Main Circuit Breaker Selection 1

[acfm]

I have a customer asking for assistance with selecting the correct main circuit breaker for a 10HP 460V triplex vacuum system. Depending on the demand, only one unit could be in operation or all three could be operating at the same time. Motor FLA is 14 each (w/0 1.15 SF).

Cutler Hammer Motor Circuit Selector says the following:

1. Instantaneous trip circuit breaker GMPC/HMCP: 30 AMPS and
2. Circuit breaker trip rating: 35 AMPS

A.) Can someone provide a briefly explain the difference between #'s 1 and 2? and
B.) Should the breaker be sized based on all three units in operation (10HP x 3 = 30HP)?

Just trying to help a guy out!

Thanks!

 

[LionelHutz]

If the electrics for this vacuum system are in an MCC then jraef is likely right. It could still be argued that the feeder to the MCC is only supplying motor loads and therefore falls under the rules for motors but you may lose this argument with an inspector.

If they controls are in one big box then the others are right. The code does allow the breaker to be sized larger than the current capability of the wire.

Be prepared for the inspector to want more appropriate protection for each individual motor too. Code does give the rules for multiple motors on one feeder but I've still run into cases where the inspector wanted protection for each motor.

If I'm building a panel with the breaker I have to follow UL508A. UL508A is very restrictive on multiple motors with one overcurrent protection means. It's basically not allowed except for some special cases. I put the main breaker with external handle for the panel and then a fuse block or smaller breaker for each individual motor.

 

[rbulsara]

OCPD for Motor Control Centers are covered by NEC 430.94 and its has to be in accordance with article 240.

This is different than a multi-motor load (a combinaiton of motors, but one assembly). Do not confuse the two.

Look up NEC 430.7, 430.24, 430.25 and 430.62. It may be invloved but not still clear enough.

I have enough of this thread now. I beleive.

 

[LionelHutz]

My point is that you really do not know how the electrical controls for this vacuum are configured. Everyone is just assuming that since it's a "triplex vacuum system" that all the motor controls are in one box.

 

[kaukau]

HERE'S A QUOTE FROM THE MASTER............
Section 240-3 contains twelve rules that modify the general requirement and permit the conductors not to be protected in accordance with their ampacities, they include:

1. Power Loss Hazard
2. Devices Rated 800 Amperes or Less
3. Tap Conductors
4. Motor-Operated Appliance Circuit Conductors
5. Motor and Motor-Control Circuit Conductors
6. Phase Converter Supply Conductors
7. Air-Conditioning and Refrigeration Equipment Circuit Conductors
8. Transformer Secondary Conductors
9. Capacitor Circuit Conductors
10. Electric Welder Circuit Conductors
11. Remote-Control, Signaling, and Power-Limited Circuit Conductors
12. Fire Alarm System Circuit Conductors

 

[kaukau]

rbulsara (Electrical)
YOUR RIGHT ON THE AMPERAGE I HIT THE WRONG (60 DEGREE COL.) WIRE SHOULD HAVE BEEN A #6 TW.
MY BAD.
THANKS FOR POINTING IT OUT.
ALOHA