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A few days ago I asked an electrician if I could run Romex through conduit - he gave me a funny look and said "No, Romex is not allowed in conduit."

When I searched online, there seems to be a wide belief that Romex cannot be run through conduit, but no one can find the code which states this. See for example here, here, or here. However, in many other places it's stated that you can, for example here or here. And here, someone claims that the NEC says it's only allowed if explicitly stated in local code!

So, which is it? Can I run Romex through conduit!?

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1  
What the myth confuses is when the outer shealth is removed from the NM-B wire, then it no longer has the ANSI markings, which negates the whole point of using ANSI in the first place. NM-B is already derated to 60° because of the sheath, even though the individual conductors are rated at 90°. Putting NM wire without the sheath is prohibited. Same goes for MC cable and any other cable which puts the ANSI markings on the sheath and not on the conductors – Kris Feb 17 at 17:36
up vote 29 down vote accepted

Yes, NM cable can be in conduit. In fact. NEC calls for it to be in conduit, when protection from physical damage is required.

National Electrical Code 2011

ARTICLE 334 Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS

II. Installation

334.15 Exposed Work. In exposed work, except as provided in 300.11(A), cable shall be installed as specified in 334.15(A) through (C).

(B) Protection from Physical Damage. Cable shall be protected from physical damage where necessary by rigid metal conduit, intermediate metal conduit, electrical metallic tubing, Schedule 80 PVC conduit, Type RTRC marked with the suffix -XW, or other approved means. Where passing through a floor, the cable shall be enclosed in rigid metal conduit, intermediate metal conduit, electrical metallic tubing, Schedule 80 PVC conduit, Type RTRC marked with the suffix -XW, or other approved means extending at least 150 mm (6 in.) above the floor. [ROP 7-94] Type NMC cable installed in shallow chases or grooves in masonry, concrete, or adobe shall be protected in accordance with the requirements in 300.4(F) and covered with plaster, adobe, or similar finish.

There's also some notes in Chapter 9, dealing with how to figure for cables when calculating conduit fill.

Chapter 9 Tables

Notes to Tables

(5) For conductors not included in Chapter 9, such as multiconductor cables and optical fiber cables, the actual dimensions shall be used.

(9) A multiconductor cable, optical fiber cable or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.

However, it depends on where the conduit is. If the conduit is underground (or any other damp or wet location), then NM cable is not allowed.

National Electrical Code 2011

ARTICLE 300 Wiring Methods

I. General Requirements

300.5 Underground Installations.
(B) Wet Locations. The interior of enclosures or raceways installed underground shall be considered to be a wet location. Insulated conductors and cables installed in these enclosures or raceways in underground installations shall be listed for use in wet locations and shall comply with 310.10(C). Any connections or splices in an underground installation shall be approved for wet locations.

300.9 Raceways in Wet Locations Above Grade. Where raceways are installed in wet locations above grade, the interior of these raceways shall be considered to be a wet location. Insulated conductors and cables installed in raceways in wet locations above grade shall comply with 310.10(C).

ARTICLE 334 Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS

II. Installation

334.12 Uses Not Permitted.
(B) Types NM and NMS. Types NM and NMS cables shall not be used under the following conditions or in the following locations:

(4) In wet or damp locations

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The heat the NEC is primarily concerned with is INTERNAL, secondary is ambient (air temp). The internal heat is generated by current flow through the wire's resistance and will add to the ambient. (larger wire = lower resistance = less heating and in wire, 14 is smaller than 12 or 10)
The NEC has tables to guide the installer on proper size of wire and conduit to reduce heat and dissipate properly.


The NEC is not an instruction manual but an installation statute (where adopted). If you are planning a DIY project do not use the code for the "how-to". There are many publications at HD, Lowes, and the bookstore that can guide you on DIY electrical projects and likely keep you within the code for those home projects while providing some of the necessary NEC information. Any project not covered in these self-help books should be left to a licensed electrician. Keep in mind the code is the minimum requirement to be followed. Also, someone with "electrician" on the side of the truck does not mean they are licensed or truly knowledgeable.

Conduit may not be required in the code for a wire type or wiring method but it might be desirable for a sense of security against physical damage in your particular installation. Oversizing wire or conduit guarantees that you will allow for heat. NEC is published by the NFPA, National Fire Prevention Assn, Much of the code is intended to prevent electrical fires.

(Bold below is emphasis added by me)

300.4 Protection Against Physical Damage. Where subject to physical damage, conductors, raceways, and cables shall be protected. (A) Cables and Raceways Through Wood Members. (1) Bored Holes. In both exposed and concealed locations, where a cable- or raceway-type wiring method is installed through bored holes in joists, rafters, or wood members, holes shall be bored so that the edge of the hole is not less than 32 mm (1 1⁄4 in.) from the nearest edge of the wood member. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by screws or nails by a steel plate(s) or bushing(s), at least 1.6 mm (1⁄16 in.) thick, and of appropriate length and width installed to cover the area of the wiring. Exception No. 1: Steel plates shall not be required to protect rigid metal conduit, intermediate metal conduit,rigid nonmetallic conduit, or electrical metallic tubing.

With all that, yes, nm and nmc can be run in conduit but it is not a typical practice since it is designed and permitted to be run exposed with some exceptions relating to PROTECTION, remember in all code questions the AHJ (local inspector) and NFPA have final authority of interpretation of correct application. PLEASE, SAFETY FIRST! google for how electricity kills/

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one reason you don't put romex in conduit is because it creates more heat and is not advised in conduit if you have conduit you can run insulated wires instead it's probably cheaper. when you put romex inside conduit The Romex cannot breathe and retains too much heat.I always thought that it was okay. And I asked a licensed electrician. he looked at me funny and said as if it was a stupid question that you wouldn't do that because the wires would get too hot and that it's redundant if you have to use conduit you don't use romex as well its one or the other romex is used in the walls and in the attic for interior locations EMT conduit flex tubing excetra is used in exposed conditions schedule 80 PVC grey conduit underground with wires not in Rome. but of course they are insulated wires.

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1  
Yes we've all heard this, but as far as I can tell it appears to be an urban legend. The conduit is already surrounded by several layers of rubber, a very good insulator of heat - the pipe it all runs through adds very little extra heat-insulation. But whether or not it's an urban legend, it's not supported by NEC code (see above answer). – BlueRaja - Danny Pflughoeft Jun 20 '15 at 1:09
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If your wire gauge releases so much heat that a thin layer of sheathing introduces a risk of overheat issues then you've got bigger problems than simply Romex in conduit. – user515655 Oct 4 '15 at 2:10
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Again code that doesn't make logical sense. Which insulates more, a conduit with air space or being buried in cellulose insulation. Romex in an insulated wall is going to get way hotter than romex in a conduit. I can see other reasons, but heat is a stupid argument. – user45194 Nov 3 '15 at 14:58

No, you can not run your Romex wiring in conduits. What you're looking at in the code book is referring to THHN and other wiring, that while insulated with a single coat is not insulated with a second covering and bound to 2 or 3 other wires. Power produces heat and heat is the main problem here.

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As mentioned in the accepted answer, this is an incorrect myth amongst electricians. The answer quotes the NEC, which explicitly states "Nonmetallic-Sheathed Cable: Types NM ...", which is not referring to THHN. – BlueRaja - Danny Pflughoeft Sep 2 '15 at 16:52
    
What the myth confuses is when the outer shealth is removed from the NM-B wire, then it no longer has the ANSI markings, which negates the whole point of using ANSI in the first place. NM-B is already derated to 60° because of the sheath, even though the individual conductors are rated at 90°. Putting NM wire without the sheath is prohibited. Same goes for MC cable and any other cable which puts the ANSI markings on the sheath and not on the conductors. – Kris Feb 17 at 17:34

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