2020 NEC 310.12

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Solar_ice_caps

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I have a home with a 200A MSP.

Our engineering company (who creates the planets and does the Electrical Stamping) is spec'ing 4/0 Copper for the service cable.

They are claiming that because there is a temperature correction factor at all, the entire section 310.12 does not apply and they can't size to the 83%:

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My interpretation is that, because the table is based on no correction factor, if you have one you can't use that specific table to size the wires, but 310.12 still applies and as long as your wire is rated for at least 83% with adjustment, you are still good to go.

They are using a temp correction factor of 0.96.

For our example, from table 310.16:
200A service @ 83% = 166A
2/0 Cu: 175A Rating (75deg) x 0.96 = 168A > 166A.
4/0 AL: 180A Rating (75deg) x 0.96 = 149.4A > 166A.

So both of these sizes that happen to be in the 310.12 table can still be used, but they cannot be assumed until the calculation above is confirmed.

Thoughts?
 
You are correct, except that the temperature correction can start from the 90C value (assuming 90C wires). So for 2/0 Cu, the computation is 195A (90C) x 0.96 = 187A > 175A (75C), so the ampacity is still 175A. Which of course is greater than the 166A required by 310.12(A).

Similarly, if 310.12 didn't apply, 3/0 Cu would suffice: 225A (90C) * 0.96 = 216A > 200A (75C), so the ampacity is still 200A.

So the engineering company is doubly wrong. : - )

Cheers, Wayne
 
Yeah, They use the 75deg values for everything which I am not exactly sure why, but our local utility also makes an assumption that all terminals are only 75C rated and expects you to size accordingly.
 
The table values based on the wire insulation rating may be used as the starting point for ampacity correction and adjustment. Then the adjusted and corrected value is compared to the table value for the termination rating, and the smaller of the two is the final ampacity.

Cheers, Wayne
 
wwhitney said:
The table values based on the wire insulation rating may be used as the starting point for ampacity correction and adjustment. Then the adjusted and corrected value is compared to the table value for the termination rating, and the smaller of the two is the final ampacity.

Cheers, Wayne
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You are a gentleman and a scholar, Wayne.

Thank you for your feedback. Cheers.
 
jaggedben said:
FWIW, I've never seen anyone ask for a temp derating on a 200A service. But I guess the temps are moderate around here.
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I went down a rabbit hole of determining the proper ambient temp for correction factor. We are in North Central FL so it's pretty temperate. I read many threads about ambient vs average temps, ASHRAE data, etc and didn't come to anything conclusive. Since 0.96 doesn't cause me much issue I just stuck with that. 4/0 AL on SE for 200A is common here so I am not overly concerned.
 
Another way to look at it:

4/0 Al has a 90C ampacity of 205A. 310.12 requires 166A ampacity for a 200A residential service. So a combined adjustment and correction factor as low as 166/205 = 0.81 is allowed. Per table 310.15(B)(1), that means you're definitely good up to 122F ambient (0.82 temperature correction) if there's no ampacity adjustment for more than 3 CCCs.

So all you need is confidence the ambient temperature to use is no higher than 122F. Which I'm pretty sure is true in all of the US, and probably the whole planet.

Cheers, Wayne
 
The ampacity adjustment and ampacity correction issues were the reasons that the table showing the permitted sizes of resident service conductors was not in the code text for a couple of cycles, however it was found in Annex D, example D7 when it was not in the actual code text.
 
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