MCC Feeder Sizing

F

FaradayFF

Senior Member
Location
California
Hi All,
This question has probably been asked multiple times already, but what is the best industry practice when it comes to sizing feeder to an MCC? Would one size the feeder based on MCC bus, total connected load, or based on demand?
I'd think the conductors feeding the board would be sized based on the board bus ampacity to account for future added loads, even if this would result in higher cable size and investment upfront.
Thoughts?
Thank you,
EE
 
There isn't one good answer because there are lots of variables.
For example is every bucket in the MCC being used? If so then use that total load to determine the feeder size.
Is only 50% of the MCC being used with the rest for possible future equipment?
What's is the customers cost tolerance? Do they want to count every nickel and not spend a penny more.

IMO if there is any potential for future expansion where the MCC will have a number of additional loads then I would size the feeder to the MCC ampacity.
 
I would not treat an MCC different than any other panelboard.
Size the feeder to the reasonably expected load. Worry about the future when the future comes. Of course empty underground conduits could be justified.
 
For the feeder, you're correct that it needs to be at least the ampacity of the MCC bus (430.94). There was a time when the feeder could be undersized based on the demand factor of the motors fed from it (I don't know the year the NEC added this). If you're looking for an MCC size, 800A is very common.
 
HarrySlother said:
For the feeder, you're correct that it needs to be at least the ampacity of the MCC bus (430.94).
Click to expand...
Does it really say that? Shall not exceed sounds like it can be less than the bus rating.

430.94 Overcurrent Protection.
Motor control centers shall be provided with overcurrent protection in accordance with Parts I, II, and VIII of Article 240. The ampere rating or setting of the overcurrent protective device shall not exceed the rating of the common power bus. This protection shall be provided by (1) an overcurrent protective device located ahead of the motor control center or (2) a main overcurrent protective device located within the motor
control center.
Click to expand...
 
Do a load calculation on the MCC. The code has a minimum feeder ampacity and maximum OCPD requirements based on the calculated load.

Personally if it was me and it had a main breaker or fuse, I would size the feeder conductors and OCPD rating to the rating of the main most of the time. But that is strictly a design decision.

Keep in mind the conductor ampacity and OCPD rating are not the same thing.
 
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