sample Test question

[MichaelJ]

I am helping a Inspector that is studying to take he IRC electrical exam. and one of the sample question is" A 240 volt single phase three wire feeder circuit has a continuous load of 65 amps. The ambient temperature of the area in which the wires are running is 70 degrees Celsius. Which of the following insulation types and wire sizes would be acceptable to feed this branch circuit if copper-clad aluminum wire was used? A. TW 3 B. UF3 C. THHN 4 D. XHHW 4 or E. C and D"

The study guide says that the answer is E. both C and D. My question is according to the the tables in the IRC E3705.1( 2014 NEC 310.15(B)(16)) but there is no allowance for the ambient temperature table IRC E3705.2 ( 2014 NEC 310.15(B)(2)(a)).

My calculation #4 THHN or XHHW at 90c is 75 amps x adjustment at 70c ambient for 90c wire is .58 = 43.5 amps.
Am I missing something?

 

[augie47]

I would agree, the guides answer is way off.

 

[d0nut]

Not that any of the answers are correct, but if you had to choose one of the given answers I would say that E would have the best chance of being correct on the test. Here are a couple of logic paths to get you to E even though the question is messed up.

1. TW and UF are 60degC conductors. The ambient temp is given as 70degC, therefore only conductors with higher than 70degC can be used. Since options C and D would have the same ampacity, if one is correct both must be correct (Answer E).

2. If you didn't trust any of the numbers given in the question, options A and B have a lower ampacity than options C and D. (A=B)<(C=D). Therefore, if A or B were allowable, than C and D would also be allowable. Since "all of the above" is not an option, A and B must not be allowable. Since the ampacity of C = D, both must be correct (Answer E).

 

[Little Bill]

Not that any of the answers are correct, but if you had to choose one of the given answers I would say that E would have the best chance of being correct on the test. Here are a couple of logic paths to get you to E even though the question is messed up.

1. TW and UF are 60degC conductors. The ambient temp is given as 70degC, therefore only conductors with higher than 70degC can be used. Since options C and D would have the same ampacity, if one is correct both must be correct (Answer E).

2. If you didn't trust any of the numbers given in the question, options A and B have a lower ampacity than options C and D. (A=B)<(C=D). Therefore, if A or B were allowable, than C and D would also be allowable. Since "all of the above" is not an option, A and B must not be allowable. Since the ampacity of C = D, both must be correct (Answer E).

You know logic has no place in most test questions!

 

[Tulsa Electrician]

I think the point of the question would be for insulation type since it did not pass the math test.
I think d0nut nailed it.
Had to edit. Almost missed copper clad al

 

[dm9289]

I am helping a Inspector that is studying to take he IRC electrical exam. and one of the sample question is" A 240 volt single phase three wire feeder circuit has a continuous load of 65 amps. The ambient temperature of the area in which the wires are running is 70 degrees Celsius. Which of the following insulation types and wire sizes would be acceptable to feed this branch circuit if copper-clad aluminum wire was used? A. TW 3 B. UF3 C. THHN 4 D. XHHW 4 or E. C and D"

The study guide says that the answer is E. both C and D. My question is according to the the tables in the IRC E3705.1( 2014 NEC 310.15(B)(16)) but there is no allowance for the ambient temperature table IRC E3705.2 ( 2014 NEC 310.15(B)(2)(a)).

My calculation #4 THHN or XHHW at 90c is 75 amps x adjustment at 70c ambient for 90c wire is .58 = 43.5 amps.
Am I missing something?

I would think differently on wire size 65 x 1.15 =75 for continuous load.
Below 100 amp so assume 60c max capacity of wire
So #2 copper THHN at 90 c 130 x .58=75.4

IMO

 

[david luchini]

I would think differently on wire size 65 x 1.15 =75 for continuous load.
Below 100 amp so assume 60c max capacity of wire
So #2 copper THHN at 90 c 130 x .58=75.4

IMO

It's 125% for continuous load. So 65 x 1.25=81A. And the question specified aluminum conductors.

So, you'd need a minimum conductor size of #2 THHN Al for the continuous load;
At least #1 THHN Al for ampacity in the 70C ambient to support the load;
and at least a #2/0 THHN Al to be properly protected by the OCPD (assuming the minimum required OCPD size of 90A.)

 

[dm9289]

It's 125% for continuous load. So 65 x 1.25=81A. And the question specified aluminum conductors.

So, you'd need a minimum conductor size of #2 THHN Al for the continuous load;
At least #1 THHN Al for ampacity in the 70C ambient to support the load;
and at least a #2/0 THHN Al to be properly protected by the OCPD (assuming the minimum required OCPD size of 90A.)

Thank you for the correction I missed the aluminum

So would it be 65X1.25=81amps then 2/0 at aluminum 150amp X.58=87amps

 

[david luchini]

Thank you for the correction I missed the aluminum

So would it be 65X1.25=81amps then 2/0 at aluminum 150amp X.58=87amps

#2/0 al would be required to protect the conductor with a 90A OCPD using the next size up rule.

#1 al would have sufficient ampacity for the load (115 x .58 = 67A), but would not be properly protected by a 90A OCPD.

 

[dm9289]

#2/0 al would be required to protect the conductor with a 90A OCPD using the next size up rule.

#1 al would have sufficient ampacity for the load (115 x .58 = 67A), but would not be properly protected by a 90A OCPD.

wouldn't the load be considered 87 amps

 

[david luchini]

wouldn't the load be considered 87 amps

No, the load is 65 amps

load of 65 amps.

 

[dm9289]

No, the load is 65 amps

I thought 65 x 125% because its a continuous load

 

[david luchini]

I thought 65 x 125% because its a continuous load

See 215.2(A)(1)

 

[dm9289]

See 215.2(A)(1)

So it is 87amps 215.2(A)(1)(a) and thank you again i need the input

 

[david luchini]

So it is 87amps 215.2(A)(1)(a) and thank you again i need the input

What about 215.2(A)(1)(b)?

The load is 65A. The ampacity after the application of the correction factor has to be at least 65A.

 

[dm9289]

What about 215.2(A)(1)(b)?

The load is 65A. The ampacity after the application of the correction factor has to be at least 65A.

Hmmmmmmmm

 

[dm9289]

Hmmmmmmmm

You are causing me to keep rereading the section lol. Good exercise

 

[augie47]

What about 215.2(A)(1)(b)?

The load is 65A. The ampacity after the application of the correction factor has to be at least 65A.

You are far far brighter lamp than I, but 210.19 seems to indicate the circuit must have an ampacity of 125% of the continuous load.
Can you 'splain the error in my thinking ?

 

[david luchini]

You are far far brighter lamp than I, but 210.19 seems to indicate the circuit must have an ampacity of 125% of the continuous load.
Can you 'splain the error in my thinking ?

210.19(A)(1)(b) says the conductor has to have an ampacity greater then the load (65A) AFTER the application of the correction factor...basically the same as 215.2(A)(1)(b).

So use the larger conductor of an amapacity of 125% of the continuous load with no adjustment/correction factor, or the actual load after any adjustment/correction factors are applied.

In this example, #2 Al would be adequate for 125% of the continuous load, but #1 Al would be required to get the 65A at 70C. #1 is the larger conductor so that would be the required size.

(One additional step is that the ampacity of the #1 Al at 70C would not be protected by a 90A OCPD, so an even larger conductor is required.)

 

[augie47]