When sizing a neutral on commercial applications, can 70% be taken? I have a 100 amp. single phase sub panel fed by # 3 copper. What size neutral can I use? I tried to find it in the code, but can't seem to locate it. Can't seem to keep up with it all. Where can I find it in the code? I usually just run the same as phase conductors, but I may need to downsize if I can, to use a smaller pipe size. Thanks a lot. Steve

Try 2002 N.E.C. 220.22

Is this sub panel supplied from a single phase service or is it supplied from a three phase service. If the feeder is from a three phase service than the grounded current carrying conductor will carry just as much current as the phase conductors.
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Tom Horne

We need to know the load and type of supply to determine if you can reduce the grounded conductor. Some panels you can reduce the grounded conductor on three phase or single phase.

The way I read it There can be no reduction in the Feeders & Service Neutrals till over 200 amps. Then after that 70% of the unbalanced load over 200amps, IF it meets the previsons of that section.And No reduction in some cases depending on type of service and type of loads.Such as an unbalanced Wye System.Some cases the Neutral Should be upsized.

Also look at 250.24b1 for additional info on ungrounded conductor sizes.

The sub panel is fed from a single phase service. The load at this point will very little, about 30 or 40 amps, but I am supplying additional spaces for future expansion. It is a business which may add more equipment. I'm thinking I will go ahead and pull the same size neutral anyway to be safe. Thanks Steve

Yoopersup,
Are you saying for feeders less than 200 amps the neutral must be the same size as the ungrounded conductors?
Don

Don
(Under cetain conditions) Yes, And sometime the Netural Should be larger then the ungrounded conductors.

Yoopersup,
Then I don't agree with your statement. 220.20 only requires that the grounded conductor of a feeder be sized to the maximum unbalanced load. If this maximum unbalanced load exceeds 200 amps, then you only need to size the grounded conductor for 200 amps plus 70% of the unbalanced load greater than 200 amps. If you have a 120/240 volt panel where there is a one 20 amp single pole breaker and ten 2 pole 40 amp breakers for 240 volt loads, the ungrounded conductor could be 500 kcmil and the grounded conductor #12 and be compliant with the rules in the NEC.
Don

You might want to look at 250.24, 250.26 and table 250.66 - you can't have that much of a difference in size between phase conductors and your grounded conductor (neutral)

Don
' If nothing else yer interesting ?? Where do you come up with this stuff I said No such thing?? I also refered you to 250.24b1 if I recall. Like I said veryyyyyy interestingggggggg

Kenny,
That would be true if we were talking about a service, but the original post specified a sub-panel and those sections do not apply. For a feeder the neutral only has to be sized for the maximum unbalanced load. My example of 500 kcmil ungrounded conductors and a #12 grounded conductor is code compliant for a feeder with the types of loads that I specified.
Don

Don- I agree with you 100%. Thats why I said we need to know the load. And Yooper, Your a sharp code man but you blew it on this one. There was a time you could only reduce the neutral 2 sizes but that rule went bye bye. Now like Don says, the grounded conductor has to be large enough to carry the neutral load plus if it's a service panel, it can't be smaller than the GEC. There is allowable reductions after 200a though like you said.

[This message has been edited by George Little (edited 10-22-2004).]

[This message has been edited by George Little (edited 10-22-2004).]

Daaaa
So you agree with Don 500 mcm with #14 netural????? Remember I always Did refer you to 250.24b.And also 250.122 gives equipment gr. size Which means 500 mcm ungrounded conductors (400amp) with #14 neutral and a #2 .copper equipment ground?? ummmm maybe we're talking about didfferent things here???Question Outright Can a Neutal be reduced under 200 amps??

Yes

Yooper- I think we might be talking about 2 different animals. If we are talking about Service- then the grounded/grounding conductor are the same conductor and the conductor might be asked to conduct fault current at the service level, which would exceed Don's 20a. If we are talking about a sub panel, then the grounding conductor must comply with 250.122 because it would be expected to conduct current to trip the subpanel main overcurrent device. But the grounded conductor only has to be large enough to serve the maximum unbalance that could be imposed on the load- in Don's sample the maximum would be 20a. IMHO

[This message has been edited by George Little (edited 10-22-2004).]

George, nice post, I agree entirely. I came back to this thread when I realized Yopper might have missed that the opening question was about a sub panel not a service panel.

Bob

Thanks for the input guys. Yes it is a subpanel in question. I will have a maximum of 30 amps unbalanced load on the neutral at this point. So if I wanted to, I could feasibly use a # 10 neutral, though I won't for future expansion sake. I believe I got it straight now, and it's helped me tremendously to understand the difference between the service and feeder conductor requirements for neutrals.. Steve

I need to ask again is the supply to the service that supplies this panel single phase or three phase. A single phase feeder that is supplied from a three phase wye connected service will have as much current on the neutral as on the most heavily loaded phase conductor. The neutral currents in a single phase feeder that is supplied from a three phase wye connected service DO NOT CANCEL.
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Tom H

My heads still buzzing from a 400 amp service with that # 12 neutral.

No, you can't do that with a service, only with a feeder.
Don

Tom, the subpanel is fed from single phase 120/240v main panel. By the way, I completed my feed to this panel. I decided to go ahead and pull the same size for the neutral ( 3 # 3's and a # 8 ground ). The pull went great and I am satisfied that whatever expansion the owner of business may do, that it will be covered. Thanks again for the replies. Steve