Q about derating

terribletomJune 28, 2010

Hi folks! Just a quickie question here...

In planning a semi-busy conduit run, I'll have a couple of pairs of switched "travelers" (i.e., alternative hot paths running between three-way switches) in the same raceway. For the purposes of derating, does a pair of travelers count as one conductor or two?

From the standpoint of heat management, it strikes me that a pair should count as one, since only one can be an active conductor at any point in time. [OTOH, code doesn't always accommodate my intuitions and divinations. :-) ]

(Note: The question is not about conduit physical fill volume...just how to count conductors for derating calcs.)

Thanks much!

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normel

The travelers should each be counted as a current carrying conductor, but remember, if this is a 15 or 20A circuit, derating will not affect wire size/circuit breaker choice until you exceed 9 current carrying conductors.

    Bookmark   June 29, 2010 at 6:30AM
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terribletom

Thank you, Norm. Groan, that's pretty much what I suspected the answer to be.

In this case, it's a mix of 15- and 20-amp circuits and the plan is right on the bubble. That is, if two travellers count as one, then I have 8 total. Otherwise, I have 10.

Common sense suggests that it's a stupid reason to pull #10s instead of 12s and 12s instead of 14s when two of the conductors are guaranteed to be dead at any given time.

But, oh well, code is code whether it always makes sense in context or not.

    Bookmark   June 29, 2010 at 7:50AM
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joed

I would count them as 1 same as when using a multiwire branch circuit the neutral doesn't count.
Don't confuse derating with conduit fill. Those are two different calculations.

    Bookmark   June 29, 2010 at 7:55AM
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brickeyee

For fill, two conductors.

For derating, a single conductor since they cannot both be active at the same time.

This is exactly analogous to a multi-wire branch circuit.

3-wires for fill, two for derating.

Remember you get to start with the actual conductor ampacity for derating (not the footnote limit of 15 A on #14 and 20 A on #12).

    Bookmark   June 29, 2010 at 8:30AM
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terribletom

joed wrote: "I would count them as 1 same as when using a multiwire branch circuit the neutral doesn't count."

Now there's an answer I like better. :-) Thanks.

and brick':

"For fill, two conductors.

For derating, a single conductor since they cannot both be active at the same time.

This is exactly analogous to a multi-wire branch circuit.

3-wires for fill, two for derating."

Makes sense to me. Thanks.

Remember you get to start with the actual conductor ampacity for derating (not the footnote limit of 15 A on #14 and 20 A on #12).

Yup, capisce. Of course now that I'll only have eight, I'm home free anyway.

    Bookmark   June 29, 2010 at 11:06AM
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