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1200 amp sevice conductor sizing #170419 11/02/07 05:38 PM
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trekkie76 Offline OP
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I have a question about conductor sizing.
I have a 1200 amp main breaker on a 120/208Y service feeding a grocery store.
I came up with (4) 500 MCM per phase.
380A x4= 1520amps.
1520A x .80(T310.15(b)(2)(a)4-6 conductors in a raceway.=1216 A
1216A Round down to next standard= 1200A.

Have I gone astray somewhere? With all the lights, computers, etc. I would consider the neutral a current carrying conductor. I have seen other topics on here and none had mentioned the wire size I came up with.

As always thanks for the help gentlemen!
Gary

Work Gear for Electricians and the Trades
Re: 1200 amp sevice conductor sizing [Re: trekkie76] #170420 11/02/07 06:22 PM
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mahlere Offline
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I would run 4 conduits, each containing 4 400MCM...A,B,C,N

For derating purposes, you use the 90deg column...so 400MCM is 380A x 4 = 1520A * 0.8 = 1216A

However, whether the Neutral is actually a CCC can be disputed. If not, then you can get away with 350MCM




Re: 1200 amp sevice conductor sizing [Re: mahlere] #170421 11/02/07 06:39 PM
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trekkie76 Offline OP
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I thought you could only use the column that corresponded to the temperature rating or the terminations?

Re: 1200 amp sevice conductor sizing [Re: trekkie76] #170422 11/02/07 06:40 PM
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mahlere Offline
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for sizing...for derating you can use the 90deg column..

Re: 1200 amp sevice conductor sizing [Re: mahlere] #170429 11/02/07 07:15 PM
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trekkie76 Offline OP
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I dont understand what you mean. I thought you sized the conductor according to ampacity required, then adjusted the selected conductor according to ambient temp, # of conductors in conduit, etc. I am not trying to sound obtuse, but I guess I was tuaght differently.

Re: 1200 amp sevice conductor sizing [Re: trekkie76] #170430 11/02/07 07:22 PM
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mahlere Offline
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Originally Posted by trekkie76
I dont understand what you mean. I thought you sized the conductor according to ampacity required, then adjusted the selected conductor according to ambient temp, # of conductors in conduit, etc. I am not trying to sound obtuse, but I guess I was tuaght differently.


nope...you size the conductors based on the terminal temp rating...but for derating purposes only, you can use the 90deg column. because you are worried about the insulation rating, not the connection rating.

without derating, 4-350's or 4-400's have enough ampacity in the the 75deg table to carry the load. but for derating only, you can use the 90deg table.

maybe someone else can explain it better.

Re: 1200 amp sevice conductor sizing [Re: mahlere] #170431 11/02/07 07:31 PM
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trekkie76 Offline OP
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but wouldnt that result in a conductor sized below the ampacity of the 75 degree column?

Re: 1200 amp sevice conductor sizing [Re: trekkie76] #170432 11/02/07 07:32 PM
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trekkie76 Offline OP
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i am going to look at this tommorrow morning and laugh im sure!

Re: 1200 amp sevice conductor sizing [Re: trekkie76] #170433 11/02/07 07:37 PM
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mahlere Offline
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nope...not in this case...

I will say this though...find out if the inspector wants you to consider the neutral as a CCC...if not, no derating necessary...then you can run 350's...

Re: 1200 amp sevice conductor sizing [Re: mahlere] #170464 11/03/07 05:34 PM
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gibbonsseabee80 Offline
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I am thinking the same as Mahlere. I was taught you size your conductors to the terminal ratings of the equipment you are hooking up, which most of the time is the 75 deg C colum. You use that colum for your basic calculation for ampacity, then if you derate due to conduit fill or a change in ambient temperature..you look right to the 90deg colum. So rather than changing wire gauge you change insulation type by looking at the 90 deg colum only for conduit fill or ambient temp derating. This was explained to me in the Mike Holt exam book. Good luck, Brian


Brian Gibbons
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