Thread: derating

42 year old house with 200 amp upgrade. They installed the new panel about 16 feet from where the old panel used to be, and ran all the wiring for the house through a single conduit to a big junction box where the old panel had been. There was no inspection sticker on anything. As far as I could tell there was no attempt to derate, although I admit I'm not completely sure what to look for. But it basically looked like a normal panel - #12 wires to 20 amp breakers, #10 wires to 30 amp breakers, etc.
Isn't major derating needed here?

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I count over 30 current carrying conductors.

NEC says that requires to 40% of their original rating.

That means a #12 AWG is no longer 20 amps, it's only 10 amps (derating is taken from the 75 degree C column because that is newer NMB cable, which would have 75 degree C rated conductors in it).

Which means those #14 AWG is only good for 8 amps and the #10 AWG is only good for 14 amps.

jeez Jerry, it took you long enough.
Not.
Thanks a bundle, I mean, bunch.

If the nipple is less than 24" long you do not have to derate your wires....

Exception No. 3: Derating factors shall not apply to conductors
in nipples having a length not exceeding 600 mm

(24 in.).

They need to run a gutter with less than 20% fill to fix the problem. They can run the wires to and from the gutter with the 2" PVC as long as the nipple is 2' or less.

James,

True, but ...

"They installed the new panel about 16 feet from where the old panel used to be, and ran all the wiring for the house through a single conduit to a big junction box where the old panel had been."

The question/statement was regarding a 16 foot long run of those wires.

I wasn't very clear was I.

I was trying to say that to be able to run all those wire 16 feet you could use a foot long nipple at each end and a 14' gutter in the middle.

Wouldn't they still be "bundled" in the "gutter" and still require "de-rating"?

Thanks,
Bob

If the gutter is not over 20% filled at any cross section then the wires do not have to be deated....at least this is my understanding.

Bob,

What James is saying is that, *with the right size wire gutter*, you could limit the raceway "bundling" at each end to 24" or shorter (why not a 2" nipple?) and not have to derated through the up-to-24"-length raceway.

However, *with the right size wire gutter* means that the 30 or so conductors, some in two conductor cable, so 'the cables', along with the conductors, would need to be spaced apart to maintain spacing.

"Bundling" is not just for "bundling", it also includes 'lack of maintaining spacing' between the cable/conductors.

Now, doing that in a wire gutter would create problems for the electrician as the conductors and cables would need to be strapped such that they have spacing maintained between them.

Now, however, let's say that the electrician goes to the trouble to do that.

Now, there is no "bundling" and 'spacing has been maintained', but ...
From the NEC. (bold is mine)
- 310.15 Ampacities for Conductors Rated 0–2000 Volts.
- - (B) Tables.
- - - (2) Adjustment Factors.
- - - - (a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a).
- - - - - - FPN: See Annex B, Table B.310.11, for adjustment factors for more than three current-carrying conductors in a raceway or cable with load diversity.
- - - - - Exception (There are several exceptions, however, none apply to this scenario.)

That wire gutter would be serving 'as a raceway', and thus the derating problem would still exist, even if the electrician went to all the trouble to use a wire gutter large enough to allow for them to maintain spacing.

The reason is for heat dissipation - all the heat from all those conductors will still be trapped in that one 'raceway' - the wire gutter.

So, the final end result is that going to all that trouble would achieve ... nothing.

I was referring to this section....


366.22 Number of Conductors.
(A) Sheet Metal Auxiliary Gutters.

The sum of the crosssectional
areas of all contained conductors at any cross section
of a sheet metal auxiliary gutter shall not exceed 20 percent of
the interior cross-sectional area of the sheet metal auxiliary
gutter. The derating factors in 310.15(B)(2)(a) shall be applied
only where the number of current-carrying conductors, including
neutral conductors classified as current-carrying under the
provisions of 310.15(B)(4), exceeds 30. Conductors for signaling
circuits or controller conductors between a motor and its
starter and used only for starting duty shall not be considered

as current-carrying conductors.

Got it, thanks!
Bob

James,

You mean this section first, right?
(bold and underlining are mine)
- II. Installation
- - 366.10 Uses Permitted. Auxiliary gutters shall be permitted to supplement wiring spaces at meter centers, distribution centers, switchboards, and similar points of wiring systems and may enclose conductors or busbars.

Sounds to me that your proposed use 'does not' fall within those "Uses Permitted". You are proposing to use the wire gutter to replace, not supplement, a raceway, and not for those above listed conditions either.

This is a proper use for a gutter. If you like section 376 better then we'll use that. Either way a gutter is ok if sized properly.