4 Pole ATS per NEC

[11Haze29]

Installation is a typical padmount xfrm (480/277) and an auxilary standby generator (480/277). Both have grounded neutrals and carry the neutrals to a Automatic Transfer switch. ATS manufacturer says install must be with a 4 pole ATS to switch the neutrals. He explained that without switching the neutrals, neutral current could split between the neutral conductor of the source, and through the ground path of the neutral conductor of the idled alternate source. I understand the logic. But where in the code does it say that the neutral must be switched to prevent the ground path from conducting current. Need a specific reference to convince the customer to justify added expense of the ATS. Any references?

 

[Dsg319]

Check 250.6

Objectionable current

 

[tom baker]

What is the amperage of the main service disconnect!

 

[Strathead]

Installation is a typical padmount xfrm (480/277) and an auxilary standby generator (480/277). Both have grounded neutrals and carry the neutrals to a Automatic Transfer switch. ATS manufacturer says install must be with a 4 pole ATS to switch the neutrals. He explained that without switching the neutrals, neutral current could split between the neutral conductor of the source, and through the ground path of the neutral conductor of the idled alternate source. I understand the logic. But where in the code does it say that the neutral must be switched to prevent the ground path from conducting current. Need a specific reference to convince the customer to justify added expense of the ATS. Any references?

simply remove the bond to ground in the generator, unless it is required, which is generally only when you have GFCI breaker on the normal feed.

 

[11Haze29]

these are large generators, usually around 800 amps. So the 4th pole add considerable cost to the ATS. Dont understand comment about removing the generator bond to ground. Its a wye output so are you saying NOT to bond the neutral to ground?

 

[don_resqcapt19]

these are large generators, usually around 800 amps. So the 4th pole add considerable cost to the ATS. Dont understand comment about removing the generator bond to ground. Its a wye output so are you saying NOT to bond the neutral to ground?

If you want to use a 3 pole transfer switch, you must remove the neutral to ground bond at the generator. That way, all of the bonding between the neutral and earth is at the service equipment. That service bonding is the bonding for both the service neutral and the generator neutral when you are using a 3 pole transfer switch for a 3 phase 4 wire generator. The neutral remains isolated from any earth connections except at the location of the main bonding jumper. A equipment bonding jumper would be installed with the generator conductors for the required bonding of the non-current carrying metal parts of the generator.

A more direct code rule that prohibits the use of a 3 phase generator with the neutral bonded at the generator and a 3 pole transfer switch would be 250.24(A)(5).

 

[augie47]

Tom's post (#3) might be relevant due to GF mains,. What size is the service disconnect ?

 

[tom baker]

Generally , using a 4 pole transfer makes the installation more complicated as you need to establish grounding and bonding at the generator. The nec does not say when a 4 pole switch is required, but by practice a 4 pole switch is required when gfp is required on for the service or feeder.
Hence why the question on size of service.

 

[augie47]

(guess we may never know)

 

[tom baker]

Maybe a question posted from work, long weekend, perhaps we will find out tommorrow

 

[11Haze29]

If the main on the generator does have GF, and you use a 3 pole ATS, won’t the GF still work even if the neutral is not bonded to ground at the generator, but is bonded at the service entrance.

 

[wyattlewis]

If the main on the generator does have GF, and you use a 3 pole ATS, won’t the GF still work even if the neutral is not bonded to ground at the generator, but is bonded at the service entrance.

If you check out this white paper you can see "improper ground fault sensing" that there exists the possibility of a ground fault that bypasses the generator's GF sensor which would not happen if there were a 4-pole ats.