Sizing 2

[Reesh14]

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!

 

[tommom]

What are the exact questions?

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.

 

[wareagle]

For a single motor 430.22A of the NEC states that the conductor shall have a rating not less that 125% of the FLA listed in 430.6.A.1 which is the tables beginning with 430.147. The overcurrent device is given in 430.52 which refers to table 430.52. If you use an inverse time breaker
(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.

 

[Reesh14]

Thanks guys. I appreciate your responses.

 

[nightfox1925]

What type of circuit breaker you are intending to use? Thermal magnetic or a Magnetic Only (Motor Circuit Protector)? Both types are having different maximum allowed ratings per NEC.

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

 

[resqcapt19]

Thermal magnetic or a Magnetic Only (Motor Circuit Protector)?

Note that you can only use the magnetic only as part of a listed combination starter per 430.52(C)(3).
Don

 

[sitl]

You should always try to use manufacturer recommended ratings. For example, if full load motor current is 1.25A, then you should use motor circuit breaker that has a setting of his ''thermal'' part (1-1.6)A (for example). Manufacturers usually also give corresponding contactor rating according to coordination type (matched to a motor circuit breaker), so you should have little dilemma.
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)