I am working on a USG project where power cables placed in a 6" conduit along with fiber optic cable are specified to be no longer than 500' from an antenna control unit to a power subsystem distribution cabinet (PSDC).The PSDC is supplied by a 30 AMP or 45 AMP breaker depending on cable lengths.
The work is being done by a contractor with specifics in what is required to be supplied.
One installation will only need 250' of power cable and fiber optic.
The contractor intends to no make at least the power cable shorter without looking for a major change to the contract issued and proposes to coil excess power and fiber optic.
I have fears of 250' or coiled power supply either creating induction, or starting a fire.
I could not find anything in NFPA 70 (National Electric Code) that says one can or cannot have an excessive amount of coiled power cable.
A general industry standard is 12' (manholes + handholes). Some say it is not a good idea to place power and comms in the same conduit.
Does anyone know of a citable authority on the maximum length that a power cable can be coiled?
The power is 208 VAC, 3 phase60 Hz 45 Amps per phase.
Best Answer
The maximum current/temperature limits for cables are specified with the cables straight in conduit. As you add more cables, the rating drops. Coiling will do exactly the same thing, though coiling is not usually specified under 'derating' as it's generally a bad thing to do.
If you can derive a new, lower, power rating for the cable when that length is coiled, then you could protect the cable with a smaller value breaker (it's not the normal consumption of the load that's important, it's the protection for the cable). I'm not sure how you could realistically estimate what the lower rating should be, to the satisfaction of anybody who is going to insure or underwrite the installation against fire risks.