NEC Requirements for Metal Conduits

The NEC Articles covering conduit start with Art. 342 and end with Art. 356. The first four conduits are the metal ones:

• Intermediate metal conduit (IMC) [Art. 342].
• Rigid metal conduit (RMC) [Art. 344] (Fig. 1).
• Flexible metal conduit (Type FMC) [Art. 348].
• Liquidtight flexible metal conduit (Type LFMC) [Art. 350].

Intermediate and rigid

You can use IMC and RMC in all atmospheric conditions and occupancies [Sec. 342.10, Sec. 344.10]. RMC, commonly called “rigid,” has long been the most common raceway for protecting conductors from physical damage. IMC is also used for this purpose.

IMC is a circular metal raceway with the same outside diameter as RMC. IMC has a wall thickness less than that of RMC, so it has more space for conductors. IMC is lighter and less expensive than RMC and is approved for the same applications. The steel alloy used for IMC makes it stronger than RMC even though the walls are thinner.

Where practicable, avoid putting metal raceway in contact with dissimilar metals; galvanic action corrodes the metal [Sec. 342.14, Sec. 344.14].

The minimum trade size is 1/2-in. for both IMC and RMC. The maximum is trade size 4 for IMC and trade size 6 for RMC [Sec. 342.20, Sec. 344.20].

For complete systems, the number of conductors cannot exceed the percentage fill in Chapter 9, Table 1. Raceways must be large enough to permit the installation and removal of conductors without damaging the insulation [Sec. 342.22 and Sec. 344.22].

Conduit bends can’t have kinks [Sec. 342.24, Sec. 344.24]. To comply, use a bender per the manufacturer’s instructions. The total bends (including offsets) between pull points cannot exceed 360° [Sec. 342.26, Sec. 344.26].

The rules for securing IMC and RMC are the same [Sec. 342.30, Sec. 344.30]. For example:

• Fasten within 3 ft of each outlet box, junction box, cabinet, conduit body, or other conduit termination.
• Support the raceway at intervals not exceeding 10 ft.
• Straight runs with threaded couplings can be supported per the distances in Table 344.30(B)(2).
• Exposed vertical risers for fixed equipment can be supported at intervals not exceeding 20 ft if the conduit is made up with threaded couplings, firmly supported, and securely fastened at the top and bottom of the riser. And if no other means of support is available.

Threadless couplings and connectors must be made up tight to maintain an effective ground-fault current path to safely conduct fault current per Sec. 250.4(A)(5), Sec. 250.96(A), and Sec. 300.10 [Sec. 342.42(A), Sec.344.42(A)]. If buried in masonry or concrete, threadless fittings must be of the concrete-tight type. Fittings installed in wet locations must be listed for use in wet locations to prevent moisture or water from entering or accumulating within the enclosure per Sec. 314.15.

Don’t use running threads for the connection of couplings (they are permitted at other locations) [Sec. 342.42(B), Sec. 344.42(B)].

To protect conductors from abrasion, install a metal or plastic bushing on conduit termination threads (regardless of conductor size) unless the box, fitting, or enclosure already provides this protection [Sec. 342.46, Sec. 344.46].

Flexible and liquidtight flexible metal conduit 

FMC, commonly called “flex” or “Greenfield” (after its inventor), is an interlocked metal strip type of raceway. It is used primarily where flexibility is necessary or where equipment moves or vibrates. To ensure the longevity of FMC and LMFC, run these in a smooth arc rather than a tight bend or (at the other extreme) a nearly straight run.

LFMC is commonly called “liquidtight.” It is similar in construction to FMC, but has an outer liquidtight thermoplastic covering. LFMC has the same primary purpose as FMC, but (when used with LFMC fittings and connectors) also provides protection from liquids and from some corrosive effects.

As you might guess, the requirements for FMC and LFMC are similar. One unsurprising difference is you cannot use FMC in wet locations [Sec. 348.10(12)(1)], but that is exactly where you’d use LFMC [Sec. 350.12] (Fig. 2).

The one usage restriction on LFMC is you can’t use it where subject to physical damage. By contrast, FMC has that restriction and five more (besides wet locations). For example, you also can’t use FMC [Sec. 348.12] in hoistways other than as permitted by 620.21(A)(1), or in battery storage rooms.

FMC and LFMC can be used in exposed or concealed locations [Sec. 348.10, 350.10], but four locations for LFMC are enumerated [Sec. 350.10]. For example, you can use LMFC if flexibility or protection from machine oils, liquids, vapors, or solids is required.

The maximum trade size is 4 for both FMC and LFMC [Sec. 348.20(B), Sec. 350.20(B)]. You can’t use FMC or LFMC smaller than 1/2 in. [Sec. 348.20(A), Sec. 350.20(A)], but you can go as low as 3/8 in. for these applications [Sec. 348.20(A) and Sec. 350.20(A) Exception]:

(1) For enclosing motor leads.

(2) Not exceeding 6 ft in length.

a. For utilization equipment.

b. As part of a listed assembly.

c. For luminaire tap connections per Sec. 410.117(C).

The number of conductors in FMC or LFMC can’t exceed the percentage fill in Chapter 9, Table 1 [Sec. 348.22, Sec. 350.22]. Raceways must be large enough to permit the installation and removal of conductors without damaging the insulation.

Cables can be installed in FMC or LFMC if the total area of cables does not exceed the allowable percentage fill in Chapter 9, Table 1 and there’s no conflict with the respective cable Article(s).

When bending, ensure the conduit will not be damaged and its internal diameter will not be effectively reduced. The radius of the curve of the inner edge of any field bend cannot be less than shown in Chapter 9, Table 2, using the column “Other Bends” [Sec. 348.24, Sec. 350.24]. The total bends (including offsets) between pull points can’t exceed 360° for these flexible conduits [Sec. 348.26, Sec. 350.26].

The cut ends of FMC and LFMC must be trimmed to remove the rough edges [Sec. 348.28, Sec. 350.28], but for FMC this is not necessary if fittings are threaded into the convolutions [Sec. 348.28].

FMC and LFMC must be securely fastened by an approved means within 1 ft of termination and at intervals not exceeding 41/2 ft [Sec. 348.30(A), Sec. 350.30(A)].

Exception  No. 1: They don’t have to be securely fastened or supported where fished between access points through concealed spaces and supporting is impractical.

Exception No. 2: If flexibility is necessary after installation, unsecured lengths from the last point where the raceway is securely fastened can’t exceed:

(1) 3 ft for trade sizes ½ through 1¼

(2) 4 ft for trade sizes 1½ through 2

(3) 5 ft for trade sizes 2½ and larger

Exception No. 4: Lengths not exceeding 6 ft from the last point where the raceway is securely fastened can be unsecured within an accessible ceiling for luminaires or other equipment. Listed fittings are considered a means of securement and support.

New with the 2023 NEC is additional text following the four exceptions for LFMC: “For the purposes of these exceptions, listed LFMC fittings are permitted as a means of securement and support.” This sentence means using LFMC fittings for securing and supporting applies only to installations made using one of the four exceptions. It does not give permission to use these fittings to secure and support LFMC in all applications.

FMC or LFMC installed horizontally in bored or punched holes in wood or metal framing members, or notches in wooden members, at intervals not more than 41/2 ft is considered supported. But the raceway must be secured within 1 ft of terminations [Sec. 348.30(B) and Sec. 350.30(B)].

Equipment grounding conductors

You can use IMC and RMC as equipment grounding conductors (EGCs) [Sec. 342.60, Sec. 344.60] with no additional requirements. IMC and RMC are excellent EGCs, because the conduit radius greatly exceeds the radius of any conductor you can run inside it (important for high frequency current, which travels in the “skin”). Ensure all connections are mechanically sound and electrically continuous.

There are three EGC rules that apply to FMC or LFMC:

1. If flexibility is necessary (to minimize the transmission of vibration from equipment or because the equipment requires movement) after installation, an EGC of the wire type must be installed with the circuit conductors per Sec. 250.118(5) [Sec. 348.60, Sec. 350.60].
2. If flexibility is not necessary after installation and vibration is not a concern, the metal armor of FMC or LFMC can serve as an EGC if the circuit conductors contained in the raceway are protected by an overcurrent protective device rated 20A or less, and the combined length of the raceway in the same ground-fault return path does not exceed 6 ft [Sec. 250.118(5)].
3. You can install equipment bonding jumpers inside or outside the conduit. If outside, a jumper can’t be longer than 6 ft and must be routed with the conduit per Sec. 250.102(E)(2).

Pro tip

Go beyond mere Code compliance, and pay attention to the aesthetics of the installation. For example, parallel runs of IMC on the same wall should be the same distance apart. Small details of workmanship send a big message about work quality.      

These materials are provided by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.