Conductor sizing

[yordanys]

Hi Guys,

I'm stuck with this question, Could you please help me to get the right answer? Thank you.

The minimum size THWN Conductor that can be used on a 48amp load in a conduit with a total of 15 current-carrying conductors is ___AWG.

A)8 B)2 C)3 D)4

My answer is 48x 0.50=24 #12 obviously is not the correct answer. looking forward for your help. Thank you.

 

[petersonra]

The derating factor of 50% applies to the conductor ampacity you start with, not what you end up with.

 

[infinity]

Divide 48 by the derating factor.

48/.5= 96 amps

 

[shaxruz1991]

Sent from my iPhone using Tapatalk

 

[Little Bill]

Sent from my iPhone using Tapatalk

You can use the 90 deg column for derating. It wouldn't make any difference in this example but could in others.

 

[infinity]

You can use the 90 deg column for derating. It wouldn't make any difference in this example but could in others.

Correct if this were a 90° C conductor but THWN is only rated for 75° C.

 

[Little Bill]

Correct if this were a 90° C conductor but THWN is only rated for 75° C.

Just going by what is available here. Nothing but THWN-2, don't know when I've seen just TWHN. But since the question specifically mentioned THWN, I guess you're correct.

 

[Dennis Alwon]

And the correct answer is #1 awg

We don't know the terminations and the load is less than 100 amps so we have to assume 60C per 110.14(C). #2 at 60C is 95 amps. Thus we need to go to #1.

 

[infinity]

Just going by what is available here. Nothing but THWN-2, don't know when I've seen just TWHN. But since the question specifically mentioned THWN, I guess you're correct.

I hear you. Then OP sounds like a test or homework question so the THWN part is relevant.

#1 AWG is not on the list of choices.

 

[Dennis Alwon]

I hear you. Then OP sounds like a test or homework question so the THWN part is relevant.

#1 AWG is not on the list of choices.

I know it is not but that is the correct answer. Of course, the instructor is looking for #2 but he needs to give more info for that.

 

[Little Bill]

I know it is not but that is the correct answer. Of course, the instructor is looking for #2 but he needs to give more info for that.

Dennis, you have split so many hairs that's why you don't have any!

 

[infinity]

I know it is not but that is the correct answer. Of course, the instructor is looking for #2 but he needs to give more info for that.

Why wouldn't C) #3 AWG be correct as outlined in post #4? #3@75° C = 100 amp*50%= 50 amps.

 

[Dennis Alwon]

Why wouldn't C) #3 AWG be correct as outlined in post #4? #3@75° C = 100 amp*50%= 50 amps.

Based on the number of conductors we need a conductor sized for 96 amps, correct?

 

[Dennis Alwon]

Dennis, you could use the 75 DEg column for derating so #2 could be used. 115A x 50% = 57.50A

Why can you use the 75C column

 

[Little Bill]

And the correct answer is #1 awg

We don't know the terminations and the load is less than 100 amps so we have to assume 60C per 110.14(C). #2 at 60C is 95 amps. Thus we need to go to #1.

Dennis, you could use the 75 deg column for derating since the conductors are THWN. 115A x 50%=57.50A

 

[Dennis Alwon]

Why can you use the 75C?

(a) Termination provisions of equipment for circuits
rated 100 amperes or less, or marked for 14 AWG through
1 AWG conductors, shall be used only for one of the following:
(1) Conductors rated 60°C (140°F).
(2) Conductors with higher temperature ratings, provided
the ampacity of such conductors is determined based on
the 60°C (140°F) ampacity of the conductor size used.
(3) Conductors with higher temperature ratings if the equipment
is listed and identified for use with such conductors.
(4) For motors marked with design letters B, C, or D, conductors
having an insulation rating of 75°C (167°F) or higher
shall be permitted to be used, provided the ampacity of
such conductors does not exceed the 75°C (167°F)
ampacity.

 

[Dennis Alwon]

On another note how did I quote you and my post end up ahead of yours, Bill?

 

[Little Bill]

On another note how did I quote you and my post end up ahead of yours, Bill?

Multi-tasking and I accidentally deleted it.

 

[Little Bill]

Why can you use the 75C?

I was confusing the deration from the more than 3 conductors with also having to be 60 Deg.

 

[infinity]

Based on the number of conductors we need a conductor sized for 96 amps, correct?

Correct, but you're assuming that the terminals would limit the conductor to the 60° C ampacity?