2- and 3-pole single phase transfer switch- differences

[Dubledox]

Greetings all. I am specifying a single-phase automatic transfer switch, to be used for back-up utility power for a critical 120/240V panelboard. The two entrance poles are fed from two small power transformers connected to the utility.

I am unsure whether I need a 2-pole or 3-pole transfer switch. There's some debate whether overlapping neutral or non-overlapping neutrals is better. My understanding is that you just sent the neutral through unswitched, so I would only need a 2-pole arrangement on this ATS.

Am I correct in my thinking, or totally wrong? Advice is appreciated.

 

[qcroanoke]

Greetings all. I am specifying a single-phase automatic transfer switch, to be used for back-up utility power for a critical 120/240V panelboard. The two entrance poles are fed from two small power transformers connected to the utility.

I am unsure whether I need a 2-pole or 3-pole transfer switch. There's some debate whether overlapping neutral or non-overlapping neutrals is better. My understanding is that you just sent the neutral through unswitched, so I would only need a 2-pole arrangement on this ATS.

Am I correct in my thinking, or totally wrong? Advice is appreciated.

Since you're doing the specifying I would not specify a switched neutral.
Also make sure you specify utility to utility transfer switch.

 

[steve66]

Two utility services?

I'd probably spec a switched neutral. You will have N-G bonds at each utility transformer, and if you spec a solid neutral, you will have 2 N-G bonds on the same system.

 

[GoldDigger]

Two utility services?

I'd probably spec a switched neutral. You will have N-G bonds at each utility transformer, and if you spec a solid neutral, you will have 2 N-G bonds on the same system.

There are several issues involving the neutral that are often confused with each other in a 120/240 three wire single phase system.
1. You can switch the neutral or wire it through solid. As noted if both feeds to the transfer switch have their own ground neutral bond on the customer side of the service point(s) but ahead of the ATS you cannot use a solid neutral.
2. Given that you will switch the neutral, two other options come up:
2a. The utility sources are synchronized and you will be using a closed transition (overlapping) switch. In this case the neutral switching should be overlapping too.
2b. Whether or not the sources are synchronized, you choose to use an open transition (open A first then close to B) switch.
In that case you can use and identical pole for neutral and take the risk of a momentary open neutral condition damaging some loads. Or you can use a switch which opens both hots before opening the neutral and then closes the neutral to the alternate source before closing the hots to that source.

If the only N-G bonds are on the utility side of the service points, as long as the utility approves I do not see an NEC problem with the multiple N-G bonds involved in tieing both neutrals together and not switching them. What I would avoid is using the neutral from one transformer only in conjunction with the hots leads from the other transformer.