How to protect a Power Supply

[tim89s]

Assuming the power supply doesn't run continuously, do you need to size the overcurrent device 125% of the nameplate rating. I can't find anything on power supplies in table 240.3. The power supply in question is 208v 3 phase 15kw on the input side. The nameplate states 54 amps. I need to know if I can use a 60 amp or 70 amp circuit.

Thank you
Tim

 

[petersonra]

the ocpd is there to protect the wiring, not the power supply.

the manufacturer probably has some guidance available for what rating of OCPD is appropriate.

personally, if there is no guidance from the manufacturer I would go with the 70 A option.

 

[tim89s]

I thought that the overcurrent protection was there for both the wire and equipment. At least that is how I interrupted 210.20

 

[petersonra]

I thought that the overcurrent protection was there for both the wire and equipment. At least that is how I interrupted 210.20

It can indeed do both. However, there is no requirement in many cases for it to protect the equipment.

For instance, with motors, the OCPD is only required to provide short circuit protection for the wiring. The motor and wiring is protected from overload by the overload relay (or whatever you are using to protect against over current). That is what you can have a CB that is set to 250% or more of the FLA of the motor.