Transfer Switch Neutrals

[jcleavit]

So I've noticed that some of the transfer switches I've installed (mostly smaller ones) seem to have solid neutral busses and others (larger switches) have switched neutrals. What is the reason some are switched and some are not?

 

[ActionDave]

Mostly it's a design issue. The only reason to have a switched neutral is if you have some kind of ground fault protection on your main.

 

[tom baker]

Yes as Dave said on ground fault. About 95% of systems don't transfer the neutral.

 

[LarryFine]

The choices are:

1. Each source's neutral is grounded ahead of the T/S, and the neutral is switched.

2. The neutral is singly grounded after the T/S, and the neutral is not switched.

#1 is often found in industrial setups, and #2 is what's found in residential setups.

 

[suemarkp]

I've only installed 2 transfer switches, but both were using portable generators in feeders and not the service. In that case, the neutral and ground are already unbonded and must remain that way. If the generator has an unbonded neutral, then you can use a solid neutral transfer switch. If the generator has a bonded neutral, and you can't undo it, then you need a transfer switch that switches the neutral too because the neutral-ground bond in the generator (when connected to the power inlet) would rebond the feeder downstream which is not permitted. Some of this becomes more obvious when you see the signage requirements in article 702 for what type of generator (bonded neutral -vs- unbonded neutral) can connect to a power inlet.

 

[Sparkdizzle]

This graphic explains the two situations pretty well I think



Sent from my iPad using Tapatalk

 

[LarryFine]

If the generator has a bonded neutral, and you can't undo it, then you need a transfer switch that switches the neutral too because the neutral-ground bond in the generator (when connected to the power inlet) would rebond the feeder downstream which is not permitted.

That also means the premises GEC would need to be relocated to ahead of the ATS, and everything after the ATS should be un-bonded (i.e., wired as a sub-panel) and the ATS need to be service-rated or have a main ahead of it.

Unless I'm mistaken.

 

[suemarkp]

No, I think you are correct (premises GEC was already ahead of the transfer switch, but you'd need a 2nd one for the generator feed side). My generator has a floating neutral, so solid neutral transfer switch is what was used.