Service ratings of conductors

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giovanni0502

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Location
PA
So I am wondering how 500mcm can be used on a 400amp service given the following in the 2017 nec:

230.42 A=conductors must be able to carry the load per article 220.
230.42 B= conductors must match the service disconnect rating.
230.90 A= conductors must have a rating equal or greater than the service disconnect. I take this as breaker or fuse disconnect (not fuse rating). Exception 2 says we can round up per 240.4. This seems to contradict 230.42 B.

Thoughts?
 
230.42(B) Specific Installations.
In addition to the requirements of 230.42(A), the minimum ampacity for ungrounded conductors for specific installations shall not be less than the rating of the service disconnecting means specified in 230.79(A) through (D).
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The maximum size disconnect specified in 230.79(A) through (D) is 100 amps. The ampacity of 500 kcmil far exceeds 100 amps.
 
In my opinion, for any of A to D installation type art.230.79 states the minimum and not the maximum. [15-60A].
 
Julius Right said:
In my opinion, for any of A to D installation type art.230.79 states the minimum and not the maximum. [15-60A].
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Yes it does say that, but does go to 100 amps in (C).

For the purposes of my answer to the OP, those are the maximum size of disconnects that are required. Calling them the maximum required makes it clear that there is no conflict between 230.42(B) and 230.90(A) for his application as he is working with a 400 amp disconnect.
 
They all basically mean greater than or equal to. "Can not be less than..."

So, 230.79 requires the service disconnect be greater than or equal to 230.79(C), which requires it to be greater than or equal to 100A, and, therefore, 230.42(B), which points us to 230.79, also requires the conductors be rated greater than or equal to 100A.

So as long as the disconnect and conductors are rated greater than or equal to 100A then you are okay.

I think Don's comment got the point across, but Julius is correct in saying that all of those sections are minimum requirements.
 
Your reference to 230.42A is correct. The conductor must have an ampacity equal or less than the calculated load. A 400 amp service with a calculate load above 380 amps would be rare. IMO iF you had a calculated load greater than 380 amps then you could not use the 500kmcil.

As to the other references, as others have stated, 230.42 addresses 230.79 which confirms the conductors must be sized to the load and specifies some minimum disconnect ratings for specific installations.
 
Ok, I agree with all that. Follow up question: If 230.42 says the conductor must have an ampacity not less than the load and 230.79 says the minimums size of service disconnects for services of 100a or less. Then why is 310.15 b7 needed for anything other than a 100a service when 230.90 A and 230.90 A exception 2 says we can round up for under 800a anyway?


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giovanni0502 said:
Ok, I agree with all that. Follow up question: If 230.42 says the conductor must have an ampacity not less than the load and 230.79 says the minimums size of service disconnects for services of 100a or less. Then why is 310.15 b7 needed for anything other than a 100a service when 230.90 A and 230.90 A exception 2 says we can round up for under 800a anyway?
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230.79 says the minimum size is 100A. I believe the "or less" at the end was typed in error in the statement.

To answer the question, 310.15(b)(7) apples to a service rating of 100A to 400A. So I would say that section 230.42(A) covers a single family dwelling. I don't think 230.42(B) is required to be met since it applies to "specific installations" (whatever that means).

310.15(b)(7) can reduce the conductor value but more than one conductor size could be the minimum when considering rounding. The section just further explains the actual minimum.

For example, you have 250kcmil (255A) and 300kcmil (285A) that round up to 300A. So which one is the actual minimum conductor size? Well look to 310.15(b)(7). That says 83%.

So 83% * 300A = 249A, so you can choose 250kcmil.


What about a 200A disconnect / OCPD?

Is it 3/0, 2/0, or 1/0? All of those land on a standard size. So do you have to go with 3/0?

Well, with 310.15(b)(7), you can take 83% of 200A, which is 166A and use 2/0 Cu. Even though 2/0 lands on a standard size of 175A. Which is okay provided 230.90(A) exception 5.
 
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