Derating Neutrals

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dino629

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NEC 310.15(4) (a)... I'm sure the answer is simple here, but is a neutral counted when you de-rate conductors, if not when does this paragraph apply???
 
Read 310.15(B)(4) (b) and (c) for when the grounded conductor would be counted as a Current Carrying Conductor, the Grounded conductor would also be counted when the circuits are two wire circuits.

Roger
 
dino629 said:
When is the neutral considered to carry the "unbalanced load" of conductors?:-?
Click to expand...

Any time it is common to two or three unbalanced line or phase conductors.


Roger
 
Just so I understand... If we packed a pipe with (12) #10awg all with 3 hots to a neutral, (not a 3 phase load, but A,B,C phases respectively to separate loads) then We are not counting the neutral as a current carrying conductor and would not have to derate accordingly? Thanks
 
dino629 said:
Just so I understand... If we packed a pipe with (12) #10awg all with 3 hots to a neutral, (not a 3 phase load, but A,B,C phases respectively to separate loads) then We are not counting the neutral as a current carrying conductor and would not have to derate accordingly? Thanks
Click to expand...
Correct, unless we're talking a high poercentage of high-3rd-order-harmonic-content loads.
 
Correct, unless we're talking a high poercentage of high-3rd-order-harmonic-content loads.
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Larry,

Can you explain this a bit more ? I was eaves-dropping on a conversation that came up at my contractor's association meeting the other night where one of the contractors claimed that he no longer uses 12/3 MC when wiring circuits where electronic ballast fixtures are installed. Supposedly there is a large harmonic load on the neutral. Normally if you have a 15 amp load on one phase and a 10 amp load on another the neutral would carry the difference of 5 amps. From what I overheard in this case the neutral would be carrying 25 amps because of the electronic ballasts. Did I hear this correctly and if so can you explain ?

Thanks,

Phil
 
Don,

I only heard parts of the conversation but the contractors involved in the discussion do primarily commercial work. So, if I had to guess the service voltage was probably 120/208 VAC, 3 phase. However, the voltage to the light fixtures, in all probability, would have been 120 VAC based on the context of the conversation.

Phil
 
goldstar said:
Larry,

Can you explain this a bit more ? I was eaves-dropping on a conversation that came up at my contractor's association meeting the other night where one of the contractors claimed that he no longer uses 12/3 MC when wiring circuits where electronic ballast fixtures are installed. Supposedly there is a large harmonic load on the neutral. Normally if you have a 15 amp load on one phase and a 10 amp load on another the neutral would carry the difference of 5 amps. From what I overheard in this case the neutral would be carrying 25 amps because of the electronic ballasts. Did I hear this correctly and if so can you explain ?

Thanks,

Phil
Click to expand...

Not on a WYE system.
Get your amprobe out and check it out sometime
 
Phil,
If you have a 3 conductor cable on a 3 phase system with 10 amps on one ungrounded conductor and 15 amps on the other, you will have 13.2 amps on the neutral and this does not include any neutral current that is the result of harmonic loads.
 
goldstar said:
Larry, Can you explain this a bit more?
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I'll try. Normally, the neutral carries only the difference current among the hot wires sharing it. Because the voltage peaks occur at different times, the total neutral current is only the imbalances; they subtract.

However, when you have harmonics, there are peaks of the same polarity that occur at the same time, so some of the currents are additive. If large enough, they can exceed the normal current on each phase.
 
Thank you all (Don, Bob and Larry). This is what I get from eavesdropping and not getting involved in the conversation in the first place. I probably could have gotten the answer right then and there.
 
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