Motor Feeder Protection Question

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biggladant

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Kansas
Trying to size a feeder overcurrent protection.
My local supply store sized my largest breaker for a 40 HP at 150 amps. Can I use this as my largest breaker or should I use table 430.52 which useing an Inverse Time breaker would make my 40HP a 300 amp Max. breaker?-- 230V 3phase

This is my Calculation for feeder protection:

1- 40HP 104 FLC X 250% 300AMP Brk
7- 10HP 28 FLC Each = 196FLC
1- 5 HP 15.2 FLC 15.2FLC
1- 2HP 6.8 FLC 6.8FLC
1- 1HP 4.2 4.2FLC
6- 3/4 HP 3.2FLC Each= 19.2FLC

=541.4 amps next size down 500 amp breaker for feeder.

Or can I use 150 as my largest breaker instead of 300 and end up with a 391.4 amps next size down 350 amp breaker?
Thanks
 
Re: Motor Feeder Protection Question

So if I am reading this right I will have to use 300 amp as my largest overcurrent device, useing Inverse Time breakers (250 %)?

What if I was useing Instantaneous trip, would I then use 800 amps as my largest breaker?

Thanks for your help
confused
 
Re: Motor Feeder Protection Question

Yes. What I believe is the intent of the section is to ensure that the feeder protection device is sized large enough to handle the largest possible startup load plus any running currents that may be on at the same time.
 
Re: Motor Feeder Protection Question

430.62(A) says the feeder OCPD device can be sized no larger than the largest branch OCPD plus the sum of the full load currents. It does not prevent using a smaller setting.

Now just a suggestion, add a "general purpose" receptacle branch circuit to your feeder load. Your feeder can now be sized under 430.62(B).
 
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