UF or NM cable?

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  #1  
Old 05-15-05, 07:30 PM
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UF or NM cable?

I'm installing a new fiberglass lamp post. I am running 3/4" PVC from the house to inside the lamp post. My question is which wire type to run, UF or NM. It will be 14 ga., but I'm not sure which type to run in conduit per code. Or, is there another type of wire that would be more desirable to use. TIA for all your help.
 
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  #2  
Old 05-15-05, 07:52 PM
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Well, one thing is perfectly clear. NM is not one of the options.

You are allowed to direct bury UF (without conduit) 24" deep if not GFCI protected or 12" deep if GFCI protected.

Despite what some people will say (based on poor wording in the 2002 NEC), you are allowed to run UF in conduit if you really want to. But almost no one would recommend that you use UF in conduit, but rather use individual THHN/THWN (dual rated) wires.

Personally, I'd recommend direct-buried (12" or more) GFCI-protected UF, not in conduit, unless local codes require conduit or unless the run will be in an area subject to frequent digging (e.g., under a garden).
 
  #3  
Old 05-16-05, 09:30 AM
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Thanks for the reply John. Here is what I did. 15-20 years ago I installed the a wooden lamp post. I tied it into an existing outside outlet, which the wire came from the house and into a weather proof receptacle box, (PVC). This is circuit is GFCI-protected, (breaker). I ran conduit from the recrptacle box to the lamp post. I ran the conduit because there are some shrubs and flowers in the area planted where the conduit goes through. I didn't want myself, or someone else, if I sold the house, to dig and hit a buried cable. I have to install new wire because I moved the new post and because of the way I have to route the wire for the new post. I did have UF in the conduit. I have a couple of other questions for you. I assume the THHN/THWN is available in the big box stores? What do you recommend as colors to purchase, (black (hot), white (netrual) & green (ground)? Is this wire stranded or solid? The receptacle that I want to tie into is only good for solid wire I think. I've done basic wiring, but I've never used this type of wire before. Just for my own information should/or can UF cable be ran in conduit? Thanks again for your help. I'll keep you posted on how my project turns out.
 
  #4  
Old 05-16-05, 09:39 AM
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You should purchase black, white and green. Since you are talking about underground and outside, the wire must be waterproof. Conduit gets wet inside, so don't think you can get away with non-waterproof wire. Most THHN is also rated THWN, so you shouldn't have a problem.

I have never heard of receptacles and switches that you cannot use stranded wire on. All I can think of is the cheap devices that only have backstabs, and don;t have screw terminals. None of us here would recommend that you use this type of device, or use backstabs at all for that matter. Even if you couldn't connect the stranded wire to the devices, you can install a wire nut and connect the stranded wire to a short pigtail of solid wire.

As John said, you will get conflicting stories on whether you can or should run UF cable or NM cable in conduit. You can certainly do so for protection for short runs, the differences in opinion come when you discuss an entire run. As we said earlier, NM is out in your case due to the outside underground water issue.

The issue with cable in conduit has to do with heat and cable fill. This is not a real problem when you are talking about a single piece of UF or NM, but does become a problem with more than one piece. However, it is much easier to pull three single wires, especially stranded wires, than it is to pull cable of any sort.
 
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Old 05-16-05, 10:26 AM
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Originally Posted by racraft
The issue with cable in conduit has to do with heat and cable fill. This is not a real problem when you are talking about a single piece of UF or NM, but does become a problem with more than one piece. However, it is much easier to pull three single wires, especially stranded wires, than it is to pull cable of any sort.
The pull will be the big issue in deciding between cable vs. wires in conduit. If you want to use cable in conduit, upsize the conduit.

For the same size conduit, the choice between cable vs. wires does not affect the heat. What affects the heat is the number of current carrying conductors (and how much current they are carrying), and the thermodynamics of the conduit's release of that heat to the environment. Two cables of 12/2 carrying 16 amps each is going to have as much heat as four individual wires of 12 AWG carrying 16 amps. Whether you have air, water, or anything else, inside that conduit, it's all going to get as hot as the wires once the conditions stabilize. This is why the code addresses cable fill derating in terms of number of conductors.

Still, a larger conduit can release the heat to the surrounding environment better than a smaller conduit, because it has more surface area exposed on the outside. And a larger conduit is easier to pull wire or cable through.
 
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Old 05-16-05, 12:04 PM
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What I meant by heat and cable fill had to do with physical cable fill and ease of dissipation. Obviously individual conductors take up less space than a cable assembly. To escape from cable and conduit, the heat must pass through the insulation, the cable jacket and the conduit. To escape from individual conductors, it only has to dissipate through the insulation and the conduit.
 
  #7  
Old 05-16-05, 12:10 PM
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Note also that nm is rated 60c and thwn is rated 90c. Also note that the conductors in nm are usually thwn, if it's fairly new, which means the conductors are rated wet conditions. If you have a short run and already have the nm and want to run individual conductors then strip it and take the conductors out of the sheath.
 
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Old 05-16-05, 03:38 PM
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Although the construction of the conductors inside of NM is generally known to be the same as THHN/THWN, the conductors are officially unrated if removed from the sheath. I would not recommend that you unsheath a cable and use the wires inside.
 
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Old 05-16-05, 05:08 PM
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Big Mac,
THHN is available in stranded or solid. The stranded wire is easier to pull through conduit. But you are probably only pulling three wires so solid wire should be fine. The big box store will have the THHN/THWN wire you need.
 
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Old 05-17-05, 09:08 AM
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Thanks for all the info. You guys are great. In my statement in an earlier post I stated that the receptacle had on it for solid wire only. I checked some spare receptacles I have on hand and it has printed on the back “For solid core”. I thought that was for all connections on the receptacle, but it is only for the “back” or “stab” connections on the receptacle which I don’t use any way. This is why I thought I couldn’t use stranded wire on a standard receptacle. I always use the screw connections anyway. As for the wires in my project does it matter if I use THHN or THHW? There has been some discussion about heat/condensation/water in conduit, so I want to use the best wire available for the application. One other question, should I use a wire lubricant on the wires when pulling them through the conduit? I know there is a special lubricant just for this purpose. Thanks again for all your help.
 
  #11  
Old 05-17-05, 09:39 AM
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One other question, should I use a wire lubricant on the wires when pulling them through the conduit?
Yes.......
 
  #12  
Old 05-17-05, 10:07 AM
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You need THWN for underground conduit, as it will fill with water due to condensation.
 
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