Motor Branch Circuit Protection

[chicomarin]

I have a 60hp 480 V 3 phase polyphase motor. I calculated the size of the breaker for it and I got FLC 77 * 2.5 = 192.5 or a 200 amp inverse time circuit breaker. But when I use the Square-D slide chart for the same motor it tells me that I need a 110 amp breaker. Which one is the correct breaker to use?

 

[augie47]

The "correct" breaker size might be the result of engineering study taking into account the starting torque needed, LRA code, etc.
430.52 provides for the MAXIMUM size breaker. Obviously, in most applications a 110 amp will work.

 

[bob]

The chart in the NEC give you the max size breaker. You may use a smaller size. However, if the motor starts under load you may trip the the 110 amp.