Guys, is there code section that prohibits neutrals from two different panels being tied together? I figure there has to be, but I don't know the section that would apply here. The issue at hand is using a spdt relay to transfer load from a normal panel to an emergency panel. I think you would need a dpdt relay for this.
Neutrals from two different panels tied together
- Thread starter jes25
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- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
Would you have a parallel neutral. This would not be allowed 300.3(B)
Thanks for the reply Dennis. I wouldn't really say the neutrals are paralleled......Almost like they are paralleled from a different panel though. If I'm understanding the specs correctly, it seems they want to power some lights from the normal panel and when power fails transfer it over to emergency power. The problem is only the hot would be switched.
I'm concerned about neutral current flowing on the solidly connected neutral that originates in the other panel. 300.3 would probably apply here, but I feel like its a bit of a stretch.
I'm concerned about neutral current flowing on the solidly connected neutral that originates in the other panel. 300.3 would probably apply here, but I feel like its a bit of a stretch.
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- Placerville, CA, USA
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- Retired PV System Designer
This is more complicated when you have the situation of a transfer switch controlling to different sources, such as POCO and generator. There you may have the choice to treat the generator as a separately derived system and switch the neutral or treat the generator as non-separately-derived and bond the two two neutrals together.
But in your situation where you are switching a load feeder or branch circuit only, you have no choice but the switch the neutrals too. As a practical matter, other than the potential hazard of a neutral in one panel remaining current carrying because it comes from another panel and circuit, the main show stopper would be if there were GFCIs involved.
But going simply by Code text, it is prohibited. Among the other prohibitions likely to be involved will be any applicable ones requiring all of the conductors supplying that load to be in one cable or raceway.
Note that if you switch the neutral in an MWBC, including a three-phase wye circuit, you may need to take special precautions not to create a momentary open neutral during the switching process. That can be a critical factor if you have sensitive electronic rather than resistive or motor loads on the switched circuit.
Gold, this circuit is to essentially switch the load (lights) for part of a branch circuit, and I understand exactly what you're saying about switching the neutral for this. I was hoping to find something in the code that was really on point for this, but it looks like I will only have the sections already cited. I sent an unofficial RFI about this to the owner, so I can get a better understanding of how I'm supposed to wire these.
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- Licensed Electrician
Are you saying the specs want only certain circuits in a lighting panel able to be powered by the generator? I don't see how such a set up would work at all.
- Location
- Massachusetts
In my opinion 250.24(A)(5) prohibits tying two or more neutrals together.
PetrosA
Senior Member
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- Lancaster, Pennsylvania
Let's say that in order to avoid paralleling the neutrals you didn't run a neutral from the emergency source to the load (technically possible if you don't switch the neutral) - now you have a load being fed from two separate circuits via different cables or raceways which violates 300.3 (B) while on emergency power. Running the second neutral and connecting it to the normally used neutral at the load splits the neutral load between the two neutrals in different cables or raceways and violates 300.3 (B) at all times. I don't think you need anything other than 300.3 (B).
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