Conductors in a Concrete Trench

[HowdyEE]

Gentleman, I have an installation where, due to some monumental blunder, wires designed to be overhead in conduit are now to be run in a concrete trench. These are feeding power to various devices using 12-250kcmil (qty 4 of 3-wires) and qty 2 4-wire of 2/0.

I understand that concrete trenches are considered a raceway and as such are subject to ampacity deratings listed in 310.15(B)(2). Is that a correct definition? If so, can some of these conductors be placed in conduit in the trench and somehow be exempt from derating?

The 2/0 bundles are feeding DC to braking resistors only a few times a month, so there should be some diversity, but still the deratings are killing us.

Any suggestions or experience with similar problems would be greatly appreciated.

 

[electricpete]

What is the duration of the loading? If not long you may be able to take credit for the fact that the cable doesn't reach thermal equilibrium using the cable "intermediate characteristics" of 9.5.2.4 of IEEE Buff Book. Additionally there is discussion of higher temperature allowance for short times, but even if you don't go there you can still increase rating above continuous rating.

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(2B)+(2B)' ?

 

[waross]

Check your code definition of "Continuous Loading". Some codes only consider sustained loads for greater than one hour to be "CONTINUOUS". This may help.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Good point – code – I forgot about that minor detail. I think you can only take credit for the IEEE Buff Book "intermediate ratings" (vs continuous ratings) if you meet the defintion on NEC 219 B (2) “Supervised Installations” – (2) Supervised Installations For supervised installations, branch-circuit conductor sizing shall be permitted to be determined by qualified persons under engineering supervision. Supervised installations are defined as those portions of a facility where both of the following conditions are met: (1) Conditions of design and installation are provided under engineering supervision.

Other than that I'm not sure what credit code allows for non-continuous loads.

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(2B)+(2B)' ?

 

[electricpete]

Forgot the 2nd half of the qualified installations:
...(2) Qualified persons with documented training and experience in over 600-volt systems provide maintenance,
monitoring, and servicing of the system."

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(2B)+(2B)' ?