Overcurrent protection - Downstream of ATS? 2

[jimioy]

I have a facility that has two sources of power available and want to put a transfer switch in. Each of my two sources are padmount transformers. Do I need overcurrent protection on each source - between the transformer and ATS?

 

[davidbeach]

Yes.

 

[Noway2]

It depends on the nature of your application, the design of the ATS and what the ATS units are sourcing. Some equipment, such as fire protectione equipment is not allowed to have up stream protection devices per NFPA code and must be classified as service entrance equipment. While the ATS used in such equipment does have a molded case switch that does provide some over current protection, it is on the order of 20 times the frame rating of the switch, which doesn't really amount of over current protection for the load.

 

[jimioy]

Thanks Guys...I guess I need to give more info to get a more detailed response. Both transformers are on our 12,470V power grid. On the loss of utility power to our Air Nat'l Guard base (which feeds these transformers) we have two 1.5 MW generators which will then feed one of the transformers (our critical branch). This is the same transformer that feeds power to the building under normal conditions. The transfer switch is only going to be utilized (the other transformer) in the event that the critical branch circuit fails (been cut or whatever). I currently have a 600A 3phase breaker (50K AIC) between the critical branch transformer and the ATS. Do I need to do the same on the other circuit between the transformer and ATS?

 

[davidbeach]

Assuming the NEC as governing code, the transformers are outdoors, and the building is indoors, and the transfer switch is not service rated; you need a breaker between the transformer and the ATS. A better application, next time, would be a breaker type ATS with a service entrance label, but given what it sounds like you have, you need another breaker.

 

[dpc]

I agree with David. We have dealt with exactly this same situation and overcurrent protection was definitely required. It could be a fused switch instead of a breaker.

If this is an ASCO-type ATS, you might also want to check the short-circuit withstand rating of the ATS.