Duct Bank Derating?

[OhioAviator]

Situation:
Installing incoming 480V AC service lateral consisting of 6 parallel runs of 6" RGS/PVC/RGS conduit, each containing 3 x 500MCM (phases) & bare 2/0 (ground) to the mains of a motor control center (MCC). The conduits will exit underground from a pad-mount switchgear lineup and be buried for approx. 75 feet. They will then turn upward inside the building to the MCC.

Question: Do I need to derate the conductor ampacities because they're buried together in a duct-bank? I can find references in the 2002 NEC for cables rated 2,000 volts and up (Fig. 310-60 and Tables 310-77 thru 86) but not for low voltage.

If derating is needed, can you guide me to the relevant NEC reference(s) and table(s)?


Thanks!

 

[dpc]

Per the NEC, if not common sense or the laws of heat transfer, derating is not required for low voltage cables.

Back around 1987, de-rating of low voltage conductors was added to the NEC, but they were overwhelmed with whining from electrical contractors, electricians, and inspectors, so quickly turned tail and retreated.

Historically, there have been virtually no problems with overheating of low voltage feeders in duct banks, even with no derating. This seems to say that the NEC ampacities are very conservative.

 

[OhioAviator]

Thanks dcp. I couldn't find any references to low voltage derating so your response confirms my suspicions. Much appreciated.

--JR.

 

[rob46]

1. Please review thread 238-44759.
2. Review article "Current-carrying capability for industrial underground cable installations" by Julius Hegyi and Alex Klestoff.