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Joined: Oct 2002
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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
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Joined: Mar 2001
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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 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. -- Tom Horne
Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison
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Joined: Jan 2004
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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.
George Little
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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.
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Also look at 250.24b1 for additional info on ungrounded conductor sizes.
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Joined: Oct 2002
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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
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Joined: Nov 2000
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Yoopersup, There can be no reduction in the Feeders & Service Neutrals till over 200 amps. Are you saying for feeders less than 200 amps the neutral must be the same size as the ungrounded conductors? Don
Don(resqcapt19)
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Don
(Under cetain conditions) Yes, And sometime the Netural Should be larger then the ungrounded conductors.
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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
Don(resqcapt19)
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