400 amp feeders

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fatuus

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Location
Virginia
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Electrician
Please help!

From the load side of a disconnect can: (2) sets of parallel 3/0 THHN in (2) separate pipes feed a 400 amp panel? 3 phase, 4 wire

Thank you very much for your help.
 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
You have to have ABC&N in each pipe, and the pipes have to be the same length (approximately). Is this a service or a feeder? If a service, you may not need a ground wire. (The grounded conductor is bonded to grounding electrodes at both ends.) If it is a feeder, you need a #3 ground in each pipe, not the #6 that you would use with a single run of 3/0.

More details please!
 
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fatuus

Member
Location
Virginia
Occupation
Electrician
JoeStillman said:
You have to have ABC&N in each pipe, and the pipes have to be the same length (approximately). Is this a service or a feeder? If a service, you may not need a ground wire. (The grounded conductor is bonded to grounding electrodes at both ends.) If it is a feeder, you need a #3 ground in each pipe, not the #6 that you would use with a single run of 3/0.

More details please!
Click to expand...

The install is good. It is being rejected because of ampacity. It is a feeder.
 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
Carultch said:
The only issue I could see, is if the neutral qualifies as a "current-carrying conductor".
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Do you mean as in derating it per 310.15(B)(3)(a)? He said THHN, so you start at 225A before derating. 225 x 0.8 = 180, and 180 x 2 = 360. Per 240.4(B) the next higher standard breaker (as listed in 240.6) is allowed, so 400A ought to be ok.

There are lots of reasons the wiring might have been shot down, other than that it's in parallel. The lack of detail is preventing my mental telepathy from working on the inspector.
 
C

Carultch

Senior Member
Location
Massachusetts
JoeStillman said:
Do you mean as in derating it per 310.15(B)(3)(a)? He said THHN, so you start at 225A before derating. 225 x 0.8 = 180, and 180 x 2 = 360. Per 240.4(B) the next higher standard breaker (as listed in 240.6) is allowed, so 400A ought to be ok.

There are lots of reasons the wiring might have been shot down, other than that it's in parallel. The lack of detail is preventing my mental telepathy from working on the inspector.
Click to expand...

Correct. Provided that the current calculation of the panel is less than 360A.


Another reason might be if temperature derating comes in to play. And if it isn't in a heated room or direct sunlight, the temperature derate would likely be 0.96, which seldom causes a change in wire size.
 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
fatuus said:
The install is good. It is being rejected because of ampacity. It is a feeder.
Click to expand...

The inspector must be counting the neutral as "current carrying" as Carultch suggested. And objecting to the use of the 90° C ampacity in the derating calculation. If that is so, the largest breaker you could protect that feeder with is 350A.

If you have an NEC handbook, the sample derating calculations on page 330 use the higher ampacity of 90° wire in the example. Maybe the inspector could be persuaded on that basis.
 
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