pre-populating knock-outs in a sub-panel

dgeistMay 30, 2012

I'm adding a flush-mount sub-panel on the inside wall of my garage that will (over time) terminate all my indoor branch circuits. I'd like to pre-plumb routes for the branch NM coming into the panel with stubs of PVC conduit that lead to a section of the wall that I don't need to close in with drywall. This will allow adding circuits over time without having to rip open the wall cavity every time.

- Is there any code restricting the use of traditional NM 12/2 or 14/2 (as opposed to individual 12/14g conductors) or is it just not typically done?

- If my branch NM lines will be attached to framing members in the traditional manner (wire stapes, etc.) all the way to the PVC "stubs" do I need to provide a clamping fitting at the panel side of the PVC, at the far end, or both?

- Can I co-locate the conductors for more than one branch in the same conduit stub (provided there's enough room for free play and that the conductors are not electrically connected in any way)?

I know this isn't a true "conduit system" per code, but I appreciate y'all assisting in defining what I DO need to adhere to.



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Using conduit to protect 'short sections' of NM is often done but it is difficult to pull/fish the cable around bends. You are not permitted to remove the NM sheathing to make this easier. The NM sheathing will also cause you to reach the max-fill of the conduit much faster (65% of the cross-sectional area).
Why not use runs of NM to junction boxes in the unfinished area? You can delay connecting them in the panel until you want to energize them.

    Bookmark   June 1, 2012 at 1:09PM
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Thanks for the info on NM sheathing. I figured as much but confirmation is nice. I don't expect to have long conduit runs, perhaps 2-3 feet, just enough to clear the wall area between the inside utility room and the garage (need to insulate the wall and run drywall for the fire rating and inspections. The City inspector has advised the last part since I'm breaking the rated fire barrier by putting a hole in the garage wall for the panel itself).

I've seen 5-8 gang switch boxes with lots of room for junctions. Is that what you're thinking, or would each branch require its own dedicated j-box?


    Bookmark   June 1, 2012 at 3:04PM
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"The NM sheathing will also cause you to reach the max-fill of the conduit much faster (65% of the cross-sectional area). "

There are no fill requirements for short sections of conduit for physical protection.

Fill rules only apply to complete conduit systems.

    Bookmark   June 2, 2012 at 9:19AM
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At the electrical supply houses - they have large junction boxes with a cover panel that would work for you - they come in various sizes. I have both a 12"x12"x6" junction box at one end of my garage and an 8"x8"x4" at the other end. Both are in the ceiling but could be put on the wall. These larger junction boxes are gray painted.

Those larger ones cost somewhere between $10-$20. Not sure if they carry them at big box stores - but they are easily available at all electrical supply houses. The nice thing about the larger boxes is they come with a cover plate. The smaller metal ones - you have to pay separate for the cover.

Depending on how many NM wires you are bringing in you might get by with what is called a 4-11-16 junction box (because it is 4-11/16"). This is much easier to work with than the standard cramped 4" junction boxes they sell at Big box stores. BTW - they do sell the 4-11-16 boxes at the Big box stores too - just ask if you can't find them.

When I moved into my house - there was a 4" junction box with an extension on top - and it looks messy - especially when a larger roomy junction is not that expensive at your local Electrical supply house. This was changed out to an 8"x8"x4" box. Even though I had a 2"x10" joist in the ceiling of the garage - I got the 4" deep one because it gave us a lot of room on top of the box to bring in all the Romex and cable connectors... and the 8"x8" was over sized already.

There is an NEC table that will help you calculate how many Cu. in you need in a junction box to hold the number of NM you are bringing in. It is different depending on if you are using 12g or 14g or 10g. Use the table... it will help you add it all up - and then get a bigger box rated for higher Cu. in. then your calculation - you may need to add to it in the future - and it's hard to work in a tight box anyway - ones you shove in all the wire nuts.

Here is a link that might be useful: Large junction box

    Bookmark   June 7, 2012 at 4:31AM
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