWG TW conductor with an ampacity of 70A at 60° 60°C. 81

Assume that an ampacity of 60A is needed in a circuit with a 75° 75°C termination at one end and a 60° 60°C termination at the other end, where the ambient is 45° 45°C.

Example: Circuit ampacity required: 60 amps, Ambient: 45 45°°C

Again, looking at the table at the bottom of Table 310.16, a factor of .87 must be used, due to the 45° 45°C ambient. This yields a new ampacity of 82.65, which is adequate for the required 60A ampacity. 82

However, in the NEC® the Correction Factors reveals that the 70A ampacity must be derated due to the 45° 45°C ambient, by a factor of 0.71. This yields a new ampacity of 49.7A, which is less than the required 60. This is where a conductor with a higher temperature 90°°C rating becomes useful. A 4 AWG THHN conductor has a 90 ampacity of 95A.


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The amount of copper associated with a 4 AWG conductor is required to bleed the right amount of heat away from the terminal terminal.. The use of less copper won’t bleed enough heat away, and therefore overheating problems could result.

However, a 6 AWG conductor of any insulation rating could never be used in this application because the 60° 60°C terminal requires that the smallest amount of copper is a 4 AWG for a 60A ampacity.

Could a 6 AWG THHN conductor be used in this application? Its 90° 45°°C 90°C ampacity is 75A. Using the factor of 0.87 for the 45 ambient gives a new ampacity of 65.25, which seems adequate for a required ampacity of 60A.


conduits & raceways, greater labor & increased material costs.

(when ampacity adjustments are needed), d d) which hi h require i llarger

The advantage of 90 deg C wire (THHN) is that it can keep the designer from using a larger wire

This is now the catch!

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2008 NEC 110.14(C)(1)

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2008 NEC 110.14(C)(1)(a)(1…4)

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2008 NEC 110.14(C)(1)(a)(4)(1…2)

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NOT QUITE…!

IS SIZING OF CIRCUITS FINISHED…? FINISHED ?

AT THIS TIME,

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How Come…?

BECAUSE THE CIRCUIT OVEROVER-CURRENT PROTECTORS MAY BE THE ONES TO START A FULLFULL-BLOWN FIRE…!

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SIZING OCPD’S BASED ON TEMPERATURE RATINGS

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If the source transformer is 1,500 KVA, the fault duty could be 70 kilokilo-amperes. amperes. In like manner that a 100 KVA threethree-phase three-phase short circuit current at 12. 12.0 transformer delivers a threekilokilo-amperes. amperes.

For instance, if a 230v 230v lighting panelboard is receiving supply from a 500 KVA transformer, the threethree-phase fault duty at its secondary terminals could be as high as 30 kilo kilo--amperes. amperes.

Fault Duties (in a simplistic view) depend largely (among other factors) on the size of the source transformer and the impedance of the cables before the points where the fault is subjecting to. to.

INTERRUPTING RATINGS FOR OCPD’s

In any case, Fault Calculations are necessary. necessary.

For source transformers that are large where short circuit levels are higher, molded case circuit breakers (MCCB’s) are available. available.

In this case, it has to be remembered is that the use mcb’s are limited only if the source transformer are small enough. enough.

Care however on the use of miniature cb’s because they have relatively low short circuit capacities. capacities. Although models are available with ratings up to 16 KA, majority of mcb’s only have a maximum breaking capacity of 9 KA. KA.

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IT IS BEST TO SHOW THE EFFECTS OF UNDERRATED CIRCUIT BREAKERS IN THE CONTEXT OF THEIR INTERRUPTING CAPACITIES…!

ALTHOUGH FAULT CALCULATION IS NOT WITHIN THE SCOPE OF THIS MODULE,

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The ‘injury’ means the condition where after interrupting a fault, the breaker ceased to be operable - or worse, the breaker disintegrated because the fault current is too much for the breaker to handle.

The IC rating is the maximum amount of current that the device will open safely to relieve a fault condition without injuring itself.

Each circuit breaker has three most important ratings – a continuous current rating, a voltage rating and an interrupting capacity (IC) rating.

What is KAIC all about?


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THE CRCUIT BREAKER AFTER A FAULT

A 3.6 KV MINIMUM OIL CRCUIT BREAKER BEFORE A FAULT

The IC rating is the maximum amount of current that the device will open safely to relieve a fault condition - without injuring itself. The ‘injury’ means the condition where after interrupting a fault, the breaker ceased to be operable - or worse, the breaker disintegrated because the fault current is too much for the breaker to handle. 96

What is KAIC all about?



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100


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This does not only cover the large breakers but must transcend to all breakers including the smallest branch circuits at the end points of the system. system.

Therefore, it is not enough to specify the Continuous Current Rating and Voltage Rating of the breaker but most importantly, the engineer must specify the KAIC ratings of these protective devices. devices.


LV Power Circuit Breakers


Molded Case Circuit Breakers can reach up to 4,000 A rating but good only for two interruptions 106 only.

LV PCB’s are recommended for mains & distribution feeder applications from 800 A to 6,300 A.

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IS SIZING OF SAFE CIRCUITS FINISHED FINISHED…? ?

AT THIS TIME,

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NOT QUITE…!

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How come when Circuit Breakers are there…?

Because, DESIGNING CIRCUITS Must Consider CONDUCTOR DAMAGE DURING FAULTS!

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D During i short h t circuits, i it t to t th the currents conductors are tremendously high that it must be removed quickly before the damage point of conductor insulation is reached. reached.

Overcurrent protection is to open a circuit before conductors are damaged when an overcurrent condition exists. exists.

DESIGNING CIRCUITS FROM CONDUCTOR DAMAGE

b) how much ‘let‘let-through’ current it allows to flow into the conductor.

a) the speed of the clearing,

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Assuming that the OCPD’s has sufficient interrupting capacities; there are still two actions of the OCPD’s that are important in protecting circuit wires & cables.


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Damage ranging from slight degradation of insulation to violent vaporization of the conductor can result if the shortshort-circuit withstand is exceeded.

Although conductors do have allowable ampacity ratings, they also have maximum allowable shortshort-circuit current withstand ratings. ratings.



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The I2t associated with the asymmetrical current is required to be reduced to the equivalent I2t of a symmetrical current or less

Current--limiting Circuit Current Beaker


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Behavior of a Typical Current Limiting Fuse

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1.85 KA

Thus the above example is safe. Opening time & current letlet-through of the fuse is far lower than the wire withstand rating. Conductor protection is not a problem when the conductor is protected by current current-124 limiting fuses which have an ampere rating that is the same as the conductor ampacity rating.

Under shortshort-circuits, the LOWLOW-PEAK® YELLOW™ DualDual-Element fuse (30 ampere CLF) is fast acting. It will clear & limit short circuit current before it can build up to a level higher than the wire withstand. The opening time of the fuse is less than oneone-half cycle (less than 0.008 seconds). In this particular example, the prospective current letlet-thru by the fuse is less than 1,850 amperes.

Case 1: A # 10 AWG conductor can only withstand 4,300 amperes for one cycle and 6,020 amperes for oneone-half cycle. cycle. In the circuit below, is the wire protected when the available shortshort-circuit current of 40, 40,000 exceeds the wire withstand?

40 KA

2) Use an OCPD which is a current-limiting type (CLF or CLCB) such as that shown in the previous case. 125

(See Chart).

1) Use a larger size conductor (i.e., from # 10 to # 1/0 AWG), one with a withstand rating greater than the short-circuit for 1 cycle

What can be done to correct the above misapplication?

Yes. The 40 KA short-circuit current far exceeds the withstand of the # 10 THW wire. The slow acting ordinary circuit breaker (clearing time of 1 cycle) makes the circuit misapplied.

40 KA

Case 2: Does the circuit below represent a misapplication? (10 AWG THW insulated copper wire can withstand 4,300 amperes for one cycle and 6,020 amperes for onecycle).. one-half cycle)

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REMEMBER GENTLEMEN, A FULL-BLOWN FIRE ONLY NEEDS AN IGNITION. AND IGNITION IS, YES – IT’S THE RESPONSIBILITY OF THE ELECTRICAL ENGINEER.

OTHERWISE, WHAT SHOULD HAVE BEEN A NICELY-DESIGNED CIRCUIT ACTUALLY TURNS OUT TO BE GROSSLY WRONG & FAULTY! 128

IN SIZING CIRCUITS & FEEDERS, WE ALSO NEED TO CONSIDER THE SIZE OF THE SOURCE TRANSFORMER;

SO THEN,

‘ARE WE DOING IT…?’

The question is…:

Therefore, circuit designing is not complete without fault calculations…

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THANK YOU THANK YOU

THANK YOU

END…

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Sizing Circuits

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