3-pole vs 4-pole transfer switch

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FaradayFF

Senior Member
Location
California
Greetings All,
It appears that a 4-pole manual transfer switch could be used for application where only the phase conductors need to be switched on a 3 phase system. If the neutrals are solidly interconnected at a neutral bus bar in the MTS enclosure, the 4th pole on the switch would be unused. I think purchasing 4-pole based breaker MTS would be slightly more expensive, but provide more flexibility. Are there any other concerns/design issues here?

Thanks,
EE
 
In my area, 3-pole discos are much more common than two pole, so I’ve used them for single phase 2-wire setups. and just not used the third pole.

I just buy the neutral lug kit if it’s the first point of disconnect and needs a N/G bond, otherwise I just split bolt the neutral in the box.
 
Other than "transfer" provisions on some smaller load centers I have not seen a breaker style manual ATS.
The 3 pole solid neutral manual switches are more common here.
 
I would think the choice would depend more on if you were designing a separately derived system or a non-separately derived system. If you were designing a separately derived system that didn't use the neutral at this time but wanted the flexibility to add the neutral in the future, I think it would make sense to provide the 4-pole switch. If you are designing a non-separately derived system, the fourth pole doesn't really gain you much as you would have to reconfigure the generator and distribution system into a separately derived system to take advantage of switching the neutral.

I have designed a 4-pole ATS on a non-separately derived system once because there was a plan to remove that building's non-separately derived generator in the future and refeed the generator distribution from a generator plant that was a separately derived system. I made sure to put signage on the ATS and in the plans explaining why I had designed the system the way I did.
 
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