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Joined: Dec 2002
Posts: 308
Edward Offline OP
NEC says if I have more than three current carrying conductors in a raceway I must derate. According to NEC HandBook 1999 Table 310-15(b)2(a)
4-6 conductors 80%
7-9 70%

and so on.

In my NEC 1999 HandBook it also says that "the table is based on no diversity. Which means that all conductors in the raceway or cable are loaded to their maximum rated load."

So if the conductors are not loaded to their maximum load capacity then I do not have to derate? Right?

Joined: Dec 2003
Posts: 524
...Wrong... You MUST ALWAYS derate for conduit/raceway fill.. That is the Code, is not selective...

.."if it ain't fixed,don't break a Licensed Electrician"
Joined: Feb 2004
Posts: 1,438
Edward wrote:

So if the conductors are not loaded to their maximum load capacity then I do not have to derate? Right?

Preventing conductors from reaching certain loads which would be permissible with 3 conductors or less.. Isn't that derating?


Joined: Sep 2003
Posts: 650
You _always_ have to derate for conduit fill. The only question is 'Can you derate with load diversity rather than absolute ampacity?', and this becomes a question of how you count current carrying conductors.

If I have 10 conductors in a raceway, but the system _guarantees_ that only 2 conductors will actually carry current at any one time, then can I count that raceway as containing 2 conductors for the purpose of figuring derating? If all of the conductors are in use at the same time, but the system guarantees that the _total_ current is less than a given amount, can I use that _total_ current to figure the nominal count of 'current carrying conductors'?

The NEC has explicit provisions for not counting certain conductors which are expected to carry current. The four circuit conductors of a three phase multiwire circuit count as _3_ wires for the purpose of derating (at least as long as there are negligible harmonics). This is because the worst case scenario for heating is with any three of the conductors carrying full current, and the other one carrying no current. I believe that the travellers for three ways switches also only count as one current carrying conductor. But I do not know of a general permission to account for load diverse current carrying conductors in a raceway.

Consider the following situation:

A room lighting system with 8 single pole switches to provide selective control of the different lights. All of the switches are located in one location, fed through a single 3/4" EMT. Some of the switches have lights which require a neutral. All conductors are 12ga THHN. In the EMT there are 11 conductors; the ungrounded conductor and the grounded conductor from the single phase supply, insulated EGC, and 8 switched conductors going back through the EMT to a junction box and then on to the various lamps.

Clearly there are 10 conductors which are expected to carry current. However I claim that for purposes of de-rating this arrangement should count as _two_ conductors, that no de-rating is required, and that the conductors are properly protected with a 20A breaker.

My reasoning is that the worst case scenario is where full current flows the ungrounded supply conductor and one of the load conductors, with all of the other conductors unused. In any other case, conductors are individually carrying less current. Since wire heating is proportional to the _square_ of the current flow, having the same total current flow in additional wires at the same time will result in less total heat production.


Joined: Feb 2002
Posts: 375
winnie ---

I love your response. That is a great example.

Edward ---

In general, you are allowed to solve the worse case heat transfer problem and derate enough to keep the temp below the wire rating.

Joined: Jan 2003
Posts: 4,391
So if the conductors are not loaded to their maximum load capacity then I do not have to derate?

No, you must derate.

Could you put a 12 AWG on a 30 amp breaker as long as you only load it to 20 amps?(forget about 430 & 440 for now [Linked Image])

Derating is just what it says, the conductor is no longer rated X it now rated less than X.

While what has been said does make sense and I agree it will work IMO it does not meet the code.

I also do not think you will sell this to an inspector.

Apparently there is no definition of load diversity from the NFPA.

When you have a derating table that is based on this undefined load diversity that means they already based the table on mixed loading.

If all the conductors where for some reason fully loaded this table may not provide enough derating.

Derating table 310-15(b)(2)(a) is no longer based on load diversity. In the 99 code there is an FPN directing you to Table B-310-11 in the rear of the book which is a derating table based on load diversity.

It does not eliminate derating it just eases it up a little.

How do you convince an inspector that you can use Table B-310-11?

I have no idea, FPNs are not code.

IMO the only chance you might have is if you have a electrical engineer sign off on it.


Bob Badger
Construction & Maintenance Electrician
Joined: Dec 2002
Posts: 308
Edward Offline OP
I guess i will have to derate. [Linked Image]

Thank you.

Joined: Feb 2003
Posts: 138
and this becomes a question of how you count current carrying conductors.

Funny, I don't see that listed in any of the 5 exceptions in 310.15(B).

You could try to have an AHJ approve the methods in 310.15(C) if you are an engineer.
Even when using (C), I don't see the logic there!


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