Downsizing neutral?
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david
Senior Member
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- Pennsylvania
220.61 feeder or service neutral load
Service neutral and "grounded conductor" sizing
I am having a discussion with an engineer regarding sizing the incoming service neutral to a service switch for a fire pump. Code dictates you need to run a neutral to the line side of the service switch, even if you do not need the neutral on the load side (i.e. a motor like a FP). Our...
forums.mikeholt.com
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Also for feeders look at 215.2(A)(1)(a)Exception #3.
kwired
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It only needs sized per the load it will carry though that can be a little tricky to calculate. But if you have a panelboard with all line to line loads other than say a lighting circuit and maybe a general purpose receptacle circuit or two then it becomes rather obvious the max load on that neutral will be limited to what comes from just those few single pole circuits.
Services do limit the minimum size to be same as the minimum supply side bonding jumper size even if calculated load is less.
Feeders do limit the minimum size to be same as the minimum equipment grounding conductor size even if calculated load is less.
I think many AHJ's will allow reduction in most cases to not exceed two conductor sizes without needing to give any kind of load calculation. Lots of cables or multiplexed conductor assemblies do come with a neutral that is two sizes smaller than the ungrounded conductors.
Services do limit the minimum size to be same as the minimum supply side bonding jumper size even if calculated load is less.
Feeders do limit the minimum size to be same as the minimum equipment grounding conductor size even if calculated load is less.
I think many AHJ's will allow reduction in most cases to not exceed two conductor sizes without needing to give any kind of load calculation. Lots of cables or multiplexed conductor assemblies do come with a neutral that is two sizes smaller than the ungrounded conductors.
Thank you for all the info!
Given what you said would you think it code compliant to run a 100 ampere dwelling unit feeder as Two # 4 AWG energized, one 8 AWG Grounded Conductor, and one #8 Equipment Grounding Conductor? I just spent a couple of hours on 215.2(A)(1) and the table in 250.122 and I cannot seem to get it clear in my head. Bad day I guess.
kwired
Electron manager
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If you can prove the neutral load is no more than ampacity of the 8 AWG why not? This is the smallest it can be regardless what your calculation is though. If there is a significant amount of 240 volt loads neutral load could easily be low.
wwhitney
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wwhitney
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More a case of I knew it and then forgot it. @jaggedben, didn't this question come up recently in a different thread? See 215.2(B).
Cheers, Wayne
jaggedben
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Ah, yes. There it is! Also a case of I knew and forgot. Was looking in the wrong places for it.
That other thread was this one.
That other thread was this one.
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You all sound like me and must have the same disease... CRS-- Can't Remember S_ _ _
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Yes that is the crux of the matter isn't it. With a clothes dryer, 240 volt portion of the kitchen range, water heater I was guessing that the neutral current might actually be less than 70. If I'm allowed to multiply that by the 83% dwelling unit factor it might actually make it to #8 AWG. But I think #6 is more likely. I suspect I'd have to run it in non metallic flexible conduit fully enclosed in 1/2 inch sheet rock to be able to run the 2 #4 AWG, 1#6AWG, and 1 #8 AWG that is the more likely result of the calculation. 4-4-4-8 cable is much more likely to be available then 4-4-6-8 would be. So I'd have to compare the cost of the 2 methodologies to see which one was worth doing.
Tom Horne
wwhitney
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The text of 2023 NEC 310.12(A), for example, is:
"For a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service 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 service rating."
Implicit in the text is that the service has a single rating, e.g. 100A, not a split rating like 100A ungrounded/70A neutral. And so the allowance is only for a conductor on a 100A residential service meeting the requirements to have an ampacity of 83A. We will have to look to other NEC sections to reduce the grounded conductor ampacity further, and those sections won't have an 83% factor.
Cheers, Wayne
winnie
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- Electric motor research
If the calculated neutral current is low enough, the #8 would suffice.
I don't think the #4s would work for the hots, unless this 'feeder' is carrying the entire load of the residence or the residence only has a 100A service with #4 conductors.
Concurring with @wwhitney : IMHO the 83% factor for a residence doesn't apply to your neutral calculations, since they by default are only a portion of the residence load.
I don't think that is a valid because the language change that clarified that the supply conductors must be supplying the entire load was to put a stop to the practice of running 2 different feeders from the Service Equipment to 2 different panels in large new homes and applying the reduced sizing to different sets of conductors. The formal interpretations made by the NFPA on the issue previously were clear that was a misapplication of the reduced size allowed for feeders. Contractors were taking advantage of the misunderstood allowed reduction in sizing, which had been taken verbatim from the allowable reduction in size for the service, which in the situation I'm describing is fully compliant, and applying that to 2 and more feeders to separate parts of the load.
I recall the comments on the proposed change to the rules language that objected to clarifying that the reduced sizing was confined it to a single feeder that carries the entire load of a dwelling unit. Major tract McMansion home contractors were looking at a sharp increase in costs because the reduced ampacity of the feeder only applied to one set of conductors and no more. That made it so that the service Entry conductors had to be 600 Kcmil or the equivalent cross sectional area and like wise the 2 200 ampere feeders had to be 250 Kcmil. The use of 2 sets of 4/0 Al was then much more clearly forbidden as it was always meant to be.
The code making panel was very clear in their rejection of the tract home contractor concerns. The reduced size of the supply conductors to a dwelling unit are based on the known diversity of use inherent in the way people use electrical power in homes. Since consumption histories verify the families do not use the entire possible load of a home at the same time the reduced size is safe because overloading the conductors is very unlikely. If that load was that of an institutional board and care home experience has shown that occupancy, even if in an identically sized and equipped home will use much more of the calculated load at the same time and cannot use the reduced size supply conductors safely.
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kwired
Electron manager
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- NE Nebraska
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- EC
True, but not everyone uses a cable at all times. They do make raceways and individual conductors.
kwired
Electron manager
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- NE Nebraska
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- EC
I agree. I also likely would seldom or never run 8 AWG neutral with a 100 amp feed to a dwelling unit. Feeder within a dwelling - maybe, would depend on what the supplied loads are.
A panel supplying mostly 240V HVAC and limited line to neutral loads - definitely would if using raceway and individual conductors. Have done the minimal sized neutral many times in non dwelling applications where there is mostly 2 and/or 3 pole breakers and limited single pole breakers involved.
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