russhammer
Member
I think I have seen in the code book that a neutral conductor can be "not more than 2 AWG sizes smaller than the ungrounded conductor". Can someone tell me if this is correct and what the article is than states this. 
Neutral sizing
- Thread starter russhammer
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- Lockport, IL
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- Retired Electrical Engineer
Re: Neutral sizing
I don?t think there is such a requirement. I?ve certainly not hear of it. But all I can offer from the NEC itself is Example D2(b). It shows a calculated service load of 122 amps, which would require an AWG #1 (for Copper USE). It also shows a calculated neutral load of 62 amps, which would only require an AWG #6 (for Copper USE). That is a difference of 4 AWG sizes.
I don?t think there is such a requirement. I?ve certainly not hear of it. But all I can offer from the NEC itself is Example D2(b). It shows a calculated service load of 122 amps, which would require an AWG #1 (for Copper USE). It also shows a calculated neutral load of 62 amps, which would only require an AWG #6 (for Copper USE). That is a difference of 4 AWG sizes.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Neutral sizing
Oh, I agree that downsizing neutrals is not a good design practice. But that was not the question.
Oh, I agree that downsizing neutrals is not a good design practice. But that was not the question.
Re: Neutral sizing
It all depends on what is being fed, and how much you downsize. Sometimes downsizing the neutral is OK. One example might be the feeder to a panel to feed 3 phase motor loads where the panel includes a neutral for service receptacles.
For neutral sizing, see 220.22. For three phase loads like dryers, ovens, etc, the neutral must be sized for at least 70% of the load on the ungrounded conductors. In the case of #8, #10, #12, and #14 wire, that does pretty much correspond to making the neutral no more than 2AWG smaller.
Steve
[ October 21, 2004, 09:10 AM: Message edited by: steve66 ]
It all depends on what is being fed, and how much you downsize. Sometimes downsizing the neutral is OK. One example might be the feeder to a panel to feed 3 phase motor loads where the panel includes a neutral for service receptacles.
For neutral sizing, see 220.22. For three phase loads like dryers, ovens, etc, the neutral must be sized for at least 70% of the load on the ungrounded conductors. In the case of #8, #10, #12, and #14 wire, that does pretty much correspond to making the neutral no more than 2AWG smaller.
Steve
[ October 21, 2004, 09:10 AM: Message edited by: steve66 ]
- Location
- Illinois
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- retired electrician
Re: Neutral sizing
Steve,
Steve,
I don't think so. Look at 220.22.
Don
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Neutral sizing
Don and Steve: You are not saying different things, but rather are saying the same thing in different ways.
First of all, we do not know whether or not the original question was related to a household. So for the moment, let us presume that it did.
Secondly, 220.22 tells us that the neutral load shall be the ?maximum unbalance. . . .? It then defines that term with the sentence that Don quoted. So we must use that number as the neutral load. But we are free to select any conductor for which the ampacity is at least that high. Put another way, the neutral conductor must be sized for at least 70 % . . . (as Steve had said).
Don and Steve: You are not saying different things, but rather are saying the same thing in different ways.
First of all, we do not know whether or not the original question was related to a household. So for the moment, let us presume that it did.
Secondly, 220.22 tells us that the neutral load shall be the ?maximum unbalance. . . .? It then defines that term with the sentence that Don quoted. So we must use that number as the neutral load. But we are free to select any conductor for which the ampacity is at least that high. Put another way, the neutral conductor must be sized for at least 70 % . . . (as Steve had said).
- Location
- Illinois
- Occupation
- retired electrician
Re: Neutral sizing
Charlie,
My point is that three phase loads do not contribute any unbalance and are not counted in the neutral sizing. Steve's post said "For three phase loads like dryers, ovens, etc,".
Don
Charlie,
My point is that three phase loads do not contribute any unbalance and are not counted in the neutral sizing. Steve's post said "For three phase loads like dryers, ovens, etc,".
Don
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Neutral sizing
I see what you mean. I recall once being able to count to three. Those were the good old days.
I see what you mean. I recall once being able to count to three. Those were the good old days.
Re: Neutral sizing
Don:
You are right in that my statement would have been more accurate if I included the word "household".
Then again, I wasn't trying to be exact. I was trying to give a very brief overview. I assumed the poster (and others) would read the paragraph for details.
Steve
Don:
You are right in that my statement would have been more accurate if I included the word "household".
Then again, I wasn't trying to be exact. I was trying to give a very brief overview. I assumed the poster (and others) would read the paragraph for details.
Steve
Re: Neutral sizing
Don:
Now I get it. Household appliances are usually two phase. You are right, this doesn't apply to 3 phase loads.
So if I have a 400 amp panel with 500KCM phase conductors feeding 3 phase motor loads and a single receptacle circuit, I could run a #12 neutral (for the receptacle) with the 500KCM's??
Steve
Don:
Now I get it. Household appliances are usually two phase. You are right, this doesn't apply to 3 phase loads.
So if I have a 400 amp panel with 500KCM phase conductors feeding 3 phase motor loads and a single receptacle circuit, I could run a #12 neutral (for the receptacle) with the 500KCM's??
Steve
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Neutral sizing
I would not call it a good design. For one thing, a three-phase load (or a set of them) does not necessarily have a balanced load amongst the three phase conductors. For another thing, I would not know how to limit the single phase loads to that one outlet. But I cannot name an NEC article that would forbid it.
I would not call it a good design. For one thing, a three-phase load (or a set of them) does not necessarily have a balanced load amongst the three phase conductors. For another thing, I would not know how to limit the single phase loads to that one outlet. But I cannot name an NEC article that would forbid it.
bphgravity
Senior Member
- Location
- Florida
Re: Neutral sizing
Doesn't 250.24(B)(1) limit the minimum size a grounded neutral can be. Well, at least the one brought to the service.
Doesn't 250.24(B)(1) limit the minimum size a grounded neutral can be. Well, at least the one brought to the service.
Re: Neutral sizing
In addition to 220.22, NEC does have a few other references about minimum feeder/service size of grounded conductor.
215.2, which I guess applies to grounded conductor as well as "hots".
250.24(B)(1), as mentioned, for service supplied systems
250.30(A)(6) for separetely derived systems
In addition to 220.22, NEC does have a few other references about minimum feeder/service size of grounded conductor.
215.2, which I guess applies to grounded conductor as well as "hots".
250.24(B)(1), as mentioned, for service supplied systems
250.30(A)(6) for separetely derived systems
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