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NEC 310.12(A) One-Family Dwelling Service Conductors & Feeders

jar546

jar546

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In 2013 I built a home. I remember talking to the electrician about his choice of conductor sizes for the following scenario:

The service conductors were fed from the POCO pad-mounted transformer between houses in the development. The service conductors came directly to a 200A combination meter/service disconnect which is typical in Florida. Technically, the only service conductors were the lateral feed wires from the POCO to the combo pack seen below.
(click to enlarge)
Screen Shot 2023-09-14 at 22.25.47.png
Coming out of the bottom lugs of this 200A combo pack were 2/0 awg copper THWNs going to my MLO panel in the garage which acted as my main panel and provided 100% of the house load. So far so good, right? After all, Section 310.12(A) & Table allow 2/0 awg copper for a 200A service & feeders for an SFR. See Table below.
(click to enlarge)
Screen Shot 2023-09-14 at 22.37.32.png

Normally, a 200A service or feeder would require 3/0 awg copper conductors per NEC 200.16

Would it make any difference if the combo pack outside had a 60A circuit added to it for a pool panel?
Would NEC 310.12(A) make a difference? Personally, I don't think it matters if there is a shared load because the verbiage of NEC 310.12(A) concerning the sharing of the load would not apply because the 2/0 feeder to the MLO is just that, a feeder and not service conductors.

Do you agree?
 
Wow, a single phase dwelling unit question. How fascinating.

First, what code cycle is that? Here we are under the 2017 and the applicable artical would be 310.15(B)(7).

And yes the install you propose would be compliant with 2/0 as the conductors to the inside panel would be a feeder and covered by this section.
 
310.12(B) Feeders.
For a feeder rated 100 amperes through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating. If no adjustment or correction factors are required,Table 310.12 shall be permitted to be applied.

Is the pool associated with the dwelling? I have encountered jurisdictions that would not allow the 83% multiplier for a feeder if the service panel had breaker positions... whether the positions were in use or not.
 
wwhitney said:
No, see (2020) 310.12(C).

Cheers, Wayne
Click to expand...
310.12(C) Feeder Ampacities.
In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B).

Do you contend that (C) negates the requirement found in (B) that the entire load be supplied?
 
ICE said:
310.12(C) Feeder Ampacities.
In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B).

Do you contend that (C) negates the requirement found in (B) that the entire load be supplied?
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It doesn't negate it, it just provides an additional allowance for the case that a downstream feeder doesn't carry the entire load. Because as you pointed out, it doesn't make any sense to require a downstream feeder to be larger (in ampacity) than the upstream feeder supplying it.

Cheers, Wayne
 
ICE said:
310.12(C) Feeder Ampacities.
In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B).

Do you contend that (C) negates the requirement found in (B) that the entire load be supplied?
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Yes, because it is a feeder.
Or is it?
 
wwhitney said:
It doesn't negate it, it just provides an additional allowance for the case that a downstream feeder doesn't carry the entire load. Because as you pointed out, it doesn't make any sense to require a downstream feeder to be larger (in ampacity) than the upstream feeder supplying it.

Cheers, Wayne
Click to expand...
Downstream...upstream... I'm not seeing the stream. (C) is straight up a "feeder for an individual dwelling unit." I take the word for to mean as the supply of. To stretch the application of (B) by invoking the literal reading of (C) and include all feeders brings in a second feeder in Jeff's example... the pool circuit. At 60 amp one would expect a minimum #6 AWG. Now if 310.12(C) applies well then #8 is large enough.

(C) clearly states that the ampacity of a feeder for an individual dwelling unit shall never be required to be greater than as specified in 310.12(B). An ampacity that is at a minimum 83% of the feeder over-current protection....if it supplies the entire load of said individual dwelling unit.

Having considered the worth of (C) prior to this discussion, I have not determined a reason for (C) being included. To assume that a literal reading of (C) throws the door open to any and all dwelling feeders is odd.

It is perhaps irrelevant semantics however, the term individual dwelling unit as used in (B) refers to "individual dwelling unit in a two-family or multifamily dwelling".

Why 310.12(C) is included in the code is not clear to me. At best is seems superfluous. At worst it causes confusion.
 
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ICE said:
Why 310.12(C) is included in the code is not clear to me. At best is seems superfluous. At worst it causes confusion.
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Look, you have a 200A nominal residential service. The service conductors need only have an ampacity of 83% * 200A = 166A, per 310.12(A). What 310.12(C) is telling you is that any feeder on that service never needs an ampacity of more than 166A. So that makes a difference for a 200A nominal or 175A nominal feeder, but makes no difference to the 60A feeder for the pool.

Cheers, Wayne
 
wwhitney said:
Look, you have a 200A nominal residential service. The service conductors need only have an ampacity of 83% * 200A = 166A, per 310.12(A). What 310.12(C) is telling you is that any feeder on that service never needs an ampacity of more than 166A. So that makes a difference for a 200A nominal or 175A nominal feeder, but makes no difference to the 60A feeder for the pool.

Cheers, Wayne
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Would the same be true for a 600 amp service?
 
ICE said:
Alrighty then why would the condition of the entire load not apply as well?
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Because it doesn't, and if you read it as applying, 310.12(C) becomes meaningless, so that's obviously the wrong reading? : - )

Less flippantly, 310.12(A) and (B) include two predicates, "rated 100 through 400 amperes" and "supplying the entire load associated with a
one-family dwelling . . ." 310.12(C) uses the description "for an individual dwelling unit."

Which of the two predicates in 310.12(A) and (B) does the 310.12(C) description resemble? It's the latter predicate about "supplying the entire load associated with a one-family dwelling. . ." So the intent in 310.12(C) is to contrast with 310.12(B) and apply when the "supplying the entire load . . . " condition isn't met, but you just have a feeder "for an individual dwelling unit."

Cheers, Wayne
 
So to get the true meaning of 310.12(C) takes cherry picking an inference. I am willing to believe that 310.12(C) is meaningless.
 
ICE said:
I am willing to believe that 310.12(C) is meaningless.
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The language in question first showed up in the 2014 NEC, and if you read the 2014 Report on Comments (second round of revisions), it is clear the intention is as I described. (https://www.nfpa.org/Assets/files/AboutTheCodes/70/70-A2013-ROC.pdf and see in particular Comments 6-43 and 6-52). At one point the text was going to be "In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that of its 310.15(B)(7)(a) or (b) conductors." Perhaps you find that clearer and they should have stuck with that?

Also, let's try this argument: Suppose 310.12(C) said "any other feeder" instead of "a feeder". Would that make it clear what 310.12(C) means? If so, consider that the 310.12(B) feeder also gets to use the 83% factor. Combine the 310.12(B) feeder with "any other feeder" and you just get "any feeder," i.e. "a feeder", the language of 310.12(C).

ICE said:
So to get the true meaning of 310.12(C) takes cherry picking an inference.
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It's hardly cherry picking, it is the obvious meaning.

Cheers, Wayne
 
OK, let's try this one more time:

310.12(A) covers services "rated 100A through 400A" and "supplying the entire load". 310.12(B) covers feeders "rated 100A through 400A" and "supplying the entire load". No problem.

310.12(C) says "In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B)." The phrasing "In no case shall a feeder for an individual dwelling unit . . ." (indefinite form) means any "feeder for a dwelling unit" is covered; there's no restriction on size or "supplying the entire load." The value referenced is the "ampacity specified in 310.12(A) or (B)."

Great, so the ampacity for what service "supplying the entire load" or feeder "supplying the entire load"? It is clearly implicit that it is the service or the feeder that is supplying the feeder that you want to apply 310.12(C) to (aka the "upstream" feeder or service).

So the upshot is that if you have a 310.12(A) service or 310.12(B) feeder, then any further feeder it supplies (aka "downstream" feeder) shall not be required "to have an ampacity greater than" the specified ampacity for your 310.12(A) service or 310.12(B) feeder.

Not the most elegant wording, but not really any room for debate.

Cheers, Wayne
 
I apologize for being too dense to let is soak in.

A conductor can't be a feeder unless the source is a service.
{Feeder. All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.}

According to 310.12(C), a feeder conductor can never be required to be larger than the service conductor as determined by 310.12(A). Therefor the requirement found in 310.12(B) that the feeder shall supply 100% of the load associated with the dwelling is moot.

Why then is the 100% factor mentioned in 310.12(B) with regards to a dwelling feeder? In what circumstance would the 100% of the load apply to a dwelling feeder?
 
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