MCC Feed

[johnmccoll]

I am working on an industrial MCC. 480V, 600A, parallel 500kcmil feeders to a 600 amp breaker. The load side of the breaker feeds a 600amp transfer switch and is fed with parallel 4/0 copper. My recommendation was to upsize these 4/0 conductors to parallel 350kcmil and have met with some resistance from the engineers. The 4/0 feeds are about 10' long, but a don't consider this a tap. They are also 120 degree rated, however the terminations on both ends are rated at 75 degrees. Am I missing something someplace which allows for this or is it how I believe and the engineers are suggesting I create a intentionally dangerous situation? I also should add that the feeder conductors show considerable heat damage and the breaker failed to trip on test which suggests to me the potential that this service has exceeded 600amps may exist. Thanks for the help!

 

[infinity]

Why are they resisting? Seems like you have 450 amps worth of conductor (225A * 2) on a 600 amp OCPD.

 

[johnmccoll]

They claim that when the MCC was installed (1983), it was factory wired (very doubtful), and this somehow gives us an exemption now.

 

[charlie b]

It seems to me that you are talking about a wire that originates within the MCC (load side of a breaker), then leaves the MCC and heads over to the transfer switch. That is not the manufacturer's wiring. The MCC was not listed with that feeder already installed. It is a field installation, and it must meet the rules of the NEC. So I agree with you (and Rob) that the 4/0 conductors are too small. Depending on the calculated load (downstream of the transfer switch), you might be able to use 250 or 300 MCM conductors. But I would tend to want to go with the 350s anyway.

 

[petersonra]

I don't see any grandfather clause applying here.

Maybe there are 450A fuses in the transfer switch.

 

[johnmccoll]

That's just what I figured. Wanted to make sure I wasn't missing something. Thanks guys!

 

[charlie b]

Maybe there are 450A fuses in the transfer switch.

I don't think that would help. I agree with the OP that this is not a tap rule situation, so protecting the conductors at their ampacity has to happen at the start of the run.

 

[augie47]

Just to clear my fuzzy brain. Are the conductors that come from the load side of the breaker to the TS ones that were factory installed as part of the MCC or otherwise ?

 

[petersonra]

I want to know why this cannot be considered a tap if there are fuses in the transfer switch.

 

[johnmccoll]

The conductors to the TS where thought by the engineers to be factory installed, however I seriously doubt that they were. As for fusing on the TS, there is none. The feed goes from the 600amp OCPD to the TS with going through any other protection. The TS is rated and will be switching the 600amp main feeder.

 

[augie47]

The conductors to the TS where thought by the engineers to be factory installed, however I seriously doubt that they were. As for fusing on the TS, there is none. The feed goes from the 600amp OCPD to the TS with going through any other protection. The TS is rated and will be switching the 600amp main feeder.

This is, IMO, the "key". Are are these components in one or adjacent enclosures ?
You mentioned to conductiors were 120 degree rated. Do you happen to know of any identifying markings on them ?
To me, it seems possible that this may be a similar situation as you have with such items as motor starters where the factory installed interior wiring is often a gauge considerably less than the NEC requirement as the wirning is a type that will withstand higher temps and is engineered and approved as a part of the factory assembly.

 

[Jraef]

This is, IMO, the "key". Are are these components in one or adjacent enclosures ?
You mentioned to conductiors were 120 degree rated. Do you happen to know of any identifying markings on them ?
To me, it seems possible that this may be a similar situation as you have with such items as motor starters where the factory installed interior wiring is often a gauge considerably less than the NEC requirement as the wirning is a type that will withstand higher temps and is engineered and approved as a part of the factory assembly.

I agree with your "key" (poet and didn't know it).

It is entirely possible that the ATS is mounted inside of the MCC, down stream of the Main CB. MCC mfrs who make both the MCCs and the ATS will sometimes have the ATS listed under UL845 inside of their MCCs, Cutler Hammer being a prime example. If that's the case, the ATS wiring may indeed have been done by the factory and is part of their UL845 listing for that unit. Generally when that is the case, the connection to the MCC main bus is a direct bus connection, but if they listed it with cable, it's valid. The higher temperature rating of the cable insulation may have allowed for the higher ampacity, but that would also have likely required lugs rated at 90C, not 75C, so that may be the only mistake.

Still a lot of maybes in there, and I would imagine that if it were factory installed, they would have used crimp lugs in which case they would be fine for 90C.

 

[templdl]

Was the MCC as described received as a UL listed product and not modified in the field. If so the NEC does not have a reason to supperceed a UL listed assembly as the NEC recognizes UL as a recognized testing agency in art 110.
The MCC has been tested and approved.
Now, where do you go from there? Do you have a reason to believe that the MCC has been modified? If so, since the MCC is no longer as shipped by the manufacturer and they have no control over any modifications then one must consider that the modifications would have to be compiant with the NEC requirements in my opinion.

 

[petersonra]

I also should add that the feeder conductors show considerable heat damage and the breaker failed to trip on test which suggests to me the potential that this service has exceeded 600amps may exist. Thanks for the help!

I am inclined to think that any typical paralleled 4/0 THHN could handle 600A without being damaged by heat in this type of install. The OP said in another post that the wire is rated at 120C. If that is the case, to get any significant insulation damage it would have had to have experienced temperatures significantly above 120C for extended periods of time. It seems to me that there is something else potentially going on here that needs a close look beyond the wire sizing.

The OP also indicated the local breaker did not function. If there was an extended overcurrent that led to the conductor damage, possibly related to the failed CB, why didn't the upstream OCPD trip? If there was enough current that lasted long enough to damage the wire insulation, something should have tripped.