Take the 2-minute tour ×
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It's 100% free, no registration required.

I've been discussing specifics of an electrical job with a licensed electrician and he mentioned that in my area, it's acceptable/recommended to feed 12 gauge NM wire through an outdoor conduit but that the PVC sheathing SHOULD be removed if the wiring is going to travel through more than 6 feet of conduit.

This doesn't make sense to me. How/why would that be better than keeping sheathing on through the conduit run? Condensation?

share|improve this question
1  
Probably more about the difficulty in fishing it through the conduit than anything else. But that's just a guess. –  Chris Cudmore Sep 16 '13 at 19:19
2  
Conductors inside most NM cable are THHN (Thermoplastic High Heat-resistant Nylon-coated) not THWN (Thermoplastic Heat and Water-resistant Nylon-coated) (if they're labeled at all), and so are not allowed in wet or damp locations (which according to NEC, outdoor conduit is). The sheath is removed to reduce friction while pulling the wires through conduit, thereby making it easier to pull. –  Tester101 Sep 17 '13 at 11:56
    
Note: If the conductors are THWN, there is no problem if the sheath is removed. –  Tester101 Sep 17 '13 at 12:01
add comment

4 Answers 4

There is a widespread belief that the NEC does not allow NM-cable in conduit, but does allow THHN (the individual wires). This belief is incorrect.

However, it is for some reason lesser-known that NM-cable cannot be used in outdoor conduit at all, stripped or otherwise.

So, the answer to your question is: stripping is a common but misguided (unnecessary) practice when using NM-cable in indoor-conduit. And it is not allowed at all when going through outdoor conduit, like in your case.

share|improve this answer
    
THHN is only allowed in conduit in dry locations. –  Tester101 Sep 17 '13 at 11:59
    
Sorry... I'm still confused. I (now) understand that NM-cable cannot be used for outdoor conduit at all. But what about UF cable? Does the sheathing for that need to be stripped? prior to routing the wiring through the conduit? Or is it just best to leave the sheathing on any/all times that UF wire is going through outdoor conduit? –  Mike B Sep 17 '13 at 15:46
    
@MikeB: If running conduit outdoors, the cable needs to be rated for wet-locations, like UF cable or individual THWN wires (not to be confused with "THHN"). I don't know if you're allowed to run stripped UF cable in conduit, but even if you can, I wouldn't recommend it - that stuff is really hard to strip, and THWN is cheaper/smaller/easier to work with. The benefit of UF is that it can be directly buried (with no conduit), which is easier than burying conduit. I would run a UF cable if you're sure you'll never need to pull another circuit through, otherwise run conduit with THWN wires. –  BlueRaja - Danny Pflughoeft Sep 17 '13 at 16:21
add comment

Its a heat build-up issue: wire in conduit must be allowed either radiate (without sheath) or the current carrying capacity must be derated. This is true also in bundles of sheathed wire running in parallel.

share|improve this answer
3  
Both THHN and NM cable are derated when many conductors are bundled together for long runs. –  BlueRaja - Danny Pflughoeft Sep 16 '13 at 19:49
    
Wouldn't the derating only apply to conduits with more than 3 current carrying conductors? Keep in mind that the EGC is not a current carrying conductor, and the grounded (neutral) conductor is not counted if it carries only the unbalanced current from other conductors of the same circuit. For example, a MWBC would only count as 2. –  Tester101 Sep 17 '13 at 17:11
add comment

It's a heat buildup issue. The outer jacket of NM cable is there to protect the internal conductors from damage. Conduit's purpose is to protect the conductors from damage. Having both is redundant. A conductor's current rating is based on its ability to shed sufficient heat to prevent damage to its insulation at that current rating. When you have NM in conduit you complicate the current rating calculation. It's like wrapping someone in more and more blankets. It may protect them from bumps and bruises, but eventually they die from heat stroke.

Consider a piece of 12ga wire. It's rated for 20amps. The resistance of 12ga wire is 1.588 milliohms per foot. At 20amps, a 12ga piece of wire needs to shed .635 watts per foot. That heat has to be radiated away. If it's allowed to build up, the conductor's insulation could be damaged.

Best practice is never to run NM inside conduit. Even if you work out the current load de-rating and verify that it's safe, you don't know what the next guy may shove into that conduit. Plus, on anything less than short runs, it's next to impossible to pull solid conductor NM through conduit, when you could pull individual conductors of stranded THHN/THWN with far less effort.

share|improve this answer
add comment

You can fit more conductors (More circuits) inside a conduit using THHN then you can with NM Cable. A NM Cable is typically 1/2" wide and carries three conductors, but NM cable is noticeably larger then three THHN wires. You can fit more circuits inside of conduit using THHN then you can with NM Cable, particularly when there are twists & turns or 90-degree bends in your conduit.

Imagine if you were an electrician who was hired to run an additional circuit through preexisting conduit. Normally, one can fit 3 circuits inside of this conduit. You open the box and discover 2 NM cables shoved down the conduit with no room to run THHN wire. Now you can't proceed without running a second conduit. Many electricians wouldn't like this as it means more work.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.