Reduced Neutral Conductors

[faresos]

Hello Everyone:

I have a question regarding replacing some existing panelboards.

While replacing 600A, 480/277V panels it was found that the panels have (2) sets-500 MCM conductors per phase, however, the neutral is only 1/0AWG per panel. The contractor claims this is an NEC code violation [NEC 310.10(H)(2) Section for parallel conductors] and he wants to pull 500KCMil for neutral feeders to make them code compliant. My understanding we can reduce the neutral conductor size based on the unbalance load, however, since they are parallel runs, the minimum size neutral shall not be less the #1/0 which what we have. Per the panel schedule, most the loads are 3-phase motors, transformers, UPS by pass and many unknown loads but all fed from 3-Pole breakers. Is this a code violation assuming we do not have a lot of 277V? I will ask the contractor to verify these unknown loads to ensure they don’t feed any 277V loads.

Thanks,

 

[augie47]

You are correct in that the neutral can be sized according to the calculated load and 1/0 would be the minimum if they are in parallel.

 

[faresos]

You are correct in that the neutral can be sized according to the calculated load and 1/0 would be the minimum if they are in parallel.

Thank you for confirming.

 

[tom baker]

The neutral size is based on the unbalanced load or the size of the OCPD protecting the circuit, and as pointed out the minimum size in parallel is 1/0.
Please see section 215.2(A)(2)
Some years ago the minimum size neutral was simply the unbalanced load, you could have a #12 neutral for a lighting circuit.A change was made make the neutral based on the size of the OCPD in case there was a line to neutral short you would have protection.

 

[kwired]

If there is no line to neutral loads you don't even need the neutral if the panel in question is supplied by a feeder.

You must always run a grounded conductor with service conductors, and an EGC with feeders and branch circuits (though metal raceway can be the EGC).

Otherwise yes you were correct and also correct with 1/0 ends up being minimum for this mainly because you can't parallel smaller then 1/0.

I done exact same thing with a 600 amp feeder before (except it was 208/120) where most the load was three phase motors and there was only a couple 120 volt circuits - one for lighting one for a receptacle. Supplied by parallel 500 phase conductors and 1/0 neutrals.

Was serving an area with HVAC compressors. Was no anticipation of any significant 120 volt load ever being added - but parallel 1/0 still would take pretty good load anyway in that situation. No way there was enough capacity to add enough 120 volt load to max out that neutral and still be able to run all the existing three phase load.

 

[Andy Delle]

And I'll throw this into the mix. If this is a data center, broadcast facility, communications center, insurance/financial company with thousands of desktop computers.... The end wye load side neutral must be oversized, often specified as 2x.

Reason is harmonic current. I have seen many a cooked neutral feeder in old 1960/70s era panels during the HDTV facility rebuilds/upgrades in the 2000s. This issue became a hot topic (no pun intended) in the 1980s when switch mode power supplies were introduced on a large scale.

 

[kwired]

And I'll throw this into the mix. If this is a data center, broadcast facility, communications center, insurance/financial company with thousands of desktop computers.... The end wye load side neutral must be oversized, often specified as 2x.

Reason is harmonic current. I have seen many a cooked neutral feeder in old 1960/70s era panels during the HDTV facility rebuilds/upgrades in the 2000s. This issue became a hot topic (no pun intended) in the 1980s when switch mode power supplies were introduced on a large scale.

If the majority of load is line to neutral non linear - very likely so. There can still be somewhat significant line to neutral non linear load, yet even more HVAC load - making total non linear load not so significant compared to all the combined load. Even if HVAC is VFD driven, if all line to line drives it is not going to add harmonics to the supply neutral.

Add: now with a large facility you may have certain feeders that you may need that large neutral for and others that you don't - depending on loads being fed.