30A breakers for motor controllers?

[VEI Dude]

Can anyone verify that there's no rule against running a victor-driven mini-cim on a 30A breaker circuit? The mini-cim is ~19A.

Thanks.

[Gregor]

The table in R46 does indeed allow this, but any reason as to why you're not running it off a 40A breaker? I assume you're not out of 40A slots on the PDB.

[VEI Dude]

Yep... we'll most likely be out of 40A slots.
Thanks.

[Al Skierkiewicz]

Breakers are sized for the wire they feed. If you choose a smaller gauge wire then you may use a smaller breaker.

[Andy Brockway]

Breakers are sized for the wire they feed. If you choose a smaller gauge wire then you may use a smaller breaker.

I thought we could use the larger gauge wire with a smaller breaker. We have been using 10 gauge for the 40 amp. This is left over from when 10 gauge was required.

The rules (R47) specifiy a minimum wire size per breaker size.

[Al Skierkiewicz]

Andy,
Yes you can, the wire size minimum is listed by the breaker feeding that branch. i.e. a 40 amp breaker can be used with #12 wire but I recommend #10 for CIM motors. The wire size is chosen for safety and voltage drop from the NEC handbook for open frame DC wiring. If #14 wire was used with the 40 amp breaker, it is possible to heat the wire to the point at which the insulation fails. #12 wire on the other hand may get warm but not to the point at which a failure would occur. A dead short of course, would be protected by the breaker. At 100 amps, the #12 drops ~0.2 volts per foot. In a typical robot wiring job I would expect to lose almost a volt at the motor input during starting and heavy loading with a fully charged battery.

[hiddenhobbit]

Can we use CIMs from the 30A? Or any motor controller for that matter?

Thanks!

[Ether]

Breakers are sized for the wire they feed. If you choose a smaller gauge wire then you may use a smaller breaker.

Isn't it the other way around?

If you choose a smaller gauge wire then you must use a smaller breaker.

Or maybe it's the ambiguity of the phrase "smaller gauge wire"...