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If I run electrical wiring under a deck, what conduit and strap material / grading do I need? It doesn't have to be the underground / metal materials, right?

I like to build something similar to this

enter image description here

I found it on this website:

http://www.thedeanfamily.com/deck_5_.htm

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Yes, PVC conduit is nice stuff. Make sure to keep water from entering it, or it may carry water to places you don't want water. (that said, all outdoor conduit is presumed to be 100% full of water all the time, so you must use wet-rated wire).

You'll be using single wires in conduit, just like most commercial buildings. You need wire marked for wet locations. There are many grades of wire, but what I find on store shelves is THHN or THWN - the latter has the "W" wet rating. The cost difference is so small that many places only stock THWN -- but check, don't take any THHN.

You can choose solid or stranded wire, stranded is a lot easier to work with, but sometimes more difficult to terminate.

You cannot shuck down Romex wire and use the white and black wires, because the interior wires are not labeled. Shucking down to bare wire for bare grounds is fine.

You will need at least 3 rolls of wire. It's most economical in 500ft rolls.

  • Green or bare for ground wires (if you use them)

  • White or gray wire for neutral (I recommend white, you can use gray to distinguish a neutral from a different circuit.)

  • Any color not mentioned above, for your hots. Most people go black, but you are welcome to install a rainbow of colors if you like, the only rule is you must be reasonably consistent within your installation. So if you use orange for switched-hot for lighting, stick with that.

You will need to run a ground wire with plastic conduit. I recommend you run a ground wire with metal conduit - in the great outdoors, that stuff will rust, even if it's galvanized, which will eventually cause grounding problems.

If you have metal (commonly aluminum) boxes in your system, there will be a hole already tapped 10-32 for a ground screw. Attach a pigtail of solid wire to that screw, and wire-nut it to your other grounds. I use stranded wire myself, and a crimp-on ring terminal.

In your photo, that guy needs more support for some of those pipes, especially the long run on the left. PVC needs support at closer spacing than steel EMT.

Don't even think about buying PVC at the big-box store or any national-chain seller, because they won't have all the parts. They'll have brand X boxes and brand Y box covers, and no, they don't play well together. BTDT. Rather than have to go to the electrical supply house 3 towns over for the missing bits, create a relationship with the electrical supply house in your town and use the system they sell and fully stock. For one thing, it'll be cheaper - big-box creates the illusion of cheap, but nails you on all but the most mainstream.

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    Actually -- most generic building wire is dual rated THHN/THWN – ThreePhaseEel Jun 20 '16 at 0:42
  • Couldn't you use UF wire for this and skip the conduit? – Sherwood Botsford Jul 18 at 14:11
  • @SherwoodBotsford Depends what's underneath it and whether it'll be subject to damage. You might just have an answer there. – Harper - Reinstate Monica Jul 18 at 16:40
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You can use any conduit material for this application. EMT will be cheaper than PVC, in my experience, but use whatever makes you happy.

You MUST use an exterior (wet location) wire. Any exterior conduit is by definition a wet location. A VERY common thing we see here is folks trying to run NM in exterior conduit, which is a violation, since NM is not rated for wet locations, (and also a pain in the behind, since NM is very hard to pull in conduit, and does horrible things to your fill calcuations.) If you strip the NM, you get unrated wires (at least with every NM cable I've ever stripped, the interior wire insulation is not marked; someone could make a real difference for their NM cable by bucking that trend, but nobody does.)

Get individual wires that include a "W" in the wire type (XHW, TWN, etc.) - they will typically have multiple ratings, but at least one of them needs to include the "W"

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  1. PVC conduit is A-OK for burial -- in fact, it's often a superior choice in burial apps because it won't rust out or otherwise degrade in the underground environment, unlike ordinary steel conduit.

  2. PVC conduit is also fine outdoors provided it is marked/specified as sunlight resistant -- although this isn't a concern in an under the deck application as yours. Make sure to support it every 3' and put in expansion joints every 12' to keep your run from doing the worm!

  3. You need to use wet location wire inside conduit when outdoors. THWN works if you want to use individual wires -- if you insist on pulling cable through conduit, you can use UF. NM is not OK though!

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    While you can certainly take the same care that you would with unprotected PVC pipe, PVC electrical conduit is made to be sunlight resistant. "electrical conduit has been tested for sunlight resistance according to UL 651 requirements and is listed for continuous outdoor exposed use. The test is based on izod impact testing, not color retention." jmeagle.com/pdfs/Technical%20Bulletins/… – Ecnerwal Jun 19 '16 at 13:12
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In many jurisdictions using UF (underground feeder) or NMWU (non-metal wet underground cable as long as it's in a situation that is unlikely to be damaged. In my jurisdiction this translates either to being a space where people aren't present, (crawl space, rafter space) or at least 6 feet above the walking surface.

While this type of cable is substantially more expensive, you don't have the price of conduit. I've seen a couple articles saying it's a wash either way.

(I am not an expert on this yet. Grab the salt shaker.)

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