Primary Overcurrent Protection ATS

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xguard

Senior Member
Location
Baton Rouge, LA
Have an existing building I'm struggling to get all the pieces and parts in to add an ATS (Actually two ATS, one for normal loads and one for emergency). Making sure I'm not crazy.

I have a 480/277 3 phase generator. Stepping down to 208/120 3ph at the building. So there will be a transformer in between the ATS and the Generator. Is there a way I'm not seeing that I can use the transformer primary over-current protection to protect an ATS? It looks like I need to add a fuse or circuit breaker between each ATS and transformer. Thanks for any input.
 
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d0nut

Senior Member
Location
Omaha, NE
In the scenario you presented, you would need overcurrent protection between the transformer and the ATS. The transformer secondary conductors must land on an overcurrent device.

Is you normal service at 480V or 208V? If it is at 480V, I would design with a 480V ATS and put the transformer on the load side of the ATS.
 
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xguard

Senior Member
Location
Baton Rouge, LA
d0nut said:
In the scenario you presented, you would need overcurrent protection between the transformer and the ATS. The transformer secondary conductors must land on an overcurrent device.

Is you normal service at 480V or 208V? If it is at 480V, I would design with a 480V ATS and put the transformer on the load side of the ATS.
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Thanks! The normal service is 208 unfortunately.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Not sure why you are thinking of two ATS here. The ATS is what separates your utility source from your generator source. But if you only want to power SOME critical loads on the generator, then it becomes a factor of where you place the ATS in the circuit. We don’t know your intent yet so we can’t help much more on that.

But if we assume that your generator is going to power the entire service, then your ATS just goes on the 208V side of that transformer. Your generator will (should) have a 480V breaker on its output, so you would follow the standard rules on the secondary protection of that transformer based on the primary side protection. (And what Texie says about the requirements in the ATS listing)
 
X

xguard

Senior Member
Location
Baton Rouge, LA
Jraef said:
Not sure why you are thinking of two ATS here. The ATS is what separates your utility source from your generator source. But if you only want to power SOME critical loads on the generator, then it becomes a factor of where you place the ATS in the circuit. We don’t know your intent yet so we can’t help much more on that.

But if we assume that your generator is going to power the entire service, then your ATS just goes on the 208V side of that transformer. Your generator will (should) have a 480V breaker on its output, so you would follow the standard rules on the secondary protection of that transformer based on the primary side protection. (And what Texie says about the requirements in the ATS listing)
Click to expand...
One ATS will supply emergency lighting and the other ATS the remainder of the loads for the building.
 
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