SPD breaker size and selection for emergency switchboard

[David Christian]

Hi Guys,

I recently design one data center project. Therefore, I need to choose SPD to protect lightning damage and overvoltage damage.

As NEC code 700.8 A list SPD shall be installed in or on all emergency systems switchboards and panelboards, therefore I think I need to choose SPD for my emergency panel.

I notice so many old drawings show 30A/3P SPD in the emergency panel. I wonder know why they choose 30A/3P SPD. I check Schneider and Eaton SPD cut-sheet, there is no 30 Amps/3P SPD.

I really appreciate any suggestion.

Thank you and have a great weekend!

David

 

[Jraef]

Th SPD listing requires that it have OCP and a disconnecting device ahead of it. That OCP and disconnect must be rated for the Available Fault Current, but the SPD mfr has no idea where it might be installed. So they just list it with Class J fuses, making it good for 200kA, covering 99.9999% of installations. 30A is the smallest fused disconnect switch you can buy, but you don't want the fuses clearing needlessly and taking your SPD off line, so they go with 30A, the max that will fit in the cheapest smallest disconnect. Once it is UL listed that way, you must use that.

 

[David Christian]

Th SPD listing requires that it have OCP and a disconnecting device ahead of it. That OCP and disconnect must be rated for the Available Fault Current, but the SPD mfr has no idea where it might be installed. So they just list it with Class J fuses, making it good for 200kA, covering 99.9999% of installations. 30A is the smallest fused disconnect switch you can buy, but you don't want the fuses clearing needlessly and taking your SPD off line, so they go with 30A, the max that will fit in the cheapest smallest disconnect. Once it is UL listed that way, you must use that.

I really appreciate your detail explanation. Have a great weekend!