Common Neutral for 2SETS-( Three Phase Ckts in one condu

[zulma]

There is the statement:

If I have 2sets(3 wire #3/0 and 1w#3/0 GRD and 1W#3/0 Neutral inside 4" PVC
COnduit) for
serve a UPS. There is any violation of the code and/or there is any problems
with the harmonic current due to a unbalanced load.

I read the code and made some calculations: derating Factor, conduit space
40% apply and ground. My problem is to calculate the neutral, if it has to
carry the 70% of the unbalanced load or it could be of the same size of th
ground.


Thanks,

Zulma I. Santiago Cabrera

 

[charlie b]

Re: Common Neutral for 2SETS-( Three Phase Ckts in one condu

The neutral and the ground are sized in accordance with different rules. You never have to size one to match the size of the other. The ground (more properly the ?equipment grounding conductor?) is sized on the basis of the overcurrent protection, using Table 250.122. Given that your phase conductors are 3/0, I would infer that your overcurrent protection will be at or below 200 amps. That would mean that your ground need not be bigger than a #6 (copper).

The neutral has to be sized to carry the minimum calculated neutral load. This is the same concept as applies to the phase conductors. We cannot tell you whether there will be excessive harmonic loads on the feed to this UPS. Only you could tell us that. But to make sure we are clear on this point, having an unbalanced load cannot, by itself, cause there to be harmonic currents. Balance and harmonics are completely different phenomena.

I normally size the neutral the same as the phase conductors, unless there is a good reason to use a double-sized neutral. I might be able to show by calculation that the neutral could be smaller than the phase conductors, but I would still choose to make them the same size. That is a design choice, not a code requirement.