breaker size for motor/conductor size

[Brian Kemp]

Ugly book tables list 200A breaker for 60Hp motor at 480V 3 phase. However, it lists #3 wire and according to my understanding, you can never run undersized wire from a circuit breaker to a load. What am I missing?

 

[augie47]

You are missing some quality time is NEC Art 430
Briefly, do to the inrush current on motors it is necessary to use protective devices greater than one would normally use to protect the wire.
The Code considers the motor overload protection adequate to prevent the supply conductors from overload. The fuse or breaker is only serving to protect the system from ground-fault short circuit and a breaker larger than the conductor ampacity will accomplish that.

The Code allows this thru 430 Part II and 240.4(D)& (G)

Spend some time looking at Part II and 430 Part IV

 

[Brian Kemp]

breaker size

breaker size

Thanks, i will do that.

 

[kwired]

Ugly book tables list 200A breaker for 60Hp motor at 480V 3 phase. However, it lists #3 wire and according to my understanding, you can never run undersized wire from a circuit breaker to a load. What am I missing?

Do what Gus said and do some reading in art 430. If you sized the breaker to the 3AWG ampacity it very well may not be able to hold during the surge of current that comes when starting a motor, though different breaker manufacturers will suggest different sized breakers for the same size motor. In fact my Square D motor slide calculator suggests a 110 amp breaker for the motor in your case, but yet NEC would allow up to 200 - which is apparently what Ugly's went off of. And there are exceptions in NEC that would allow further increase if this doesn't allow the motor to start.