Circuit Breaker Sizing with Motors

[taylorh]

Hello all,

We recently had to upgrade all of our products to run with Design B energy-efficient motors. After a few weeks we started getting customer complaints of nuisance tripping. After investigating we discovered the new motors have much higher inrush ratings.

For example: Motor FLA was 13.1, with inrush spikes up to 194 A. Our thermal magnetic breaker was rated 10...16 A FLA with instantaneous trip of 208 A (+/-20%) As a quick fix we simply installed the next size up breaker, 13...20 A with instantaneous of 260 A. This seems to have solved the issue.

However, we are concerned about NEC 430.52. In the table it states for Design B energy-efficient motors, breakers should not exceed 1100% of FLA for instantaneous trip. 430.52 (C) (3) Exception 1 states that if breaker settings are not sufficient for Design B motor starting current, you shall be permitted to use no more than 1700% of FLA instead. So, it seems the max instantaneous trip rating we can use for this motor is (13.1x17) 222.7 A.

Since our max inrush is 194 A, and the next highest instantaneous rating for our breaker model is 260 A.. would we fall into this exception? 430.52 (C) (1): "Exception No. 1: Where the values for branch-circuit short-circuit and ground-fault protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal protective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere rating shall be permitted."

Meaning we could use the 260 A breaker and still be within code?

 

[Jraef]

Are you using MCPs (Instantaneous Trip-Only) or Thermal-Mag breakers?

 

[taylorh]

We are using thermal-magnetic breakers.

Our electrical engineer responded to some of my questions. It turns out the FLA ratings you multiply by the chart is not your actual motor FLA, but a value found from table 430.250. It states a 5 hp motor at 230 volts (voltage customer was using) has a max FLA rating of 15.2. So 15.2(x17) = 258.4 A

This rating is still just shy of the 260 A breaker we are wanting to use. Since our motor is actually 5.5 hp I wonder if it will be acceptable?

 

[Jraef]

We are using thermal-magnetic breakers.

Our electrical engineer responded to some of my questions. It turns out the FLA ratings you multiply by the chart is not your actual motor FLA, but a value found from table 430.250. It states a 5 hp motor at 230 volts (voltage customer was using) has a max FLA rating of 15.2. So 15.2(x17) = 258.4 A

This rating is still just shy of the 260 A breaker we are wanting to use. Since our motor is actually 5.5 hp I wonder if it will be acceptable?

The NEC table is purposefully conservative, as born out by the fact that the chart says 15.2A and your motor, despite having a non-standard HP rating, is still only 13.1A. You can use the chart values and the correct rules, no problem.

260A breaker? I've never heard of such a thing, unless you are using maybe an IEC type "Motor Protective Circuit Breaker" (MPCB)? If so, that 260A level is just for the MAG trips, not the breaker itself. Typically that type of device doesn't have adjustable magnetic trips. But you should be fine with that, so long as the THERMAL part of it can be turned down to 13.1A, because that is the actual motor FLA.