MCC Extension

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jtb

Senior Member
Location
Pennsylvania
I got an email about another questionable design.
Let me clarify that I am now the Electrical Designer responsible, as the original engineering firm has broken up.

The one-lines (verified in field) have an 800A disconnect with 800A fuses feeding dual 500kcmil/phase feeders to a 600A MCC.

The MCC was extended with 1/0 THHN by the electricians in the field. Unsure if this was
part of the engineering design.

Obviously several potential problems here, and like other instances during this same project, I have inherited some bad design issues and must correct them, as well as take the blame (I am sure all of us have had the pleasure).

My question is Code References! I have scanned through the NEC Handbook for applicable violations. I need more detail to verify compliance with Tap rules 240.21, but also need Code references for the other potential problems like inadequate protection of the MCC. I am searching as I write this, for those who think I am not trying.

I wouldn't ask if time was available to research on my own, and I apologize if you see me as taking advantage of your knowledge. If so, don't answer. My sincere gratitude for those who do.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: MCC Extension

See 430.94 for the MCC protection. I don't see how you can use the tap rules in 240.21(B) for the 1/0 unless the rating of the second MCC is less than 150 amps, or unless it has a 150 amp main.
Don
 

bennie

Esteemed Member
Re: MCC Extension

Number #1. Make an accurate load calculation. Work backward from there.

Remember the only way to protect a conductor from overload, is to not overload it.
 

ron

Senior Member
Re: MCC Extension

I agree with don about 430.94, but depending on the arrangement of the MCC extension in relation to the MCC, and the rating of the extension (expected loads), you might be able to use the 240.21(B) for the 1/0awg. Tell us more about it.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: MCC Extension

Ron,
I don't see how the tap rule can be used unless the second MCC has a rating equal to or less than the permitted ampacity of the 1/0. Look at 240.21(B)(1)(1)(a) and (b). Both (a) and (b) must be complied with to use the rule. A MCC with a bus rating, which would be the device rating, of 150 amps or less would be rare.
Don
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: MCC Extension

Ron,
Have you seen a MCC section with a bus rating of 150 amps or less? The lowest I've ever seen is 300 amps.
Don
 

ron

Senior Member
Re: MCC Extension

I have never seen a MCC with a bus rating that low, but I have seen single section MCC's with each bucket filled with NEMA Size 0 starters, which would prohibit high ampacity loads.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: MCC Extension

Ron,
The small starters in the MCC wouldn't satisfy the code rule. The main bus rating for the MCC section would be the "device rating" for the purpose of 240.21(B)(1)(1)(b).
Don
 

ron

Senior Member
Re: MCC Extension

I would consider it a judgement call, if the MCC section can only receive NEMA size 0 starters, then the bus rating is only as limited as the column of small starters permits. You can't put a NEMA 2 starter in a size 0 spot.
But I see your perspective.

[ May 15, 2003, 10:47 PM: Message edited by: ron ]
 
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