230.79(C) - one-family dwellings

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lielec11

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Electrical Engineer (PE)
So I read this as all single family dwellings require a minimum 100A service disconnecting means.

What about individual units within a two or multi family dwelling? Typically I provide minimum 100A regardless of the diversified load but I have another engineer who is trying to make the OCPDs for apartment units 80A.
 
luckylerado said:
I think you are in the wrong section. What you are describing is not the service but a feeder and is based on the calculated load. You should be looking in 220
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So you answered my follow up question... I agree with your response that it is not a service, I just typically provide 100A because my loads tend to be close to that anyway with the type of units I design around.'

Another question is in regards to section 225.39(C), which states, "For a one family dwelling, the feeder disconnecting means shall have a rating not less than 100A, 3-wire"

I know that 225 is for "outside branch circuits and feeders" but then why would you include such a broad statement?
 
lielec11 said:
So you answered my follow up question... I agree with your response that it is not a service, I just typically provide 100A because my loads tend to be close to that anyway with the type of units I design around.'

Another question is in regards to section 225.39(C), which states, "For a one family dwelling, the feeder disconnecting means shall have a rating not less than 100A, 3-wire"

I know that 225 is for "outside branch circuits and feeders" but then why would you include such a broad statement?
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Got me.. IS this a single family dwelling? Each of the separate units are not considered single family dwellings unless they stand alone.
 
luckylerado said:
Got me.. IS this a single family dwelling? Each of the separate units are not considered single family dwellings unless they stand alone.
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It is just a general question that came up from a friendly discussion. For the sake of your question let's just say it is part of a multifamily building. If fed from a common meter center then the unit load centers can be whatever size the calculations in 220 dictate, correct?

Edit: I just looked up the code definition of "dwellin unit" and "dwelling, single family" .... I think I was mistakenly lumping these into the same category.
 
Agreed...you were lumping the two together. The application as expressed lead to the following:

225.39(D) All Others. For all other installations, the feeder or branch-circuit disconnecting means shall have a rating of not less than 60 amperes.

Assuming the engineer provides the calculated load calculation in accordance with Art.220 the disconnection means aspect of your question is answered again in 225.39 (D).

Comments based on the 2017 National Electrical Code.
 
Let's be clear, if this is a multifamily building then, IMO, the feeder does not have to be 100 amps to each apartment. 225.39 is for single family dwelling.
 
225 is also for outside feeders. If you are supplying a stand alone single family dwelling that 100 amp disconnect capacity applies.

If you are feeding a dwelling unit within a building you only need to supply it with whatever art 220 load calculation applies to the dwelling. A very basic dwelling will have at least 2 - 1500 VA small appliance branch circuits plus 3 VA per square foot. If you get all that into 400 square feet you may only have a load calculation of only 4200 VA, and could supply it with a 35 amp 120 volt feeder or a 20 amp 120/240 feeder.
 
The op was mentioning perhaps multi-family dwelling. I don't see the nec states what size must be run to the units.
 
kwired said:
225 is also for outside feeders. If you are supplying a stand alone single family dwelling that 100 amp disconnect capacity applies.
If you get all that into 400 square feet you may only have a load calculation of only 4200 VA, and could supply it with a 35 amp 120 volt feeder or a 20 amp 120/240 feeder.
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You just described every new apartment unit in NYC, Brooklyn, etc., etc... :D

Dennis Alwon said:
The op was mentioning perhaps multi-family dwelling. I don't see the nec states what size must be run to the units.
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I don't think it does, this was the basis for my original post to see if any of you code guru's on here knew of a certain section I haven't come across yet...
 
Do keep in mind that 230.79(C) is specifically addressing the service disconnecting means. It is not addressing any feeder disconnecting means, and it is not addressing the service conductors, the feeder conductors, or the OCPD. If the service disconnecting means happens to be a circuit breaker, then it and the OCPD are one and the same. But they need not be. I can envision a "tiny house" that has a calculated load of 80 amps. I can use a 100 amp fused disconnect as the service disconnecting means, and thereby satisfy the minimum requirement of 230.79(C). But then I can fuse it at 80 amps. The OCPD would be the fuses, and they don't have to satisfy the 100 amp minimum of 230.79(C).
 
There's no reason that a breaker can't be both a service disconnecting means and a feeder disconnecting means. I see it all the time, a 2-6 gang meter bank with breakers that's the service equipment. Each meter has it's own single breaker that feeds a unit or a common sub remote from the service.

I submit that 230.79 and 230.80 are too vague to definitively settle the question, and that there ought to be a revision to clarify what's intended.
 
Dennis Alwon said:
225.39 is for single family dwelling.
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Not sure I can agree with that statement....However, the fact 225.39(D) is dealing with outside feeders and branch circuits it indeed may not apply in this case if the issue has nothing to do with an outside "feeder or branch circuit"....if that is the case then neither 225.39 nor 230.79 apply.

Now.....I honestly can't remember if the question was based on an "outside feeder" or not.....so I guess that would be important to look back at...aaaahhh but who has the time...:slaphead:

If the feeders do not originate outside or the disconnect is outside then feeds inside then again Article 225 or in this case Article 230 don't apply. Size it per the calculated load found in Article 220.
 
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