Service conductor sizing question

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paullmullen

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Electrical Engineer & Master Electrician
I'm working my way through the Electrical Exam Prep (2020 edition). In the review questions for chapter 9 there is a Dwelling Unit Optional Load calculation. The result of the load calculation is 112A. Using the next size up rule, that gives 125A service.

For reference, this is question 9.3 in the exam prep book, but you don't need to know the details to answer this question:



Now, the question asks for what size Aluminum conductors to use. The answer given in the answer key is 1/0. At 125A, according to the Ampacity table I think it should be 2/0. So what am I missing?

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A guess: The service is 125A, but the actual load calculation is 112A so we use the actual load calculation rather than the service size in the ampacity tables... thus 1/0 (based on 112A demand) vs 2/0 that would be required for 125A.

But this doesn't make sense to me. The feeder conductors should be sized to the match the overcurrent protection since changes in the demand in the home (another air conditioner perhaps) might change the load.
 
Oh... this is dumb... I'm using the wrong column... should be T310.12, not T310.16. duh.
 
paullmullen said:
Oh... this is dumb... I'm using the wrong column... should be T310.12, not T310.16. duh.
Click to expand...

Yep the dwelling table is a usually one size smaller. The nec started with a Table then went to 83% of the service size then it went back to the table. The fact is that the 83% is the same as the table, at least what I have seen of it.
 
paullmullen said:
Now, the question asks for what size Aluminum conductors to use. The answer given in the answer key is 1/0. At 125A, according to the Ampacity table I think it should be 2/0. So what am I missing?
Click to expand...
Even without the dwelling table with a calculated load of 112 amps you could use #1/0 Al (120 amps, 75 degree column) with a 125 amp OCPD due to the next size up rule.
 
infinity said:
Even without the dwelling table with a calculated load of 112 amps you could use #1/0 Al (120 amps, 75 degree column) with a 125 amp OCPD due to the next size up rule.
Click to expand...
Hmmm. There are next size up/down rules in several places in the code. Which reference are you considering for this one? I suspected this, but couldn't find it in the code/notes/exceptions around 310.12 and .16.

Also wow @infinity and @Dennis Alwon, extra points for how fast you responded. Really appreciate it.
 
Dennis Alwon said:
look at 240.4(B)
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OK. I read 240.4(B)(1) to disqualify the use of next size up because downstream from the main breaker are more than one receptacle... but technically, this is a feeder/service, and not a branch circuit, so I guess (1) does not apply. I guess I have to read more literally.
 
paullmullen said:
OK. I read 240.4(B)(1) to disqualify the use of next size up because downstream from the main breaker are more than one receptacle... but technically, this is a feeder/service, and not a branch circuit, so I guess (1) does not apply. I guess I have to read more literally.
Click to expand...

You confused me...hahaha 240.4(B)(1) states if it is not a branch circuit..... Clearly you are not talking about a branch circuit but rather a service so rounding up does apply.
 
For services, note that 230.90(A) Exception #2 explicitly allows the use of 240.4(B) when sizing the OCPD.

Cheers, Wayne
 
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