NEC Code Question Feeder - Circuit Breaker - Motor Control Center

[chriskei]

I have a Motor Control Center with the following

503.4 Full Load Amps

I also need to add a lighting and airconditioning circuit that requires 55 Amps.
This will be feed to a separate circuit breaker box but the feed to this will be after the main circuit breaker.

Total Load - 558.3 Amps.

I am using individual Square D Magnetic Trip Manual starters that are specified for each motor in a range of 75 HP down to 6 HP. What is the rating neeed for the Feeder Protection? I would like to use a circuit breaker for the protection device. Is a 600 AMP circuit breaker large enough? I need this to meet NEC code requirements.

 

[coulter]

I have a Motor Control Center with the following

503.4 Full Load Amps

I also need to add a lighting and airconditioning circuit that requires 55 Amps.
This will be feed to a separate circuit breaker box but the feed to this will be after the main circuit breaker.

Total Load - 558.3 Amps.

... Is a 600 AMP circuit breaker large enough? I need this to meet NEC code requirements.

I'd have a few questions:
1. This MCC, is it existing? If so, what is the rating, and what is the feeder CB going to it?

2. The 503FLA, is that an NEC calculation or measured?

carl

 

[chriskei]

Carl,

This is a new installation and I am building a new MCC enclosure. The FLA is calculated from the motor list of the motors that are being supplied for the project. I believe the numbers are pretty close to the NEC HP numbers. The feed will be from the outdoor weatherhead into the new feeder CB located inside the MCC I am building. After the CB in the enclosure I am going to branch out of that enclosure into a spearate enclosure for the light and air conditioning requirements of 58 amps. I have to supply the feeder CB and all of the motor protection devices and contactors that will mounted in an enclosure inside the building.

Thanks,

Chris

<<I'd have a few questions:
1. This MCC, is it existing? If so, what is the rating, and what is the feeder CB going to it?

2. The 503FLA, is that an NEC calculation or measured?

carl>>

 

[coulter]

This appears to be a pretty straight forward industrial process design. Not something I could do from this side of your keyboard

... The FLA is calculated from the motor list of the motors that are being supplied for the project. I believe the numbers are pretty close to the NEC HP numbers. ...

You will need to do an NEC load calc. Don't forget to toss out the coincident loads.

... The feed will be from the outdoor weatherhead into the new feeder CB located inside the MCC I am building. ...

I'm a little confused here. Is this your service entrance disconnect? If so, it isn't a "feeder" and the equipment has to be rated for "service"..

a 600 AMP circuit breaker large enough?...

I doubt it. Molded case CB are generally only good for 80% (480A) continuous. I really don't have enough information to tell you. But the real question is: Is a 600A service enough?

Get your one-line. Start the load calc from the outer ends, work back toward the service. Generally industrial loads are considered continuous. I've never designed any industrial installation close or marginal.

The NEC is not a design guide. It is a list of minimums that may keep the system from burning up. These minimums are not necessarily efficient nor reliable. Sizing the conductors and OCP has alot to do with the intended use.

carl

 

[chriskei]

I went with an 800A CB. Thanks for the help.