Is it necessary to derate cables when installed within equipment, like switchboards or equipment cabinets? It seems silly to think that if I pop up and run, say, 3 cables each through 4 conduit sections consisting of two elbows joined end-to-end, I can use 100%, but if I run the 12 cables directly from one cabinet to the next (>24" total length), and strictly adhere to 310.15(B)(2)(a) I have to derate 50%, or even more considering the other cables in this equipment. I know I could pretend they're spaced, or separate them at points to pretend they're only bundled for 24" at a pop, but I'm pragmatic enough to know that's not meeting the intent of the code, and I'm far more interested in safety than I am in technicalities.
Are there special rules covering cable troughs and equipment that allow for lesser derating for short cables in equipment?
[This message has been edited by SteveFehr (edited 03-06-2007).]
The NEC, in general, applies only to the "house" wiring, and not to the insides of equipment. After the disconnect, you're on your own.
This is not always as simple as it sounds. Take the example of an assembly line - many machines wired to act as one. One might argue that the NEC stops at the main control panel; yet, one would also be foolish to ignore the NEC for the power feeds to the sundry motors.
One NEC provision, in particular, can be tricky to follow: the ban on conductors passing through motor starters. Complex machines are often so interconnected that it's impossible not to have control wires going everywhere. At some point that collection of equipment has to become a single "appliance."
In short, there's still a role for professional judgment.
#101315 - 03/07/0708:01 AMRe: Derating in Equipment
I have a question about derating also. I have raintight trough outdoors, I go throught the wall and LB up to a trough indoors, the total length of the conduit penetrating the wall including the LB is less than 2 feet, derate or not??
#101316 - 03/07/0712:59 PMRe: Derating in Equipment
Ah, just found a nice paragraph I'd never noticed- 366.23(A) allows us to ignore derating when there are less than 30 conductors in a sheet metal auxillery gutter. No exception for plastic, though. Very handy exception that could be easily extended to apply internal cabling inside most cabinets, I'd think.
310.15(B)(2) allows you to ignore derating factors on cables < 24" not in raceways, but gives no such exception for cables in raceways.
Copper cables act as their own heat sinks, transferring heat within the cable. Annex B.310.15(B)(3) goes more in depth into this, allowing you to use the greater ampacity if the higher derating factor is no greater than 10% of the run (max 10'). Which is VERY handy! Unfortunately, Annex B only applies to engineers, though, sorry! At any rate, this isn't additive with other exceptions, but the engineering rationale that the 24" bundle/nipple, cable gutter, etc, exeptions are based on.