60C or 75C column

[dalowe]

I have been taking your classes for years, thanks! I am dealing with an engineer that has given me drawings that I believe are wrong. He has a 100 amp panel at a barn and a 100 amp fused disconnect at the house. This all part of a solar installation. The wire that he has coming from the barn he has listed as a #4 CU. I would normally put in a #3. But after watching your videos anything 100 amps or less should be sized using the 60C column, which would be a #1 CU. Could you please clear this up for me? With code references. I have been at this all morning.

 

[MAC702]

Most every post in this website is a "help." Put the actual subject in the subject line.

 

[charlie b]

110.14(C)(1)(a)(3) allows the use of the 75C column, "if the equipment is listed and identified for use with such conductors." What this means in this situation is that if the terminations are rated for 75C (and they usually are), then you can use the 75C column. That means that #3 is good enough, but #4 is not.

 

[charlie b]

Most every post in this website is a "help." Put the actual subject in the subject line.

Good catch. I fixed it for the OP.

 

[electrofelon]

I would add that it is very common to (incorrectly) apply 310.15(b)(7) for any feeder in/around a dwelling. Another possibility is that it is a tap situation.

 

[dalowe]

Follow up

Follow up

Thanks for changing my subject, I couldn't find how to edit. Now let me throw a wrench in the problem. I now have another engineer saying that since the #4 is rated for 95 amps you can go up to the next breaker size. I know this is completely wrong but I can't find what I need to prove it. I keep telling him that the breaker is dictating the wire size and you can go up in breaker size when you do load calculations to size your wire.

 

[electrofelon]

He is correct about the next size up rule, but incorrect in using the 90 degree collum ampacity, unless the conductors transition to 75 degree rated ampacity before terminating on the equipment.

 

[ron]

Thanks for changing my subject, I couldn't find how to edit. Now let me throw a wrench in the problem. I now have another engineer saying that since the #4 is rated for 95 amps you can go up to the next breaker size. I know this is completely wrong but I can't find what I need to prove it. I keep telling him that the breaker is dictating the wire size and you can go up in breaker size when you do load calculations to size your wire.

See if 240.4(B) helps your situation

 

[kwired]

Thanks for changing my subject, I couldn't find how to edit. Now let me throw a wrench in the problem. I now have another engineer saying that since the #4 is rated for 95 amps you can go up to the next breaker size. I know this is completely wrong but I can't find what I need to prove it. I keep telling him that the breaker is dictating the wire size and you can go up in breaker size when you do load calculations to size your wire.

#4 is only good for 85 amps @ 75C, which is what you must use for 75C terminations. It is good for 95 amps @90C which you can use as starting ampacity when making ampacity adjustments presuming you have 90C insulation.

As mentioned 310.15(B)(7) allows us to use #4 protected at 100 amps for the supply to a dwelling unit, but that is not what you are supplying so (B)(7) does not apply.

 

[dalowe]

#4 is only good for 85 amps @ 75C, which is what you must use for 75C terminations. It is good for 95 amps @90C which you can use as starting ampacity when making ampacity adjustments presuming you have 90C insulation.

As mentioned 310.15(B)(7) allows us to use #4 protected at 100 amps for the supply to a dwelling unit, but that is not what you are supplying so (B)(7) does not apply.

That is exactly what I am thinking. But I still don't know what code section says that if you have a 100 Amp breaker you have to use this wire. I know it's the right way, it's just something I know from years of working. I just can't prove it.

 

[kwired]

That is exactly what I am thinking. But I still don't know what code section says that if you have a 100 Amp breaker you have to use this wire. I know it's the right way, it's just something I know from years of working. I just can't prove it.

General rules start with 240.4 which starts off saying condutors shall be protected against overcurrent in accordance with their ampacities specified in 310.15.

Sub sections that follow begin to cover specific situations and allowances, such as (B) allowing you to use next higher standard overcurrent device up to 800 amps.

So if you want to approach it from the perspective of I have a 100 amp breaker, then you must have at least 91 amp conductor because if you only had 90 amp conductor that is a standard overcurrent device and you would have to protect it at 90 amps. But at same time if you only had a 91 amp conductor you can not have more than 91 amps of load on it either when protected at 100 amps.

If you would happen to already have 4 AWG run to your barn, and have a load calculation of less than 85 amps, an easy code compliance fix would be to install a 90 amp breaker to protect it.

 

[dalowe]

General rules start with 240.4 which starts off saying condutors shall be protected against overcurrent in accordance with their ampacities specified in 310.15.

Sub sections that follow begin to cover specific situations and allowances, such as (B) allowing you to use next higher standard overcurrent device up to 800 amps.

So if you want to approach it from the perspective of I have a 100 amp breaker, then you must have at least 91 amp conductor because if you only had 90 amp conductor that is a standard overcurrent device and you would have to protect it at 90 amps. But at same time if you only had a 91 amp conductor you can not have more than 91 amps of load on it either when protected at 100 amps.

If you would happen to already have 4 AWG run to your barn, and have a load calculation of less than 85 amps, an easy code compliance fix would be to install a 90 amp breaker to protect it.

Thanks, now that makes sense. I did not that I could do it that way, but I now understand. Thank you very much.