3 pole versus 4 pole automatic transfer switches

[Jim in Ashland]

I am including a 300kW generator to handle a fraction of the loads for a new building. Approximately 2/3 of that generator will be on an ATS for non-emergency loads. The rest will be on an ATS dedicated for emergency lighting. The service is 1500kVA 480V grounded wye. I understand that places me in the position for providing ground fault protection at the service entrance - and I have done so. I seem to be chasing my tail to find the answer to whether or not the ATS' should be a 3 pole or a 4 pole devices. It is obvious that if the generator replaced the entire service then 4 pole devices are required. But in this instance we are a layer removed from the Service Entrance equipment and thus the breakers requiring ground fault protection it seems a 3 pole ATS is suitable. It then follows that the generator is NOT a separately derived system, and therefor can be grounded accordingly.

 

[charlie b]

I think your description is correct. In your situation, I would use 4-pole ATS's for both the emergency and the optional standby systems. I generally prefer a 4-pole ATS anyway. But when you have two or more of them, it becomes trickier to get the grounding right. One thing you must do, however, it make both ATS's the same. If you use one 3-pole and one 4-pole, you will not be able to get the grounding right, for the generator would be an SDS in one case and not an SDS in the other.

Welcome to the forum.

 

[jxofaltrds]

This may help. Check to see if the generator comes bonded.