service neutral calculation

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knoppdude

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Location
Sacramento,ca
I don't know if this question has been asked before, but does anyone know why the service neutral conductor for a residential dwelling is sized by dividing the calculated VA for the neutral load by 240 volts instead of 120 volts? I ask this because the VA load for the neutral load is determined from 120 volt loads, and because it does not seem that the neutral would ever have 240 volts imposed on it. The answer is probably obvious, but I am not seeing it. Thanks.
 
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suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
Because if you divided by 120V, you'd get a number larger than the service which doesn't make sense. Lets say you have 24KW of 120V appliances. If you divide by 240V, you get 100A, and that would be the size 240V feeder you install. The 120V loads should be (in theory) equally split between the two phases. If you divided 24KW by 120V, you'd get 200A. Wouldn't make sense to install a 200A neutral when the ungroundeds are 100A.

The balanced loads assumption should be safe, as it would be really difficult to only use all the loads that are on just one leg.
 
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gndrod

Senior Member
Location
Ca and Wa
*/*240

*/*240

I agree with what Mark says. One other thing using residential optional methods is that the final neutral sizing also requires a standard method load calculation for the neutral size. [220.61(A)] Unbalanced load rbj
 
Smart $

Smart $

Esteemed Member
Location
Ohio
knoppdude said:
...but I am not seeing it. ...
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I agree, because I don't see what you said in the code either. ;)

It's real simple. Add up your 120V loads per leg, i.e. those on L1 separate from those on L2. Use calculation values, not the breaker sizes. Whichever leg totals the highest is the ampacity value which determines the minimum size of the neutral, except that it cannot be smaller than the minimum size required for the GEC per Table 250.66.

PS: If your 120V-loads calculation values are in VA units, then you divide your per leg totals by 120, not 240.
 
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Smart $

Smart $

Esteemed Member
Location
Ohio
LarryFine said:
Yes, but don't forget that half of them should be on each phase.
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...but there is no such thing as on each phase for 1? ;)

That aside, yes, the loading should be approximately equal per leg if all loads are on at the same time. In a perfect world, yes it would be exactly half... but how often does that perfect world exist?
 
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Smart $

Esteemed Member
Location
Ohio
gndrod said:
I think Larry meant phase leg. In past experience, panel schedules are usually required on large projects for one aspect of balance reference. rbj
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Ohh! :grin:

Actually I knew what he meant... just having some fun with word play ;)
 
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knoppdude

Senior Member
Location
Sacramento,ca
Ok, I understand this issue now. As the neutral is designed to carry unbalance, it will never carry more load than either one of the ungrounded conductors, in most cases. In addition, if one of the ungrounded conductors is opened, then it will only be carrying the load of one ungrounded wire, which was sized using 240 v. I know it will be sized with a reduction on certain loads (cooking equipment, dryers, etc.), 240 v loads will not function in this senario, and the neutral is not given the 70 percent rating for 120 loads. Thank you all for your help.
 
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