4-Pole vs 3-Pole ATS

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PP26

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If my system utility voltage is 208Y/120V, and I have an emergency standby generator 480Y/277V. Do my ATS's needs to be 4-pole?
The way I understand in choosing between 3-pole and 4-pole ATS's depends on how I want my system to be grounded and how I want my OCPD's to react during a ground fault scenario in both utility and emergency side.

Additional information is that I have multiple separately derived system (transformer) in the building, the secondary side neutral of all this transformers will be connected to a common grounding electrode conductor which is connected to my main grounding electrode (in this case, a metal underground water pipe) with an additional supplementary ground as per code requirements. Note that this main grounding electrode is the one I am utilizing for my utility source.

Back to my question, do I need to have a 4-pole ats? or a 3-pole will be perfectly fine.
Please note: I don't intend to trip my generator breakers if I have ground fault when my system is being fed from the emergency standby generator.
Thanks in advance for those who will share their thoughts and knowledge.
 
My understanding is that the main difference is whether or not the neutral is switched.

If it is switched, then each source should be bonded and the load should not be bonded.

If it is not switched, then the sources should not be bonded and the load should be bonded.

Thus, you decide based on whether you can make both sources either bonded or not bonded.

(By bonded, I'm referring to the neutral/grounding bonding. The two sources should match.)
 
LarryFine said:
My understanding is that the main difference is whether or not the neutral is switched.

If it is switched, then each source should be bonded and the load should not be bonded.

If it is not switched, then the sources should not be bonded and the load should be bonded.

Thus, you decide based on whether you can make both sources either bonded or not bonded.

(By bonded, I'm referring to the neutral/grounding bonding.)
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Hi Larry,

Thank you for responding, as I've said on my OP, I don't have the intention of tripping my generator breakers, which is equivalent to not switching the neutral and not installing a system bonding jumper on the generator.
 
Also something to consider is whether or not your system has GFPE protection. We see designs that only use 4 pole transfer switches when the system has GFPE protection.
 
infinity said:
Also something to consider is whether or not your system has GFPE protection. We see designs that only use 4 pole transfer switches when the system has GFPE protection.
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I agree. In my case, my system voltage is 208Y/120V, and so thus not require to have a GFP for service disconnect switches more than 1000 amps.
So, back to my question, using 3-pole ATS is fine?
 
For a single ATS, the decision is almost entirely a design decision. The problem you run across is when you have a non-separately derived generator system with multiple transfer switches you end up with multiple ground fault paths back to the source on the multiple ATS unswitched neutral conductors. These multiple paths will cause any GFPE in the system to not function properly.

My personal preference is to use separately derived generator systems so that my neutral-ground bond is located in the equipment that is powering the system, whether on utility or generator power. That way if you remove power to one source and power the loads from the other source, you can work on the dead source without worrying about current flowing through it in a fault condition or you can remove the neutral-ground bond in the equipment without worrying about creating an ungrounded system.
 
d0nut said:
For a single ATS, the decision is almost entirely a design decision. The problem you run across is when you have a non-separately derived generator system with multiple transfer switches you end up with multiple ground fault paths back to the source on the multiple ATS unswitched neutral conductors. These multiple paths will cause any GFPE in the system to not function properly.

My personal preference is to use separately derived generator systems so that my neutral-ground bond is located in the equipment that is powering the system, whether on utility or generator power. That way if you remove power to one source and power the loads from the other source, you can work on the dead source without worrying about current flowing through it in a fault condition or you can remove the neutral-ground bond in the equipment without worrying about creating an ungrounded system.
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Thank you for responding. In my case, the 3-pole ATS's are already on site. The electrical contractor just want to make sure that his ATS's will be fine since the original design intent is not to trip the generator breakers even if there's a ground fault on the system.
 
PP26 said:
Thank you for responding. In my case, the 3-pole ATS's are already on site. The electrical contractor just want to make sure that his ATS's will be fine since the original design intent is not to trip the generator breakers even if there's a ground fault on the system.
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Is that permissible to do intentionally? Would a fault-detection setup be adequate?
 
If your scenario generator were 120/208 to match the utility, and also with neutral connected loads at the dual source bus, probably yes you might have a 4 pole ATS.

In the scenario you state with the generator is 480 and the utility is 120 ./ 208 Y, there is a scenario to avoid the 4 pole switch (which could be better or much better. The generator would have no neutral connected load, be solidly grounded from the utility an shared building GEC system, and so the generator would be non separately derived.

The ATS could be 3 pole, solidly connected grounding, (not a neutral, a grounding or GEC conductor). The load on the generator after the ATS is the step down transformer primary winding ( I am assuming here but not stated in the OP), a delta connected no neutral load, to get to the 120/ 208 Y at the secondary to match the utility supply. Again solidly grounded to the building neutral GEC as separately derived there. The generator is non separately derived and solidly grounded to the building system, the transformer secondary is separately derived and solidly grounded to the building system, 3 pole switch only with no neutral connected loads on the generator.

If you have any neutral connected loads on the generator and a neutral at the dual source bus, likely you could end up with neutral and ground in parallel and objectionable neutral current flowing on the grounding. So 4 pole switch would be advised.

If the generator load *is* 3 phase 3 wire, with ground, but no neutral or neutral connected load (the transformer primary), yes 3 pole switch is possible and likely desirable.
 
The generator is solidly grounded non separately derived in that scenario and would, must, trip the breaker for the generator supplying power into a fault.
 
LarryFine said:
Is that permissible to do intentionally? Would a fault-detection setup be adequate?
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Yes. The incoming utility voltage is 208Y/120V, which does not require to have a GFP for service disconnect rated 1000 amperes or more as per 230.95 of NEC (2008).
 
__dan said:
If your scenario generator were 120/208 to match the utility, and also with neutral connected loads at the dual source bus, probably yes you might have a 4 pole ATS.

In the scenario you state with the generator is 480 and the utility is 120 ./ 208 Y, there is a scenario to avoid the 4 pole switch (which could be better or much better. The generator would have no neutral connected load, be solidly grounded from the utility an shared building GEC system, and so the generator would be non separately derived.

The ATS could be 3 pole, solidly connected grounding, (not a neutral, a grounding or GEC conductor). The load on the generator after the ATS is the step down transformer primary winding ( I am assuming here but not stated in the OP), a delta connected no neutral load, to get to the 120/ 208 Y at the secondary to match the utility supply. Again solidly grounded to the building neutral GEC as separately derived there. The generator is non separately derived and solidly grounded to the building system, the transformer secondary is separately derived and solidly grounded to the building system, 3 pole switch only with no neutral connected loads on the generator.

If you have any neutral connected loads on the generator and a neutral at the dual source bus, likely you could end up with neutral and ground in parallel and objectionable neutral current flowing on the grounding. So 4 pole switch would be advised.

If the generator load *is* 3 phase 3 wire, with ground, but no neutral or neutral connected load (the transformer primary), yes 3 pole switch is possible and likely desirable.
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Thank you Dan for your response. I really appreciate it.
 
Do you have a one-line? It's hard to give an answer with so many unknowns, Where is the ATS relative to the transformer? Is the high voltage side (480V) of the transformer delta? Why are you noting that there are other transformers in the building? Where are these transformers connected to relative to the ATS? Why is your generator a different voltage than your incoming utility service? Is there more than one ATS? You don't need ground fault protection because your L-G voltage is below 150V so why is there ground fault protection/indication? Is it because the generator breaker has ground fault protection/indication? I think a lot of these may be answered (and in fewer words) if you had a one-line.
 
Sorry there is a problem with the above scenario (working on something else). The generator output is likely Y and can be grounded up to the first disconnect which is likely at the generator So the generator would be separately derived with a system bonding jumper, solid equipment ground, and GEC.

You can still carry 3 wire with ground from the generator as long as the loads, all the loads, are 3 phase 3 wire with ground.
 
Question is if the existing ATS is rated 480 or or only 208 since the rest of the existing is apparently 208. If the ATS 208 only and you have neutral connected loads on it, which is likely, you could be looking at a 4 pole ATS.

Alternately if the ATS is 480 rated, there is a scenario to step up the utility 208 to 480 and run that through the ATS, 480 V 3 pole 3 wire, solidly grounded no neutral, then step back down after the ATS to 208 Y for the EM loads.

If the generator were very close to the gear and the EM loads were long distance away, there could be an advantage to bringing 480 close to the load with remote step down transformers near the loads, one scenario.

If the existing ATS is not 480 rated, that's a different scenario. KVA makes a difference. Small kVA I might want to buy transformers and carry 480 to the loads. large kVA I would try to avoid buying extra transformers and just step down once, probably.

Many factors to consider, how much kVA and how far the distances.
 
Sorry again. Head is in a different place, classes all day.

The generator is separately derived, system bonding jumper, ground and GEC at the generator.

Run the 480 close to the load which is the existing ATS, 3 ph 3 wire plus ground should be possible. Step down transformer between the ATS and generator.

At the step down transformer, the Y secondary could be non separately derived with the the 3 pole ATS and a solidly connected insulated neutral. You would be depending on the main or system bonding jumper of the 208 V supply to ground the transformer secondary. I would verify that there is a main bonding jumper I am depending on, and not a second neutral to ground connection elsewhere.

The truly clean way to do it is to not have any neutral connected loads at the dual source bus, which is likely not applicable to your existing. And I know in the past I have looked at similar situations where 3 pole ATS was not possible, but I forget the details. Head is still elsewhere but I guess this would be the third draft so far..

If it were me I would try to make the 3 pole ATS work if possible and prefer a 3 pole non switched neutral if I could make it work, meaning meets code and solidly grounded supply, so any fault on the supply will pass sufficient fault current to trip quickly.

Always better to make the mistakes at the design stage where they can be fixed cheaply. I would look it over to see if that scenario fails somewhere.
 
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