Can Romex (NM-B) cable be run through conduit?

Question

A few days ago I asked an electrician if I could run cable 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!?

Answer 1

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

Answer 2

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.

Answer 3

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.

Answer 4

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/

Answer 5

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.

Answer 6

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.

Answer 7

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.