5.5 Raceways: RMC, IMC, EMT, FMC, LFMC, PVC and ENT

The Common Rules

Every raceway article in Chapter 3 shares a structure: .10 Uses Permitted, .12 Uses Not Permitted, .20 Size, .22 Number of Conductors, .24 Bends — How Made, .26 Bends — Number in One Run, .28 Reaming and Threading, .30 Securing and Supporting, .42 Couplings and Connectors, .46 Bushings.

Two rules are nearly universal:

• Bends — number in one run: there shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points such as conduit bodies and boxes. This applies to RMC, IMC, EMT, FMC, LFMC, PVC, RTRC, and ENT.
• Bushings: where a raceway terminates at a box, fitting, or enclosure, a bushing shall be provided to protect the conductors from abrasion unless the design of the box, fitting, or enclosure provides equivalent protection.

Rigid Metal Conduit — Article 344

RMC is the heaviest and most damage-resistant raceway, permitted under all atmospheric conditions and occupancies per 344.10(A). Galvanized steel RMC may be installed in concrete, in direct contact with earth, and in severe corrosive influences where protected by corrosion protection and judged suitable for the condition. Aluminum RMC in concrete or earth requires approved supplementary corrosion protection.

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

344.30(A) Securely Fastened. RMC shall be secured within 3 feet (900 mm) of each outlet box, junction box, device box, cabinet, conduit body, or other conduit termination.

344.30(B)(2) Supports. Per Table 344.30(B)(2):

An exception permits straight runs of RMC made up with threaded couplings to be supported per this table, and the 3-foot rule may be increased to 5 feet where structural members do not readily permit fastening within 3 feet.

Intermediate Metal Conduit (Article 342) has the same uses and support rules as RMC but a thinner wall and lighter weight, with the same outside diameter and thread specification.

Electrical Metallic Tubing — Article 358

EMT is unthreaded thinwall tubing, lighter and cheaper than RMC, and by far the most common commercial raceway.

358.10 Uses Permitted. Exposed and concealed work; in concrete, in direct contact with earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable; in wet locations where all supports, bolts, straps, and screws are protected against corrosion.

358.12 Uses Not Permitted. Where subject to severe physical damage; for the support of luminaires or other equipment except conduit bodies no larger than the largest trade size of the tubing; where practicable, dissimilar metals in contact.

358.30(A) Securely Fastened. EMT shall be securely fastened in place at least every 10 feet (3 m) and within 3 feet (900 mm) of each outlet box, junction box, device box, cabinet, conduit body, or other tubing termination. An exception permits unbroken lengths of unthreaded EMT to be fastened within 5 feet of termination where structural members do not readily permit 3 feet.

358.42 Couplings and Connectors must be made up tight; where buried in masonry or concrete or installed in wet locations, they must be concretetight or raintight respectively.

PVC Conduit — Article 352

352.10 Uses Permitted includes concealed work in walls, floors, and ceilings; corrosive influences; cinder fill; wet locations; dry and damp locations; and 352.10(F) Exposed — PVC conduit is permitted for exposed work where not subject to physical damage if identified for such use. Schedule 80 PVC is identified for use where subject to physical damage, which is the practical reason it appears on exposed exterior risers and in parking structures.

352.12 Uses Not Permitted. In hazardous locations except as permitted; for the support of luminaires or other equipment; where subject to ambient temperatures exceeding those for which the conduit is listed; for conductors whose insulation temperature limitations would exceed those for which the conduit is listed; in theaters and similar areas except as provided; where exposed to physical damage unless identified for such use; and where subject to ambient temperature exceeding 50 degrees C (122 degrees F) unless listed otherwise.

352.30(B) Supports. PVC requires much closer support than metal raceways because it sags:

PVC must also be secured within 3 feet of each box, cabinet, or other termination, and 352.30(A) requires supports to allow for movement resulting from thermal expansion or contraction — the tie-in to 352.44 covered in section 5.2.

352.44 Expansion Fittings and 352.48 Joints (all joints made by an approved method) complete the article. Electrical Nonmetallic Tubing (Article 362) is the corrugated, flexible nonmetallic cousin, limited to buildings not exceeding three floors above grade and prohibited in hazardous locations, for support of luminaires, and where ambient exceeds 50 degrees C.

Flexible Metal Conduit — Article 348

348.10 Uses Permitted. FMC is permitted in exposed and concealed locations.

348.12 Uses Not Permitted. In wet locations; in hoistways other than as permitted in 620.21(A)(1); in storage battery rooms; in hazardous locations except as permitted; where exposed to materials having a deteriorating effect; underground or embedded in poured concrete or aggregate; where subject to physical damage.

348.20(A) Minimum Size. FMC less than trade size 1/2 shall not be used except as permitted for enclosing motor leads, in lengths not exceeding 6 feet as part of a listed assembly or for tap connections to luminaires, in manufactured wiring systems, or in hoistways.

348.30(A) Securely Fastened. FMC shall be secured at intervals not exceeding 4.5 feet and within 12 inches of each box or fitting. Exceptions allow lengths not exceeding 3 feet at terminals where flexibility is necessary, 6 feet from a luminaire terminal connection for tap connections in accessible ceilings, and 6 feet from the last point where securely fastened for connections within an accessible ceiling.

348.60 Grounding and Bonding. Where used to connect equipment where flexibility is necessary after installation, an equipment grounding conductor shall be installed. Where flexibility is not required, FMC may qualify as an equipment grounding conductor under the conditions of 250.118(5).

FMC as an Equipment Grounding Conductor — 250.118(5)

FMC may serve as the equipment grounding conductor only where all of the following are met:

1. The conduit is terminated in listed fittings;
2. The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less;
3. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 6 feet;
4. Where used to connect equipment where flexibility is required after installation, an equipment grounding conductor shall be installed.

Liquidtight Flexible Metal Conduit (Article 350) has parallel rules. 250.118(6) permits LFMC as an EGC in trade sizes 3/8 through 1/2 with a 20 A overcurrent device, and trade sizes 3/4 through 1-1/4 with a 60 A device, subject to the same 6-foot combined length and listed-fitting conditions. LFMC is permitted in wet locations where listed and marked for the purpose, which FMC is not.

Liquidtight Flexible Nonmetallic Conduit (Article 356) contains no metallic path at all and always requires a wire-type EGC.

Physical Damage and Cold-Weather Selection

On Alaska exteriors the raceway choice is usually driven by two constraints that pull in opposite directions: PVC is immune to corrosion but moves a great deal thermally and becomes progressively less impact resistant as temperature falls, while galvanized steel RMC barely moves thermally and takes impact well but must be protected against corrosion in aggressive soils.

The Code hooks are 352.12 (PVC not permitted where subject to physical damage unless identified for such use), 352.10(F) with Schedule 80 for damage-prone exposed locations, 344.10(B) for RMC corrosion protection in concrete and earth, and 300.6 for the general protection-against-corrosion requirement. Where a manufacturer publishes a minimum installation or handling temperature for a conduit or fitting, 110.3(B) makes that limit enforceable — that is the correct Code basis for cold-weather material limits, rather than any Alaska amendment, because Alaska adopts the NEC without technical amendments.