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Or should I use outdoor Romex? I heard that outdoor Romex doesn't need conduit as you are just supposed to bury it.

Edit:
Ack, I should have mentioned I have existing conduit that I want to pull the wire through. It currently has individual wires like user14408 has indicated and it looks shady. That's why I was thinking of replacing it with NM-B or UF cable.

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See this answer for code verbiage. –  Tester101 Sep 17 '13 at 11:59

4 Answers 4

up vote 3 down vote accepted

Since @Tim's post didn't make this clear: under the current code, you are not allowed to run NM-B wire outside, even through a conduit.

I don't have the NEC to verify this; I read it in this book (pg 155, "Wiring Outside"):

Under the 2008 code, you must use UF cable anytime NM-style cable (even in conduit) exits the side of the house. That is, you are no longer allowed to put NM-B cable in watertight conduit outside of the house in wet locations.

The code is vague [..] [about] NM-B cable under the house in a crawl space, or under a deck.


[Edit] The proper NEC code reference is 334.12 Nonmetallic-Sheathed Cable - Uses Not Permitted. See this answer for more details.

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Does the book site the code sections that it's referencing? –  Tester101 Aug 9 '13 at 20:14
    
@Tester101 No, or else I would have listed them here. This page on outdoor NEC requirements agrees with what I said, though. On a side note, is the NEC really not available freely online? That seems insane - you should not have to pay to know what your safety requirements are! –  BlueRaja - Danny Pflughoeft Aug 9 '13 at 20:59
    
@Tester101 Though apparently you know which section it's referencing ;) –  BlueRaja - Danny Pflughoeft Aug 28 '13 at 22:33

What you're calling outdoor Romex is actually called type UF (underground feeder) cable, it resembles Romex, but it's not. UF cable is THHN conductors plus a ground encased completely in PVC. Romex is a brand name for non-metallic paper bonded cable (NM-B, as printed on the jacket) with a PVC jacket. This is why type UF is suitable for burying and wet locations (or dry) where NM-B cable is suitable only for very specific dry locations.

It's rather common to protect or shield Romex with PVC in locations where (wet or not) it might be harmed by objects or people, but if you're going to be running high voltage (120 V+) outdoor lighting or receptacles it's a much better idea to just use UF, or PVC conduit and pull the THHN conductors through it as you need.

For this project you'll want to:

  • Have someone show you how to strip UF cable, it takes a bit of skill to do it and not damage the THHN insulation when removing the PVC casing (or your fingers in the process)

  • Call a utility locating service before you dig to bury the cable, it's got to go at least 18 inches in the ground depending on your location

  • Perhaps hire someone to trench for you if you've got long runs, or intend to just bury PVC conduit and pull your own conductors.

When using UF, please take my advice on getting someone to show you how to strip it, the folks at your local electrical supply or home center will probably be more than happy to show you. Your hands and fingers will thank you. Almost every time I was called to troubleshoot a tripping buried circuit it was because the UF cable wasn't stripped properly, and was shorting due to it.

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Also, Romex® is a brand name and trademark of Southwire (Like saying Kleenex® or Jet Ski®.) Currently the Romex® line is exclusively NM(mostly -B) intended for indoor applications, it has become the go to word to describe all NM-B in general discussions. –  Jason Aug 7 '13 at 13:04
    
Good suggestions folks, incorporated into the answer - feel free to tweak! –  Tim Post Aug 7 '13 at 13:45

I'd check local code.

The state/county I live in allows it, but recommends individual wires rather than sheathed (romex). Also remember sheathed(romex) wire has a lower amp rating than separate wires in conduit. (Romex has lower temperature range in the Amp chart, giving it a lower max amps rating)

Ex: power feed to my workshed thru 1" conduit. #6 romex == 45amps. Same exact wires w/o the vinyl sheath == 50+amps (got my 50A feed)

Personally, I very much prefer conduit to direct burial. Always check local codes. City where my office is located requires a 16g tracer wire 6"+ above buried electric, gas, water, sprinkler (color: green, yellow, blue, purple). Was a surprise to me.

You might want to give more info about location and what your feeding with the line. I'm sure others will be along with better answers.

RJ

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You cannot use nonmetallic sheathed cable (Type NM, NMS) in conduit in any damp, or wet location (NEC 2011 300.5(B), 300.9, 334.12(B)(4)). Instead, you'll want to use individual conductors rated for wet/damp locations.

Thermoplastic Heat and Water-resistant Nylon-coated (THWN) insulated conductors are commonly used in this situation, as well as directly buried underground feeder (UF) cable. Article 310.10(C) of the 2011 National Electrical Code (NEC), lists the types of insulated conductors allowed in wet locations.

National Electrical Code 2011

ARTICLE 310 Conductors for General Wiring

II. Installation

310.10 Uses Permitted. These conductors shall be permitted for use in any of the wiring methods recognized in Chapter 3 and as specified in their respective tables or as permitted elsewhere in this Code.
C) Wet Locations. Insulated conductors and cables used in wet locations shall comply with one of the following:
(1) Be moisture-impervious metal-sheathed
(2) Be types MTW, RHW, RHW-2, TW, THW, THW-2, THHW, THWN, THWN-2, XHHW, XHHW-2, ZW
(3) Be of a type listed for use in wet locations

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