Derating Wire Ampacity

[Harry H]

In an industrial situation, when installing several currant carrying conducters in a trough, we have to derate the ampacity of the wires. In sizing the wires, do we use the actual load in the plant, or the full amp rating of the circuit breaker controlling the load, or 80% of the circuit breaker's amp rating which would be the maximum continuous load on the circuit?

 

[chris kennedy]

In an industrial situation, when installing several currant carrying conducters in a trough, we have to derate the ampacity of the wires.

When you say several, you mean 30?

Welcome to the forum.

 

[Smart $]

In an industrial situation, when installing several currant carrying conducters in a trough, we have to derate the ampacity of the wires. In sizing the wires, do we use the actual load in the plant, or the full amp rating of the circuit breaker controlling the load, or 80% of the circuit breaker's amp rating which would be the maximum continuous load on the circuit?

If over 30 ccc's you have to derate. At a minimum, the derated ampacity must equal or exceed the calculated load being supplied (note: calculated load does not include extra 25% for continuous loads). However, the final ampacity cannot equal or be less than the next smaller standard ocpd rating (general case). The minimum size for continuous load factoring is [typically] not an issue because determination is based on the non-derated ampacity of the conductor.

 

[Harry H]

The particular case in question is 18 600 MCM THHN copper wires 3 wires per phase from a main to a transfer switch and 3 wires per phase back to a distribution panel in an 8"x8" wireway. The main breaker is 800amps. My starting ampacity would be 1425 amps and at 50% would derate to only 712 amps. Actual plant running load is just under 600 amps but connected load is higher. Not all the motors can run at once though because of the way the plant operates.

 

[infinity]

Derating begins at 30 CCC's at one cross section. I'm guessing that you have more than 30 CCC's (18+18)?

 

[LEO2854]

The particular case in question is 18 600 MCM THHN copper wires 3 wires per phase from a main to a transfer switch and 3 wires per phase back to a distribution panel in an 8"x8" wireway. The main breaker is 800amps. My starting ampacity would be 1425 amps and at 50% would derate to only 712 amps. Actual plant running load is just under 600 amps but connected load is higher. Not all the motors can run at once though because of the way the plant operates.

Derating begins at 30 CCC's at one cross section. I'm guessing that you have more than 30 CCC's (18+18)?

I may be wrong but it looks like he said 18-600 MCM conductors in the 8x8 wire way.


Welcome to the forum Harry H....:thumbsup:

 

[Smart $]

Derating begins at 30 CCC's at one cross section. ...

Pitnicking, but derating actually begins at 31 ccc's

 

[Harry H]

It is a total of 18 wires (9 over and 9 back) so I think I am okay without having to derate. I was believing they would treat my 8 x8 wireway trough as they would conduit or raceway.

Thank you all for your help. This appears to be a great resource when I have questions.

 

[Smart $]

It is a total of 18 wires (9 over and 9 back) so I think I am okay without having to derate. I was believing they would treat my 8 x8 wireway trough as they would conduit or raceway.

Thank you all for your help. This appears to be a great resource when I have questions.

Given that you don't have to derate, 2 sets of 600kcmil (6 ccc's over, 6 back, 12 total) ccc's would be sufficient if voltage drop does not warrant upsizing. 110.14(C) consideration would put you at 840A.

You could even drop to 2 sets of 500kcmil if your calculated load (without extra 25% for continuous loads) is less than 760A.


BTW, welcome to the forum !!!