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

George Little ---

Tightness does matter ...

I expect you are commenting on 3 NM cables going through a 1" hole.

I expect you allow the same 3 MN cables to be run through a 10"x14" joist bay.

Will you allow 3 NM cables though a 4" hole? A 3" hole? A 2" hole?

When the code contains a section that states a specific spacing for cables, I will change my position as to what the code allows. Until then anyone who requires derating because cables pass through the same hole is simply wrong.

George


That is simply your opinion, it is not a fact.

As you stated there is no definition of bundling, that being the case it is up to the AHJ or Inspector to make the call on what is or is not a bundle.

Either way the AHJ is not 'wrong' it is no different than "exposed to physical damage" it's an inspectors call.

Bob


[This message has been edited by iwire (edited 09-08-2005).]

iwire ---

When the NEC is made law, there is no "inspector's call."

In any case we have forgotten that bundling for less than 24" does not require derating. At worse the holes in joists bundle wires for 3" or so. (I guess that makes my "opinion" right and yours "wrong".)


[This message has been edited by George (edited 09-08-2005).]

George


We will have to agree to disagree on that.



What is "subject physical damage"?

What is "adequately protected"

Who makes those calls?

You won't find a guy that dislikes 'inspectors calls' more than myself, however even I have to admit that there are items in the NEC that are left up to the judgment of a single individual.

Bob

Example to ponder:
Hall with 12 light fixtures, each controlled by its own switch. All switches are located in a single (very ugly) multigang box, all fixtures are fed from a single raceway. Each fixture has a ballast rated at 1.3A. All fixtures are supplied by a single 20A circuit. The raceway from the switch box to the first fixture is carrying a total of 14 conductors: 12 switched 'hot', 1 grounded, 1 equipment ground.

For purposes of derating, how many current carrying conductors are considered to be in the raceway, and is 12ga THHN suitable for this install?

If we assume single phase service 240V/120, and replace the above install with a multiwire circuit, where 6 of the switches are supplied by one supply leg, 6 by the other supply leg, with a single shared neutral, does the 'official' number of current carrying conductors go up or down?

In this latter case, is the more or less chance of overheating the conductors?

-Jon

Jon given the installation you describe you have 13 current carrying conductors.

This means a 50% downward adjustment of the conductor ampacity resulting in the need for 10 AWG 90 C conductors.

In the second version of the circuit the current carrying conductor count remains the same as the majority of the load is non-linear.

That is my opinion of the NEC rules.

From a practical stand point no derating should be needed in this installation.

You would derate based on all 14 conductors but since the load on each is 1.3a you are well within the acceptable range.

Bob why would I need to increase the wire size if the applied load is only 1.3a each?
You might be able to argue that the grounded conductor should be #10.

Greg the load is not the issue with the NECs present rules on derating.

Let me ask this.

If this was only one 1.3 amp fixture on a dedicated circuit could I run 14 AWG to it but still use a 20 amp OCP?

No, because the rules of 240 require a conductor for this application to be rated higher then the OCP.

Now when we derate the ampacity of the conductor we must not use an OCP above that adjusted rating.

Thirteen 12 AWGs in a raceway have a rating of 15 amps at best, so we would need to drop the OCP down to 15 or increase the wire size to 10 AWG.

Bob

[This message has been edited by iwire (edited 09-08-2005).]

"Now when we derate the ampacity of the conductor we must not use an OCP above that adjusted rating."

Do you have that code section? I may be missing somerthing ... it happens