•346-1. Use. The use of listed rigid metal conduit shall be permitted under all atmospheric conditions and occupancies subject to the following:

(a) Protected by Enamel. Ferrous raceways and fittings protected from corrosion solely by enamel shall be permitted only indoors and in occupancies not subject to severe corrosive influences.

(b) Dissimilar Metals. Where practicable, dissimilar metals in contact anywhere in the system shall be avoided to eliminate the possibility of galvanic action.

Exception: Aluminum fittings and enclosures shall be permitted to be used with steel rigid metal conduit, and steel fittings and enclosures shall be permitted to be used with aluminum rigid metal conduit.

(c) Corrosion Protection. Ferrous or nonferrous metal conduit, 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 and judged suitable for the condition.

(FPN): See Section 300-6 for protection against corrosion.

•346-2. Other Articles. Installations of rigid metal conduit shall comply with the applicable provisions of Article 300.

A. Installation

•346-3. Cinder Fill. Rigid metal conduit 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 2 in. (50.8 mm) thick; where the conduit is not less than 18 in. (457 mm) under the fill; or where protected by corrosion protection and judged suitable for the condition.

(FPN): See Section 300-6 for protection against corrosion.

•346-4. Wet Locations. All supports, bolts, straps, screws, etc., shall be of corrosion-resistant materials or protected against corrosion by corrosion-resistant materials.

(FPN): See Section 300-6 for protection against corrosion.

•346-5. Size.

(a) Minimum. Rigid metal conduit smaller than 1/2-in. electrical trade size shall not be used.

Exception: For enclosing the leads of motors as permitted in Section 430-145(b).

(b) Maximum. Rigid metal conduit larger than 6-in. electrical trade size shall not be used.

(FPN): Metric trade numerical designations for rigid metal conduit are the same as those found in Extra-heavy Duty Rigid Steel Conduits for Electrical Installations, IEC 981-1989; namely, 1/2 = 16,3/4 = 21, 1 = 27, 1 1/4 = 35, 1 1/2 = 41, 2 = 53, 2 1/2 = 63, 3 = 78, 3 1/2 = 91, 4 = 103, 5 = 129, 6 = 155.

•346-6. Number of Conductors in Conduit. The number of conductors permitted in a single conduit shall not exceed the percentage fill specified in Table 1, Chapter 9.

•346-7. Reaming and Threading.

(a) Reamed. All cut ends of conduits shall be reamed or otherwise finished to remove rough edges.

(b) Threaded. Where conduit is threaded in the field, a standard conduit cutting die with a 3/4-in. taper per ft (1 in 16) shall be used.

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

•346-8. Bushings. Where a conduit enters a box, fitting, or other enclosure, a bushing shall be provided to protect the wire from abrasion unless the box, fitting, or enclosure design provides equivalent protection.

(FPN): See Section 300-4(f) for the protection of conductors at bushings.

•346-9. Couplings and Connectors.

(a) Threadless. Threadless couplings and connectors used with conduit shall be made tight. Where buried in masonry or concrete, they shall be of the concretetight type. Where installed in wet locations, they shall be of the raintight type.

(b) Running Threads. Running threads shall not be used on conduit for connection at couplings.

•346-10. Bends — How Made. Bends of rigid metal conduit shall be so made that the conduit will not be damaged, and that the internal diameter of the conduit will not be effectively reduced. The radius of the curve of the inner edge of any field bend shall not be less than shown in Table 346-10.

Exception: For field bends for conductors without lead sheath and made with a single operation (one shot) bending machine designed for the purpose, the minimum radius shall not be less than indicated in Table 346-10, Exception.

•346-11. Bends — Number in One Run. There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points, e.g., conduit bodies and boxes.

•346-12. Supports. Rigid metal conduit shall be installed as a complete system as provided in Article 300 and shall be securely fastened in place. Conduit shall be supported at least every 10 ft (3.05 m). In addition, conduit shall be securely fastened within 3 ft (914 mm) of each outlet box, junction box, device box, cabinet, conduit body, or other conduit termination. Fastening shall be permitted to be increased to a distance of 5 ft (1.52 m) where structural members do not readily permit fastening within 3 ft (914 mm).

Exception No. 1: If made up with threaded couplings, it shall be permissible to support straight runs of rigid metal conduit in accordance with Table 346-12, provided such supports prevent transmission of stresses to termination where conduit is deflected between supports.

Exception No. 2: The distance between supports shall be permitted to be increased to 20 ft (6.1 m) for exposed vertical risers from industrial machinery, provided that the conduit is made up with threaded couplings, is firmly supported at the top and bottom of the riser, and no other means of intermediate support is readily available.

Exception No. 3: Where approved, conduit shall not be required to be securely fastened within 3 ft (914 mm) of the service head for above-the-roof termination of a mast.

Exception No. 4: Horizontal runs of rigid metal conduit supported by openings through framing members at intervals not greater than 10 ft (3.05 m) and securely fastened within 3 ft (914 mm) of termination points shall be permitted.

•346-13. Boxes and Fittings. Boxes and fittings shall comply with the applicable provisions of Article 370.

•346-14. Splices and Taps. Splices and taps shall be made in accordance with Section 300-15. See Article 370 for rules on the installation and use of boxes and conduit bodies.

B. Construction Specifications

•346-15. General. Rigid metal conduit shall comply with (a) through (c) below:

(a) Standard Lengths. Rigid metal conduit as shipped shall be in standard lengths of 10 ft (3.05 m), including coupling, one coupling to be furnished with each length. Each length shall be reamed and threaded on each end. For specific applications or uses, it shall be permissible to ship standard lengths or lengths shorter or longer than 10 ft (3.05 m) with or without couplings and with or without threads.

(b) Corrosion-Resistant Material. Nonferrous conduit of corrosion-resistant material shall have suitable markings.

(c) Durably Identified. Each length shall be clearly and durably identified in every 10 ft (3.05 m) as required in the first sentence of Section 110-21.