Running electrical wire below joist in boxed in ductwork???

[newsaboutgod]

I am finishing my basement.

I had to box in about 30 inches wide of duct work and water pipes they ran across the celiling joists.

I boxed in the pipes and ducts with 2x2s.

I want to run some electrical wire (12 gauge) in this boxed in area below the joists. That saves me from drilling the joists and will greatly simplify the wiring.

I know normally it cannot run below the joists but since this is going to be boxed in with sheetrock and nothing can damage the wire or stuff can't hang on it can I run below the joists in this case? I thought you could run below the joists if you put wooden runners on each side of the wire. I would think a boxed in area would count the same.

Thanks for any help!!!

Sheila

[Joe]

Consider using EMT large enough for this and maybe some future circuits you may need (3/4"). It would avoid any code conflicts and potect wiring from future mshaps during remodels. HTH

Joe

[Doug Miller]

The Code says you can't do that in "unfinished basements." It doesn't specifically address whether it's permissible in a finished portion of an otherwise unfinished basement (which is how I'd describe what you propose).

Probably does, but check with your local electrical inspector to be sure.

[dpb]

...

Sure. Once you boxed it in and it is finished, it no longer is exposed and no problemo...

Somebody else suggested pulling them in conduit for possibly adding something in the future which isn't a bad idea, but can get the same effect more inexpensively by simply laying a 1/4" ply 2- or 3-sided box in there as a tray if that were desired.

In a former house w/ open trusses I wished for a similar trick to have been used when wired originally when the need for some additional work and repair became necessary...

--

[gfretwell]

As others have said since this is boxed in it is not exposed to damage. The only issue is that it is 1 and a quarter inches back from the "finish" surface of the framing so the dry wall guy won't hit it shooting in drywall screws. If it is safely behind the 2x2 (1.5" thick) it should be fine.

[Chris Lewis]

According to :

Note that electrical code does require that there be a thermal break between hot air ducts and wire. In Canadian code, it's usually expressed as a 1" air gap, but they're perfectly happy with a wad of fiberglass jammed in place. This also avoids vibration cutting the wire insulation and shorting to the duct.

[gfretwell]

That must just be a Canadian thing. There is nothing in the NEC specifically about it.

[M Q]

Not explicitly. However, if you are running the wire in contact or nearby a hot air duct, if even in a box with the hot air duct, your ambient temperature will be much higher than the 30 C ambient temperature that the ampacity tables are based upon and you would have to derate the ampacity of the the wire. You will probably no longer be able to use 12 gauge for a 20A circuit. Check the "Ampacity Correction Factors" tables.

BTW. If it is enclosed in the same box, neither an "air gap" or a "wad of fiberglass" would solve the ambient problem.

[gfretwell]

60c (the lowest rating for any NEC wire) is 140F, just how hot do you think forced air heat ducts get? I doubt they ever have a 20 degree temperature rise and even if the ambient air was 80f, the air in the plenum would only be 100f.

Wire in the attic in the summer is a lot more of a problem. Even with that the 90c rating of modern wire is what you use to derate.

[Steve Barker LT]

I think you might be surprized about the temp of furnace discharge. (not that i think having a wire next to it would be a problem) We have one furnace that I can't hold my hand on the discharge ductwork. And I sure as heck wouldn't have a problem holding my hand on a 100 degree object.

steve

[M Q]

...

Don't confuse ambient temperature with the temperature rating of the insulation. At an ambient of 30C you can exceed the insulation rating if you put enough current through the wire.

The ampacity tables are based on an ambient temperature of 30C (86F). They give the max currents at which the wire (with some safety margin) will not exceed the insulation rating temperature. Those tables have columns for wires with insulation ratings of

60, 75, 85, 90C. If you ambient is different than 30C you have to use an additional table "Ampacity correction factors".

[Chris Lewis]

According to :

That's true with heat pumps, but with forced-air-gas, the delta above ambient will usually be higher than that.

This may be a carry over from older heating systems. I seem to remember reading something about a "worstcase/design max" assumption of the furnace end of the ducts hitting 170F.

It's quite explicit in our codes (probably amplified in building code). Inspectors and most contractors take this one pretty seriously.

[Chris Lewis]

According to :

To add to my other posting: in many cases, ducts are subject to blower vibration transmitted via the ducting. Metal + plastic + vibration -> insulation wears out, wires short to ducting. Especially if it's a metal edge.

[M Q]

... My Carrier 58MVP furnace is spec'd for a 60-70F temperature rise during low stage operation. If the inlet air is 72F, that is

132-142F in the duct. If you put that duct in a box, that is too high an ambient for NM or any gauge, no matter what current you want to put through it. If your ambient was only 124-131, you would need 8 gauge NM to handle 15 amps.

Best to not put wire in contact or in the same enclosure as a hot air duct.

I notice that OP seems to be absent from these discussions.

[gfretwell]

These days we use NM-b with a 194 degree rated conductor. You derate from the 90c column. The derating for 123-131 is .76

14 ga 90c conductors are good for 25a times .76 equals 19a. 240.4(D) still limits that to 15a (with 4 a to spare). You have to get over 140f before you really have to derate beyond 240.4(D) with up to 3 current conducting conductors in a cable or raceway. BTW that is not unusual in an attic in the summer.

[David Combs]

What's "EMT"? (probably obvious to all but me!)

Thanks

David

[Joe]

Electrical Metallic Tubing

Joe

[jim]

Don,t know where you live but here it is against code here even if emt is used to run power in duct work.

[David Combs]

Thanks for the def!

David