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Article 344

Rigid Metal Conduit: Type RMC

Part I. General

  1. Scope. This article covers the use, installation, and construction specifications for rigid metal conduit (RMC) and associated fittings.
  2. Definition.

Rigid Metal Conduit (RMC). A threadable raceway of circular cross section designed for the physical protection and routing of conductors and cables and for use as an equipment ground‐ ing conductor when installed with its integral or associated coupling and appropriate fittings.

344.6 Listing Requirements. RMC, factory elbows and couplings, and associated fittings shall be listed.

Part II. Installation

344.10 Uses Permitted.

  1. Atmospheric Conditions and Occupancies.
    1. Galvanized Steel and Stainless Steel RMC. Galvanized steel and stainless steel RMC shall be permitted under all atmospheric conditions and occupancies.
    2. Red Brass RMC. Red brass RMC shall be permitted to be installed for direct burial and swimming pool applications.344.10 ARTICLE 344 — RIGID METAL CONDUIT: TYPE RMC
    3. Aluminum RMC. Aluminum RMC shall be permitted to be installed where approved for the environment. Rigid alumi‐ num conduit encased in concrete or in direct contact with the earth shall be provided with approved supplementary corro‐ sion protection.
    4. Ferrous Raceways and Fittings. Ferrous raceways and fittings protected from corrosion solely by enamel shall be permitted only indoors and in occupancies not subject to severe corrosive influences.
  2. Corrosive Environments.
    1. Galvanized Steel, Stainless Steel, and Red Brass RMC, Elbows, Couplings, and Fittings. Galvanized steel, stainless steel, and red brass RMC elbows, couplings, and fittings shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection approved for the condition.
    2. Supplementary Protection of Aluminum RMC. Aluminum RMC shall be provided with approved supplementary corrosion protection where encased in concrete or in direct contact with the earth.
  3. Cinder Fill. Galvanized steel, stainless steel, and red brass RMC shall be permitted to be installed in or under cinder fill where subject to permanent moisture where protected on all sides by a layer of noncinder concrete not less than 50 mm (2 in.) thick; where the conduit is not less than 450 mm (18 in.) under the fill; or where protected by corrosion protec‐ tion approved for the condition.
  4. Wet Locations. All supports, bolts, straps, screws, and so forth, shall be of corrosion-resistant materials or protected against corrosion by corrosion-resistant materials.

Informational Note: See 300.6 for protection against corrosion.

344.14 Dissimilar Metals. Where practicable, dissimilar metals in contact anywhere in the system shall be avoided to eliminate the possibility of galvanic action. Aluminum fittings and enclo‐ sures shall be permitted to be used with galvanized steel RMC, and galvanized steel fittings and enclosures shall be permitted to be used with aluminum RMC where not subject to severe corrosive influences. Stainless steel RMC shall only be used with stainless steel fittings and approved accessories, outlet boxes, and enclosures.

344.20 Size.

  1. Minimum. RMC smaller than metric designator 16 (trade size 1∕2) shall not be used.Exception: For enclosing the leads of motors as permitted in 430.245(B).
  2. Maximum. RMC larger than metric designator 155 (trade size 6) shall not be used.

Informational Note: See 300.1(C) for the metric designators and trade sizes. These are for identification purposes only and do not relate to actual dimensions.

344.22 Number of Conductors. The number of conductors shall not exceed that permitted by the percentage fill specified in Table 1, Chapter 9.

Cables shall be permitted to be installed where such use is not prohibited by the respective cable articles. The number of

cables shall not exceed the allowable percentage fill specified in Table 1, Chapter 9.

344.24 Bends — How Made. Bends of RMC shall be so made that the conduit will not be damaged and so that the internal diameter of the conduit will not be effectively reduced. The radius of the curve of any field bend to the centerline of the conduit shall not be less than indicated in Table 2, Chapter 9.

344.26 Bends — Number in One Run. There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points, for example, conduit bodies and boxes.

344.28 Reaming and Threading. All cut ends shall be reamed or otherwise finished to remove rough edges. Where conduit is threaded in the field, a standard cutting die with a 1 in 16 taper (3∕4 in. taper per foot) shall be used.

Informational Note: See ANSI/ASME B.1.20.1-1983, Standard for Pipe Threads, General Purpose (Inch).

344.30 Securing and Supporting. RMC shall be installed as a complete system in accordance with 300.18 and shall be securely fastened in place and supported in accordance with 344.30(A) and (B).

  1. Securely Fastened. RMC shall be secured in accordance with one of the following:
    1. RMC shall be securely fastened within 900 mm (3 ft) of each outlet box, junction box, device box, cabinet, conduit body, or other conduit termination.
    2. Fastening shall be permitted to be increased to a distance of 1.5 m (5 ft) where structural members do not readily permit fastening within 900 mm (3 ft).
    3. Where approved, conduit shall not be required to be securely fastened within 900 mm (3 ft) of the service head for above-the-roof termination of a mast.
  2. Supports. RMC shall be supported in accordance with one of the following:
  1. Conduit shall be supported at intervals not exceeding 3 m (10 ft).
  2. The distance between supports for straight runs of conduit shall be permitted in accordance with Table 344.30(B)(2), provided the conduit is made up with threaded couplings and supports that prevent transmis‐ sion of stresses to termination where conduit is deflected between supports.
  3. Exposed vertical risers from industrial machinery or fixed equipment shall be permitted to be supported at intervals not exceeding 6 m (20 ft) if the conduit is made up with threaded couplings, the conduit is supported and securely fastened at the top and bottom of the riser, and no other means of intermediate support is readily availa‐ ble.
  4. Horizontal runs of RMC supported by openings through framing members at intervals not exceeding 3 m (10 ft) and securely fastened within 900 mm (3 ft) of termina‐ tion points shall be permitted.

344.42 Couplings and Connectors.

  1. Threadless. Threadless couplings and connectors used with conduit shall be made tight. Where buried in masonry or concrete, they shall be the concrete tight type. Where installed in wet locations, they shall comply with 314.15. ThreadlessARTICLE 348 — FLEXIBLE METAL CONDUIT: TYPE FMC 348.24Table 344.30(B)(2) Supports for Rigid Metal ConduitimageMaximum Distance Between348.2 Definition.Flexible Metal Conduit (FMC). A raceway of circular crossMetric Designator
    Trade Size

    m
    ft16–211∕2–3∕4
    3.010271
    3.71235–4111∕4–11∕2
    4.31453–632–21∕2
    4.91678 and larger3 and larger
    6.120Conduit SizeRigid Metal Conduit Supportssection made of helically wound, formed, interlocked metal strip.348.6 Listing Requirements. FMC and associated fittings shall be listed.
    couplings and connectors shall not be used on threaded conduit ends unless listed for the purpose.
  2. Running Threads. Running threads shall not be used on conduit for connection at couplings.

344.46 Bushings. Where a conduit enters a box, fitting, or other enclosure, a bushing shall be provided to protect the wires from abrasion unless the box, fitting, or enclosure is designed to provide such protection.

Informational Note: See 300.4(G) for the protection of conduc‐ tors sizes 4 AWG and larger at bushings.

344.56 Splices and Taps. Splices and taps shall be made in accordance with 300.15.

344.60 Grounding. RMC shall be permitted as an equipment grounding conductor.

Part III. Construction Specifications

344.100 Construction. RMC shall be made of one of the following:

  1. Steel with protective coatings
  2. Aluminum
  3. Red brass
  4. Stainless steel

344.120 Marking. Each length shall be clearly and durably identified in every 3 m (10 ft) as required in the first sentence of 110.21(A). Nonferrous conduit of corrosion-resistant mate‐ rial shall have suitable markings.

344.130 Standard Lengths. The standard length of RMC shall be 3.05 m (10 ft), including an attached coupling, and each end shall be threaded. Longer or shorter lengths with or with‐ out coupling and threaded or unthreaded shall be permitted.

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