MOTOR BREAKER & CABLE SIZE

[m sleem]

Please confirm:

Breaker size of motor is equal to 2.5*rating current...(assume 100A)

Feeder size should carry out 100A or less ?

disconnect switch will be sized to 100A OR more ?

 

[infinity]

Start with the motor FLA from the proper table in Article 430. Multiple that number by 125% to size the conductors. Multiply that number by 250%, or other value in T430.52 to size the OCPD. Multiply that number by 115% to size the disconnecting means.

 

[m sleem]

that's clear infinity>>>

but do you see the risk when we are sizing the disconnect by less

than the breaker ?

 

[infinity]

Not really. The OCPD is just there for short circuit and ground fault protection, the overload device should also be sized at 115% so there shouldn't be an issue with the disconnecting means.

 

[tom baker]

Or, the breaker protects the wire and the overloads protect the motor. If you use dual element time delay fuses they can be sized much closer to the motor FLA.