adding cable to existing trough

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recooper8

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It seems I used the wrong forum (NEC) to post my question originally. Since calculation is the major question I post in this forum.

There is an existing installation to which the customer wants to add two new circuits. The existing installation has a trough with key taps feeding four (4) 200 AMP disconnects. 120/208 3 ph. the trough is 12" x 12" x 60"(LENGTH). The existing cable in the trough is 4#3/0 for each 200 amp breaker (total 16 3/0) and 2 sets of 500 MCM feeders from the transformer (total 8 500MCM). New cable to be added to the tough is to be 4 #4 feeding a new 80 amp disconnect and 4 350 MCM feeding a new 300 amp disconnect. These new cables will be terminated on the key taps in the trough. Is the trough size acceptable per NEC? Can the existing feeders (2 sets 500MCM per phase) carry the additional load being added? Any suggestions?
 
Re: adding cable to existing trough

additional information. This is an auxiliary gutter beneath the disconnects. The cable is type THHN. My calculations are that with all the cables in the gutter (8-500MCM, 16-3/0, 4-#4, and 4-350MCM) the total sq. inch area of the cable would be 12.37 sq in. Then using 20% fill the area required would be 61.85 sq. in. Since the gutter is 12x12 (144 sq in.) it is sized large enough for the additional cable. Am I calculating correctly?

It seems that the 500 MCM feeders will not carry the additional loads being added. It seems two additional sets of 500 MCM will be needed to carry the added load.

Please comment
 
Re: adding cable to existing trough

As far as the gutter goes, yes your calculations are correct.

You need to keep in mind if you have enough space to account for wire bending radius and deflection as per 312.6(A)

As far as tha appropriate ampacity goes, you two feeders are able to provide 760 AMPS (without any de-rating), which are already consumed by the 4-200 AMPS disconnects. You definitely have to add capacity...also keep in mind derating, if neeeded.
 
Re: adding cable to existing trough

molotov We actually dont have a load calc for the 4 200 amp disconnects so we dont know how much is being "consumed" by these 4 200 amp panels. If the load calc is within the ampacity of the conductor then all is well in electric land.
 
Re: adding cable to existing trough

Thanks. Molotov's reply is most helpful. There are no load calcs available. I am using the max possible in my calcs. The decision was made to run two new feeders from the transformer to the new loads. This will give a total of 6 disconnects in one location and meet nec

THANKS AGAIN
Rick
 
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