ECN Forum Forums Code Related Discussion NEC & other Code issues Derating NM cable

Greg - Wire has to be protected at it's ampacity 240.4 and if their is an adjusted ampacity the overcurrent must match that new ampacity and sometimes you can round up.

I think a more accurate statement than that would be "conductors aren't allowed to melt", citing section 310.10.

Citing 240.4 as an all-encomposing provision is little bit untrue, when you consider the applications of 240.4(A), (E) and (G)

Ryan- I like that (310.10), can't say as I've used that before in my travels. That's the beauty of this forum, you can always learn something. even an old geezer like me.

How could the wire possibly "melt" if the maximum current spread across 12 conductors is 20a.
If this was 12 20a branch circuits I would see your point.

If we didn't enforce the code, what would happen when the fixtures are changed?
The load could be higher than the load at the time of installation.

I posted the example of 12 'switched hot' conductors as a thought experiment.

I agree with iwire: it is a violation of code not to derate these conductors, but isn't really a problem to protect them at 20A.

The worst case scenario for heating is if all of the current flows in a single one of these 12 wires. All other conditions result in lower I^2R heating. Clearly having multiple conductors sharing the current should be allowed whenever a single conductor is allowed (from a heating point of view....there are current sharing issues that would make it a bad idea to use small conductors as actual parallel conductors; but having them run in parallel to _different_ loads does not cause such problems).

IMHO a reasonable additional exception to the rules used in counting current carrying conductors would be: For the purpose of counting current carrying conductors, a set of two or more conductors supplied by the same branch circuit and carrying net current in the same direction shall be considered a single conductor.

I think that the wording would need some cleanup in order to avoid switch loops counting as a single conductor, but you get the drift.

I doubt that this is actually a frequent enough situation to actually warrant a code change, however.

-Jon

I also have a problem with a residential situation where several cables leave a multi gain switch box that is fed by the same circuit. There can be no more heat at this point than the breaker itself will allow.

I was invited into a situation a couple of months back where I knew both the electrical contractor and the inspector. The electrician was turned down on rough and 334.80 were citied as the violation. The electrician trying to repair the problem got in trouble with the building inspector for destroying the top plate.

After meeting with the inspectors and contractors we all agreed that 334.80 at a switch was a little ridiculous. This home was on a pad and had two four gain switch boxes in one stud bay. The outside bracket lights was the only single pole switch.

Made my heart feel good that I was able to mitigate this situation to a peaceful and understanding end with all parties in agreement.

"If we didn't enforce the code, what would happen when the fixtures are changed?
The load could be higher than the load at the time of installation"

You still are limited by a single 20a O/C device. How much bigger can the load get?

BTW the code does not protect against the next stupid thing an installer might do. You can put in a 400a fused disconnect with 225a fuses and you only have to serve it with a 225a rated conductor.
Sure someone could come along and shove in 400a fuses. That is not addressed.

(editted, found answer to my dilema)


Dnk......

[This message has been edited by Dnkldorf (edited 09-10-2005).]

Table 310.15(B)(2)(a) doesn't have "qualifiers", that permit field modification of the rule, so derating should apply.
It seems to me that derating these conductors to 50% (which would result in an ampacity of 15 amps), would be similar to using #14 wire as the switch legs to the lights on a 20-amp ckt. That is something we all have agreed that would be prohibited.