Sub Panel

[Little Bill]

Thanks for the correction. My calculation was done extremely hastily and I may not have even had the right kind of wire. Nor am I gonna check to see if yours are right because the point of it was the idea not really the numbers.

I don't rave to correct anyone, especially when I may not be correct. You just said you wanted to get the gist of it, and your numbers didn't seem right so I tried them myself and that's what I got according to the way I learned. You are correct, that wasn't the question.

 

[dana1028]

a #3 wire is rated at 100a 75C cu. Why is a #2 required?

Some things to consider: Very few supply houses in my area carry #3...so electricians are forced to use a #2.

As an electrical inspector, knowing this, I would never hassle an electrician [i.e. 250.122(B)] for not upsizing his EGC.

Also - if you follow 110.14(C)(1)(a) to the letter - conductors #14 thru #1 are supposed to be rated at 60 degrees.

I know that 110.14(C)(1)(b) allows you to use the 75 degree rating if all terminations are rated 75 - however this section does not mandate you use the 75 column to rate your conductor.

I think perhaps this is what Dennis was suggesting. How can you ding a guy for not upsizing his EGC when the code limits your conductors to the 60 degree column [unless....]

 

[Twoskinsoneman]

Some things to consider: Very few supply houses in my area carry #3...so electricians are forced to use a #2.
I can see your point and reading through lots of ROPs I see that the panel often takes things like "standard cable configurations" into account. Possibly the availability of #3 THHN could be considered. Changes have tried to be made, and actually one 2008 proposal (5-276 Log #345 NEC-P05)on the issue was even accepted then reversed in the comment period. I think a change is required.
As an electrical inspector, knowing this, I would never hassle an electrician [i.e. 250.122(B)] for not upsizing his EGC.
I want to agree but don't know enough to say for sure it isn't necessary. It seems to me that the code making panel thinks it is...

Also - if you follow 110.14(C)(1)(a) to the letter - conductors #14 thru #1 are supposed to be rated at 60 degrees.

I know that 110.14(C)(1)(b) allows you to use the 75 degree rating if all terminations are rated 75 - however this section does not mandate you use the 75 column to rate your conductor.

Just to be clear this is not the point. The point is if you have 75C terminations then the minimum the code requires is the #3. In any other situation if someone tried to tell you you couldn't use the 75C column to size your wires you would fight it and be right. Accordingly if you upsize from that minimum you are stuck with 250.122(B) IMO.

I think perhaps this is what Dennis was suggesting. How can you ding a guy for not upsizing his EGC when the code limits your conductors to the 60 degree column [unless....]

As the code is written now I think the panel statement "any increase over the minimum" is the key to this.

 

[Dennis Alwon]

As the code is written now I think the panel statement "any increase over the minimum" is the key to this.

I think we may agree with you but what is the minimum 60C or 75C . There are many other cases where it is not black and white. That section is a bit strange as the use of a smaller EGC when the run is 5 feet will not affect the effectiveness of the egc. I believe the loss of impedence in long runs is what is at issue here.

 

[Twoskinsoneman]

I think we may agree with you but what is the minimum 60C or 75C . There are many other cases where it is not black and white. That section is a bit strange as the use of a smaller EGC when the run is 5 feet will not affect the effectiveness of the egc. I believe the loss of impedence in long runs is what is at issue here.

Yeah I hear what you are saying. In fact that proposal that I listed had good points about increasing wire size due to derating. Is that an increase? Yeah, but an increase over the minimum? I don't think so because the wire size is no longer allowed to be used due to the derating.

However I don't think any would argue that in the OP's situation he could use #3s. So to me that is clearly the minimum.