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- New Jersey
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- Journeyman Electrician
175 amps is the next standard size so next size up rule would require a 175 amp OCPD ahead of the feeder not the 200 amps that I mentioned in my last post.
sub panel and 310.15(B)(7)2
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wwhitney
Senior Member
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- Berkeley, CA
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- Retired
Negative, assuming we are talking about this scenario:
Residential 120/240V service -- 200A service conductors -- 200A service OCPD -- 200A panel with various loads -- 200A branch breaker -- 166A feeder conductors -- 200A subpanel.
310.15(B)(7)(1) would allow 166A ampacity for the service conductors, but we can choose to install larger. 310.15(B)(7)(3) allows any feeder on that 200A residential service to be no larger than 166A ampacity, whether or not we've chosen to take advantage of the 310.15(B)(7)(1) allowance.
(And of course the 200A branch breaker there is superfluous given the 200A service OCPD (assuming no parallel power sources), it could be feed thru lugs instead).
Cheers, Wayne
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- Location
- New Jersey
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- Journeyman Electrician
The sub-panel feeder is that scenario requires 200 amp conductors when using a 200 amp OCPD. That means #3/0.
wwhitney
Senior Member
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- Berkeley, CA
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No, 310.15(B)(7)(3) says they only have to be 166A. That is why I brought it up in this thread, this is often overlooked:
"(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2)."
And what does 310.15(B)(7)(1) specify? It specifies that the service conductors on a 200A 120/240V residential service may have an ampacity as low as 83% * 200A = 166A. So any feeder on that service need be no larger than 166A, regardless of what size service conductors are actually installed.
Cheers, Wayne
tank728
Senior Member
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- New Jersey
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- Electrician
I missed that in your first post. I love discovering gems in the code book to aid contractors in installation. Good find. Although I wonder how inspectors will treat this.
- Location
- New Jersey
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Are we discussing the same thing? I made no mention of a feeder for an individual dwelling unit.
wwhitney
Senior Member
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- Berkeley, CA
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The word "for" there does not mean "carrying the entire dwelling unit load." That's already covered in 310.15(B)(7)(2), so that interpretation would render 310.15(B)(7)(3) meaninglessly redundant. Rather, the word "for" there means "within".
And it only makes sense: if the service conductors are allowed to be 166A, why require anything downstream of them to be of larger ampacity?
Cheers, Wayne
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
I think Wayne is saying that if 2/0 copper is good for the service than 3/0 copper would not be required for the feeder just 2/0
- Location
- New Jersey
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- Journeyman Electrician
The 83% rule is optional. If you have 200 amp SEC's and a 200 amp service then a feeder to a sub-panel not carrying the entire load cannot use the 83% rule.
wwhitney
Senior Member
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- Berkeley, CA
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310.15(B)(7)(3) says the opposite in black and white. It makes no reference to the actual SEC size, only the minimum size that would be allowed. So whether or not you took advantage of the 83% rule upstream has no bearing on whether it may be used downstream.
Wayne
wwhitney
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P.S. The 83% formulation is new as of the 2014 NEC. Prior to that, the closest thing to 310.15(B)(7)(3) was the sentence in 310.15(B)(7) that read "The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors."
That obviously does refer to the size of the service-entrance conductors that are actually installed, so perhaps that is what you are thinking of. However, that changed in the 2014 NEC to the current language, which refers to the size specified in 310.15(B)(7)(1), not the size installed.
Cheers, Wayne
- Location
- New Jersey
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- Journeyman Electrician
Again I'm not speaking of the "feeder conductors to a dwelling unit". If I were discussing that I would agree with you.
wwhitney
Senior Member
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- Berkeley, CA
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So what do you think 310.15(B)(7)(3) means? I.e. what would it apply to if not the feeder conductors in the example in post #23?Again I'm not speaking of the "feeder conductors to a dwelling unit". If I were discussing that I would agree with you.
Cheers, Wayne
- Location
- New Jersey
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- Journeyman Electrician
It means then you have the service disconnect on the outside of the dwelling and the panel on the inside the feeder between the two is not required to be larger than the SEC's which can utilize the 83% rule since it supplies the entire load. I'm not talking about the "feeder for an individual dwelling unit" so I think that maybe were not discussing the same thing. Either way whether we agree or disagree it's a good conversation.
wwhitney
Senior Member
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- Berkeley, CA
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In your example, 310.15(B)(7)(1) says the SECs may use the 83% rule. But you don't have to use it.
310.15(B)(7)(2) says that the feeder between the outside service disconnect and the inside main distribution panel, i.e. "feeder conductors supplying the entire load associated with a" dwelling unit, may use the 83% rule. You can use that allowance even if you didn't use it for the SECs (i.e. for 200A, 3/0 Cu SECs, and 2/0 Cu from the service disconnect to the inside main distribution panel.)
All of that is clear without 310.15(B)(7)(3). So what is 310.15(B)(7)(3) adding to the picture?
Answer: it applies to any feeder that isn't "supplying the entire load associated with a" dwelling unit, using the apparently open to misinterpretation phrase "feeder for an individual dwelling unit." Any downstream feeder is also allowed to be sized with ampacity no larger than 83% of the service disconnect rating.
If "feeder for an individual dwelling unit" meant the same thing as feeder "supplying the entire load associated with a dwelling unit," then we could just delete 310.15(B)(7)(3), which makes no sense. Rather, as 310.15(B)(7)(1) and (2) use one phrasing, and 310.15(B)(7)(3) uses a different phrasing, the meaning in 310.15(B)(7)(3) is different--it means any downstream feeder.
Cheers, Wayne
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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If you have a 200a service, and it supplies a panel that supplies a 200a sub-panel, the feeder need not be full sized.
If you have a 200a service, and it supplies a panel that supplies a 100a sub-panel, the feeder must be full sized.
If you have a 200a service, and it supplies a panel that supplies a 100a sub-panel, the feeder must be full sized.
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