Article 344 Rigid Metal Conduit Type RMC

I. General





Rigid metal conduit is a raceway that is listed, has a cross section that is circular, has integral or associated couplings, is used with listed fittings to give electrical continuity, and is approved for the installation of electrical conductors.


Listing Requirements

This type of conduit, associated fittings, factory elbows and couplings must be listed.

II. Installation


Uses Permitted

(A) All Atmospheric Conditions and Occupancies. It is permitted in all atmospheric conditions and all occupancies. If the conduit's protection against corrosion is only enamel, it can be used only indoors and in areas that do not have severe corrosive problems. Aluminum enclosures and fittings can be used with steel conduit and vice versa.

(B) Corrosion Environments. If the conduit is protected from corrosion and is suitable, the conduit, elbows, couplings, and fittings can be installed in concrete, in areas where there is a severe corrosive problem, and in direct contact with the earth.

(C) Cinder Fill. It cannot be used in or under cinder fill when there may be permanent moisture unless it is suitable for this type of installation, there are 50 mm (2 in.) or more of noncinder concrete all around it, it is at least 450 mm (18 in.) under the fill, or where it is suitable for the condition and has corrosion protection.

(D) Wet Locations. Everything must be corrosion-resistant (bolts, straps, etc.).


Dissimilar Metals

Contact between dissimilar metals must be avoided where possible to eliminate galvanic action. Aluminum fittings are permitted with steel and vice versa where not subject to severe corrosion.



Metric designator 16 (trade size ½) is the minimum size permitted unless used for enclosing motor leads [see Section 430.145(B)]. The maximum size permitted is metric designator 155 (trade size 6). See 300.1(C) for metric designators.


Number of Conductors in Conduit

Refer to Chapter 9, Table 1. Cables can be used if not prohibited by cable articles. Cable cannot exceed fill requirements of Table 1, Chapter 9.


BendsHow Made

The conduit must not be damaged and the internal diameter will not be reduced. Refer to Table 2, Chapter 9, for the radius to the center line of the conduit for field bends.


BendsNumber in One Run

The maximum that is permitted is not more than 360° in bends between pull points.


Reaming and Threading

All rough edges must be removed when the conduit is cut. When threaded in the field, a tapered cutting die 1 in 16 taper (¾ in. taper per foot) must be used.


Securing and Supporting

The conduit must be supported at intervals no longer than 3 m (10 ft.) and fastened not more than 900 mm (3 ft.) from a box, fitting, etc. It must be installed as a complete system in accordance with 300.18. Fastening can be increased to 1.5 m (5 ft.) where structural members do not permit fastening within 900 mm (3 ft.). For supports for straight runs with threaded couplings, refer to Table 344.30(B)(2). No stress is to be transmitted to the terminations. Vertical risers from industrial machinery or fixed equipment can have the distance between supports at 6 m (20 ft.) if the conduit uses threaded couplings and is properly supported at the top and bottom. Where approved, the conduit does not have to be fastened within 900 mm (3 ft.) of a service head for a termination of a mast above the roof. Horizontal runs that are supported by openings through framing members not less than 3 m (10 ft.) and fastened within 900 mm (3 ft.) of termination points are permitted.


Couplings and Connectors

Threadless couplings and connectors can be used and made tight. Running threads cannot be used. Use concrete type in masonry or concrete. Comply with 314.15(A) in wet locations. Do not use threadless couplings and connectors on threaded conduit unless it is listed for that use.



A bushing must be used at the point where the conduit goes into a box, fitting, or other enclosure. This is to protect the wire. An exception is made if protection is provided with the box or fitting.


Splices and Taps

Refer to Article 300.15.



It is permitted as an equipment grounding conductor.

III. Construction Specifications



If the conduit is nonferrous of corrosion-resistant material, it must be marked to indicate it. At every 3 m (10 ft.) the conduit must be marked with identification (refer to Section 110.21).


Standard Lengths

This type of conduit has a standard in length of 3 m (10 ft.) including one attached coupling. It has to be threaded at each end. Smaller and larger lengths can be used with or without couplings and threads.

Article 90 Introduction


Wiring and Protection

Wiring Methods and Materials

Equipment for General Use

Special Occupancies

Special Equipment

Special Conditions


Annex C. Conduit and Tubing Fill Tables for Conductors and Fixture Wires of the Same Size

Annex D. Examples

Annex E. Types of Construction

Pocket Guide to the National Electrical Code 2005
Pocket Guide to the National Electrical Code(R), 2005 Edition (8th Edition)
ISBN: 0131480014
EAN: 2147483647
Year: 2004
Pages: 120 © 2008-2020.
If you may any questions please contact us: