Sizing
Sizing
(OP)
Hi guys. I have a general questions regarding the sizing of circuit breakers and disconnect switches when given the FLA of a motor. I have read and heard a few different ways to determine what sized breaker and what size disconnect switch to use for equipment based on the motor FLA, so I thought I'd ask here. Please advise. Thank you!






RE: Sizing
In NEC-land, thermal-magnetic circuit breakers are limited to 175% of motor full-load current as indicated in the applicable NEC tables for most motors.
Switches are rated based on the locked-rotor current of the motor per NEC 430.
Anyplace else (ie not NEC), no idea.
One common rule of thumb is to size CB to 150% of motor full-load current. That seems to work for most applications except perhaps high-torque apps, in which case the mfr often gives the max. CB rating - in that case, that's what I use.
RE: Sizing
(the typical breaker used) you may use a rating of up to 250% of the FLA of the motor. You may use a smaller rating. The disconnect means is given in 430.101. 430.110 requires a rating of at least 115% of the FLA of the motor.
RE: Sizing
RE: Sizing
In the US (at present), thermal magnetic circuit breakers is having a minimum ampere trip rating 15A while MCPs are as low as 3A. Check manufacturer recommended published MCB and MCP ratings for all motor HP ratings. Manufacturers like cutler-hammer or squareD have these published ratings as per US standards and complying with the related provisions of NEC.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: Sizing
Don
RE: Sizing
Problem may exist if you don't have FLC but only electric power of the motor (not every motor is a typical one).
On the other hand, not every manufacturer will give switch ratings when motors are concerned. Be aware of several issues:
- is that switch for isolation only (motor control through contactor) or will it be used for motor control
- if it is for motor control:
- transients during switching-on/off should be addressed - dependent on the type of the load
- switches for motors should have appropriate utilization category (this is connected to 1. the number of commutations and 2. whether the switch is for motor control or isolation; 1. concerning number of commutations - A suffix attached to the utilization category (frequent operations) and B suffix (infrequent operations) 2. if switch controls motor, it should be of AC23 utilization category, otherwise AC3)