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!?

  • 4
    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 '16 at 17:36
  • I always thought NM had a 60 then Nm-B came along and romex went to a 90 rating – Robert Moody May 24 at 2:43

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

  • 1
    What's the fill ratio -- if the sheath is on, how many are allowed for a given size? – Bryce Jan 31 at 23:28
  • 1
    @Bryce You have to calculate the cross-sectional area of the cable, and then use that to determine fill. – Tester101 Feb 1 at 18:25
  • 1
    A single wire / cable is allowed to be 53% fill where more than 2 wires the max fill is 40%. – Ed Beal Apr 15 at 13:53
  • @Tester101 Can you confirm that this hasn't changed in the 2017 code? I can't find any of these sections in the 2017 code book (redline version). – cryptic0 Jun 23 at 2:58

In addition to Tester101's excellent answer on when NM-B can be run through conduit, there is a section in the NEC which indirectly prohibits running NM-B through conduit in specific scenarios:

From the 2014 NEC:

312.5 Cabinets, Cutout Boxes, and Meter Socket Enclosures. Conductors entering enclosures within the scope of this article shall be protected from abrasion and shall comply with 312.5(A) through (C).

(C) Cables. Where cable is used, each cable shall be secured to the cabinet, cutout box, or meter socket enclosure. Exception: Cables with entirely nonmetallic sheaths shall be permitted to enter the top of a surface-mounted enclosure through one or more nonflexible raceways not less than 450 mm (18 in.) and not more than 3.0 m (10 ft) in length, provided all of the following conditions are met:

(a) Each cable is fastened within 300 mm (12 in.), measured along the sheath, of the outer end of the raceway

(b) The raceway extends directly above the enclosure and does not penetrate a structural ceiling.

(c) A fitting is provided on each end of the raceway to protect the cable(s) from abrasion and the fittings remain accessible after installation.

(d) The raceway is sealed or plugged at the outer end using approved means so as to prevent access to the enclosure through the raceway.

(e) The cable sheath is continuous through the raceway and extends into the enclosure beyond the fitting not less than 6 mm (1⁄4 in.).

(f) The raceway is fastened at its outer end and at other points in accordance with the applicable article.

(g) Where installed as conduit or tubing, the cable fill does not exceed the amount that would be permitted for complete conduit or tubing systems by Table 1 of Chapter 9 of this Code and all applicable notes thereto. Informational Note: See Table 1 in Chapter 9, including Note 9, for allowable cable fill in circular raceways. See 310.15(B)(3)(a) for required ampacity reductions for multiple cables installed in a common raceway.

The fittings used with NM cable to enter a breaker panel also secure the cable to the panel. When conduit is used, I am not aware of another approved method to secure the cable to the breaker panel as required for conduit runs longer than 10 ft. So practically speaking, you cannot have an entire run of NM-B (or any other non-metallic sheathed cable assembly) enclosed in conduit from a breaker panel because it then limits you to a maximum length of 10ft from a surface-mounted cabinet, cutout box, or meter socket enclosure (including breaker panels/boxes in their definitions).

The only direction that NM-B can exit the panel in conduit is out of the top of the panel.

The conduit for runs attached directly to the surface-mounted panels also have to be a nonflexible conduit per this clause as well. I believe flexible conduit is out of the picture because it offers no protection from physical damage anyways and that is the primary practical reason to run NM-B in conduit, so you might as well be directly attaching the cable to the breaker panel.

This clause also implies that runs of NM-B fully enclosed in conduit attached to a recessed-mount panel are prohibited. I believe this is the same reason that non-flexible conduit is not specified because NM-B should be protected from damage by the wall covering anyway for a recess-mount panel and there is no longer need for non-flexible raceways to protect the cable.

  • 1
    It looks like you're reading it wrong. This is about securing the cable, not about whether the cable is allowed. The Code says, "Where cable is used, each cable shall be secured..." with the exception as described in the following conditions, including first and foremost that the cable be secured on the other end of the conduit. Essentially it's saying that if the cable enters the box from the top through a short run of conduit, you're fine securing the cable to the other end of the conduit instead of the inside of the box, provided you follow all the other details. – tylerl Apr 14 at 20:05
  • @tylerl For conduit run greater than 10’, how do you secure NM cable to a breaker panel if it is entering via conduit? Normally, the fittings to enter the panel also do the securing. My point is that if you can't secure the cable via an approved method, then it prohibits use of the cable. – statueuphemism Apr 14 at 22:42
  • The conduit attaches to the box and no clamps are required when entering through conduit. – Ed Beal Apr 15 at 13:56
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    @Ed Beal Exactly, except for the clause cited in my answer which states: “Where cable is used, each cable shall be secured to the cabinet, cutout box, or meter socket enclosure.“ If you are running NM-B in conduit greater than 10’, then you need an approved method to secure the cable to the box. If there is a method for doing this, I am not aware of it. – statueuphemism Apr 15 at 21:50
  • What is going to happen? Thhn in conduit you fasten conduit wires dont fall into panel .Your NM not going to fall in – Robert Moody May 24 at 3:17

You can, but it really isn't worth it...

It's a nightmare to pull

NM (Romex) is already solid wire except in the large sizes, and now you have 3-4 wires bound together. You are trying to pull this around maybe 14" radius elbows, and if the NM gets twists in it (it always does), it really drags in those places. It's just a stiff, miserable pull - enough to "put you off conduit forever" - which is a shame, because conduit is wonderful when using easy-to-pull stranded wire.

Pulling this stiff cable increases the risk of damaging the cable. What's more, "cable in conduit" is often a newbie who chooses cable simply because he is unaware that wire comes in any other forms. Which leads to more risk of damage still.

It requires ridiculously large conduit

You still have to calculate and respect conduit fill rules, and those are very punishing toward cable. Most cable is oblong, and the rules say you use the largest dimension and treat it as a circular wire of that dimension.

  • For a single "wire", you are allowed 53% conduit fill. That means the conduit ID must be at least 137% of the large cross-section of the cable. That's not so bad, but...
  • For two "wires", you're only allowed 31% fill. That's to protect the wires from binding. That means the conduit ID must be at least 2.54 times the cable large dimension.
  • For three "wires", you're allowed 40% fill. That means the conduit must be at least 2.74 times the cable width (at 3 wires), larger for more wires.

For instance, someone wanted to squeeze three #6 cables through a 1" conduit. Now this is "easy peasy" with 6 individual #6 THHN wires and 3 bare #10 grounds. However, with three #6 UF cables, the conduit size needed to be 3" instead of 1". Whoa.

You are certainly allowed to do it, but you will be using rather enormous conduit, and you will have a hard time pulling.

  • Yes not smart to run in long pulls.Guys would run NM short runs to protect it. – Robert Moody May 24 at 3:28

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/

  • Yes you can in a garage to feed a garage door recpt. through bored holes up in the joist fine. It is not subject to harm, Now you start coming down the walls. Things change tools can hit wire ect . Run your NM sieeve down to protect NM in conduit .IT is code and fine . – Robert Moody May 24 at 3:42

EMT to NM 1/2-Inch, 3/8-Inch Coupling To run Romex, non-metallic NM-B sheathed cable in EMT conduit use a listed fitting, as shown above, per the manufacturers instructions.

There's not a lot of cooling wind passing through conduit like aerial power lines or when buried and the dirt carries the heat away so it's a reasonable concern to worry about heat degrading the insulation over time. Then again, the i squared r ohmic heat buildup of cable in conduit isn't substantially worse than ambient conditions of NM surrounded by insulation in a hot summer noon attic.

Install wiring for a ceiling fan on a muggy humid windless day in Georgia in August to appreciate the double H, 194 °F rating.

To make it easier to pull NM in conduit, don't. Strip the sheathing, leaving only the THHN insulated wires to pull, as you would transitioning your circuit from 3/8" MC to EMT.

  • 4
    The wires in NM aren't marked individually, so they are not legal for use outside their sheath... – ThreePhaseEel Mar 13 '18 at 22:16
  • I just stripped back some ROMEX sitting on the floor of the truck. Sure enough, no individual marking on its individual conductor insulation. So you're suggesting NM-B and 3/8" MC should be run in EMT with the sheathing and armor, respectively, left on? – Jules Bartow Mar 14 '18 at 23:16
  • 3
    If you're using the EMT for sleeving to protect from damage, then yes. If it's a box to box conduit run, just transition to THHN for the duration of the conduit run. – ThreePhaseEel Mar 14 '18 at 23:23
  • Yes why would you want to pull NM in along run of conduit ,sleeving yes and protection..But is code , – Robert Moody May 24 at 4:31

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.

  • 3
    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
  • 4
    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
  • 6
    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
  • Does every one agree running NM in long runs of conduit not code .Why would you. NM in short runs for protection allowed. NM in hot attics ok. If you sleeved one #14-2 in 1/2 conduit in a unheated garage how hot is that wire going to be.And if you add one more put it in 3/4. – Robert Moody May 24 at 4:07
  • Its is fine short runs,May start leaving jacket on. Never a problem in NH. If you did a whole garage in emt .Though it is a good way you would not be working much. Your bid would be nuts – Robert Moody May 24 at 4:15

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.

  • 4
    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 '16 at 17:34
  • Actually modern NMB is 90 degree rated. Most panel boards. And devices have lower temp ratings that limits the ampacity of a particular wire gauge. You can use the higher 90deg ampacity to calculate derate If you need to bundle or run in a nipple for more than 24". – Ed Beal Mar 13 '18 at 19:17

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