How to size neutral

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codequestion

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I have 120/208v single phase A AND C tapped from three phase 120/208V trough to Panel F. Lanel F A phase has 32.25 amps and C phase has 48.17 amps. What would the neutral size be per nec 210.61?

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For that application, the neutral needs to be the same size as the phase conductors. The maximum unbalanced neutral current is the current with only loads on the highest loaded phase. Even with equal loads on the two ungrounded conductors, you will have the same current on the neutral. It does not cancel where you are using only two phases of a 3 phase wye system.
 
don_resqcapt19 said:
For that application, the neutral needs to be the same size as the phase conductors. The maximum unbalanced neutral current is the current with only loads on the highest loaded phase. Even with equal loads on the two ungrounded conductors, you will have the same current on the neutral. It does not cancel where you are using only two phases of a 3 phase wye system.
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Didn't read code to verify - but logic tells me it should only need to be as large as maximum line to neutral load possibilities. Say you fed an apartment (common place to see 208/120 three wire) and had some electric space heating @ 208 volts, water heating @ 208 volts range (gets a little more complex because many have some 120 volt load), then you have already taken up some of the feeder capacity with loads that will not utilize the neutral. I don't see you needing full capacity neutral in that case but rather one that can handle the maximum possible current that may be imposed on the neutral. Keep in mind if there is extremely limited neutral load the feeder neutral still needs to be at least as large a the required EGC.
 
kwired said:
Didn't read code to verify - but logic tells me it should only need to be as large as maximum line to neutral load possibilities. Say you fed an apartment (common place to see 208/120 three wire) and had some electric space heating @ 208 volts, water heating @ 208 volts range (gets a little more complex because many have some 120 volt load), then you have already taken up some of the feeder capacity with loads that will not utilize the neutral. I don't see you needing full capacity neutral in that case but rather one that can handle the maximum possible current that may be imposed on the neutral. Keep in mind if there is extremely limited neutral load the feeder neutral still needs to be at least as large a the required EGC.
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You are correct...the maximum line to neutral load that is on any of the phase conductors. Thanks for catching that.
 
codequestion said:
I have 120/208v single phase A AND C tapped from three phase 120/208V trough to Panel F. Lanel F A phase has 32.25 amps and C phase has 48.17 amps. What would the neutral size be per nec 210.61?

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There is no 210.61. (at least in 2014 and 2017), you type the wrong section you are questioning?
 
codequestion said:
Huh oh its 220.61 NEC 2014

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Just wanted to make sure what section you were asking about - replies so far are in accordance with that section.

So you have 32.25 on one line and 48.17 on the other. If all the loads are 120 volt then you need at least 48.17 amp neutral conductor.

If you have some straight 208 volt loads mixed in you can subtract those, they don't impose any current on the neutral.

You can not have neutral any smaller then the required EGC though. (have to look for where that is but I know it is in there)
 
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