Multiple Motor Feeder Protection 1

[timm333]

I need to size the circuit breaker for a 208V feeder that supplies power to three 5HP motors in parallel. Vendor will take care of the overload and short circuit protection of individual motors. I only have to size the thermal magnetic circuit breaker for the main feeder that supplies power to all three motors. Thanks for your help…

 

[dpc]

See NEC 430.24

David Castor

http://www.cvoes.com

 

[timm333]

NEC 430.24 is for determining the size of the conductor that feeds multiple motors.

I want to size the breaker that will protect the conductor (the conductor that feeds multiple motors)

 

[healyx]

I'm not familiar with NEC regulations, but generally speaking if you are going to power a single motor Direct On Line (DOL) (e.g. not starter combo etc), you should allow the breaker to be rated 125% of full load current and 600% of FLC for 10 seconds, plus no trip at 1000% of FLC. You usually size the breaker to be about 2.5x FLC to satisfy these conditions (as a rule of thumb). Checking the curves and consulting breaker manufacturer is best way to be sure.

The worst case in the situation you describe would be all three motors starting simultaneously, this is probably unlikely, but the most conservative solution would be if you assume this to be the case.

So for 15Hp total at 208V and assuming 0.85 pf, efficiency 0.9, I get a total FLC for the combined motor load of 40.6A ((15*1000*0.746)/(1.732*208*0.85*0.9)). So 2.5*40.6 = 101.5A. So a breaker of about 100A should be OK given it is unlikely all 3 will start together. In IEC land, you could use a larger D type MCB to provide some comfort. At 100A, you can probably use a MCCB which might come with some adjustment. BTW, I have assumed everything is 3 phase.

You will need to make sure the guys providing the protection for the motors can coordinate with this feeder breaker properly.

Hope this helps.