MCC Feeders

[jt27]

I've been an electrical engineer for about 12 years now, but until recently worked in a sector where the electric code wasn't relevant to what I was doing, so I've just gotten knee deep into NFPA 70 for the first time. I'm seeing some things with our MCC feeders all over the plant I'm at that don't seem to jive with what I'm reading in the book and wondered if someone can point me to some justification that I'm not seeing. As an example...

I've got a feeder made of 2 500MCM conductors (THHN) per phase (6 total) being all run in a 4 inch conduit being fed from an 800A breaker to an MCC. Table 310.15(C)(1) tells me I need to derate the ampacity to 80% if they all run in the same conduit. Per table 310.16, this would give me an ampacity of 688A per phase.

240.4(B) tells me I can use up to the next higher standard breaker size above 688A, which would be 700A.

Is there any way to justify using an 800A breaker in this scenario rather than a 700A breaker?

 

[david luchini]

See Article 430 Part II, and Part V.

 

[augie47]

See Article 430 Part II, and Part V.

(We are assuming motors are involved thus the Art 430 reference. If there loads do not include motors obviously those references are null )

 

[jt27]

There it is, thank you very much.

 

[jt27]

When I read in 430.21 it says

Articles 250, 300, and 310 shall not apply to conductors that form an integral part of equipment, such as motors, motor controllers, motor control centers, or other factory-assembled control equipment.

I take this to apply to the conductors within the MCC, but not the feeder conductors feeding the MCC. Is that correct?

 

[Dennis Alwon]

I would agree with that..

 

[petersonra]

I think it should have been a 700 A CB.

 

[don_resqcapt19]

It is my opinion that the feeder conductors to the MCC are sized per 430.24 and protected per 430.53. However you also have to look to 430.94 for the protection of the MCC itself.

Correction 430.53 should read 430.63.

 

[petersonra]

It is my opinion that the feeder conductors to the MCC are sized per 430.24 and protected per 430.53. However you also have to look to 430.94 for the protection of the MCC itself.

430.53 refers to multiple motors on the same BC. I don't see how that applies to an MCC.

 

[david luchini]

430.53 refers to multiple motors on the same BC. I don't see how that applies to an MCC.

430.62 or 430.63 would apply.

 

[don_resqcapt19]

430.53 refers to multiple motors on the same BC. I don't see how that applies to an MCC.

Typo...430.63