9.2 RMC and IMC

9.2 RMC and IMC

Quick Answer: Rigid Metal Conduit (RMC) is Article 344; Intermediate Metal Conduit (IMC) is Article 342. Both are threaded steel (or other permitted metals) raceways built for strength. Fasten within 3 ft of boxes/cabinets (with article-specific relief to 5 ft when structural members make 3 ft impracticable), and support at intervals not exceeding 10 ft. Use them where physical protection, wet locations, or classified-location Chapter 5 wiring methods demand more than EMT can give.

Arkansas Prov’s Raceways and Enclosures items often contrast thin-wall EMT with rigid/intermediate conduit. If the stem mentions exposed warehouse columns, service masts, corrosive industrial areas, or Class I threaded conduit systems, you are usually in 342/344, not 358.

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RMC vs IMC — what the exam expects

IMC is not “flexible” and not EMT. Treat IMC like RMC for most journeyman lookups: threaded, robust, and Code-recognized as a wiring method and grounding path.

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Uses permitted — where rigid wins

344.10 and 342.10 permit RMC/IMC in a wide range of conditions:

• Exposed and concealed work.
• Dry, damp, and wet locations.
• Concrete embedment and direct burial (with corrosion protection as required by 300.6 and the listing).
• Locations subject to severe physical damage—this is the strategic contrast with EMT’s 358.12 ban.
• Many hazardous-location Chapter 5 methods that specify threaded rigid or intermediate metal conduit.

Service masts and outdoor risers commonly use RMC (sometimes IMC where accepted) because of mechanical strength and weather exposure. When a Prov stem asks for the raceway on a building service mast, think RMC first unless the problem states another listed method.

Corrosive atmospheres still require suitable materials or protective coatings—stainless, PVC-coated RMC, or other listed systems—not bare steel left to rust through.

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Uses not permitted / practical limits

RMC and IMC have fewer “never” lists than EMT, but they are still constrained by:

• Material suitability (dissimilar metals, aluminum in concrete, corrosive chemicals).
• Chapter 5 special occupancy rules that may demand explosionproof fittings, sealing, or other methods beyond ordinary threaded conduit.
• Support and coupling rules—threaded joints must be made up wrench-tight for mechanical and grounding integrity.
• Fill, bend, and pull-point rules shared with other raceways (total bends ≤ 360° between pull points under 344.26 / 342.26).

Do not assume “RMC can go anywhere.” Assume “RMC can go where physical protection is required and the environment matches the material.”

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Securing and supporting — 344.30 / 342.30

Baseline rule (same numbers as EMT, different article)

• Securely fasten within 3 ft of each outlet box, junction box, device box, cabinet, conduit body, or other termination.
• Support at intervals not exceeding 10 ft.

The 5 ft fastening relief

Where structural members are spaced so that fastening within 3 ft is impracticable, 342.30 / 344.30 language commonly allows fastening within 5 ft of the termination. Read the exact subsection in your 2023 NEC—Prov may quote the condition. This is not a free pass to skip supports on an open bay with plenty of steel to strap to.

Straight-run support tables (RMC)

Table 344.30(B)(2) permits increased distances between supports for straight runs of RMC with threaded couplings, based on trade size. Larger conduit can span farther when the installation meets the table’s conditions. Exam strategy:

1. Confirm the run is straight and uses threaded couplings as the table requires.
2. Open Table 344.30(B)(2) for the trade size.
3. Still fasten near terminations per 344.30(A).

Do not apply this table to EMT or to zig-zag flex runs.

Vertical risers

Both articles address vertical support. Under stated conditions (threaded couplings, fastened at top and bottom, no intermediate structural support available), supports may be farther apart—commonly discussed as up to 20 ft for qualified risers. Always verify the exact Exception text rather than treating 20 ft as universal.

Worked examples

Example A — Exposed column run: 1½ in RMC up a warehouse column, 40 ft. Fasten within 3 ft of the enclosure at the bottom and top; place intermediate supports ≤ 10 ft apart unless a straight-run table or riser Exception clearly applies.

Example B — IMC feeder in a wet process room: Use fittings and conduit suitable for the wet/corrosive location; support on the 3 ft / 10 ft pattern; maintain threaded continuity for the EGC path.

Example C — Trade size 4 RMC across an open plant bay: Check Table 344.30(B)(2) for a longer support interval on a straight threaded run; still hit termination fastening rules.

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Fittings, threads, and the EGC path

Field threads on RMC/IMC must be cut clean and tapered per standard conduit threading practice so couplings engage fully. Running threads that defeat the coupling’s tapered seal are a workmanship failure and can impair bonding. Listed threadless fittings exist for specific applications; follow the listing—do not mix EMT setscrew fittings onto rigid pipe as a general practice.

342.60 and 344.60 recognize IMC and RMC as equipment grounding conductors. With wrench-tight threaded joints, these raceways are among the most reliable metallic EGC paths in the Code. Expansion fittings, flexible sections, or impaired joints may need bonding jumpers—tie back to Article 250 when continuity is broken.

Where conductors leave the conduit into an enclosure, bushings protect insulation. Locknuts and bonding locknuts must be tight; concentric knockouts on the supply side of service equipment may still need bonding bushings even when the conduit is rigid.

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Bends, cutting, and reaming

Field bends must not reduce the internal diameter. Factory elbows are common on large trade sizes. Ream cut ends to remove burrs (344.28 / 342.28). The 360° total bend limit between pull points applies just as it does for EMT—add a conduit body or box when the run would otherwise exceed the budget.

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Choosing RMC/IMC vs EMT on exam stems

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Timed open-book tips

Tab 342.30, 344.30, Table 344.30(B)(2), 344.10, and 344.60. First decide which article (342 vs 344 vs 358). Then apply termination fastening (3 ft / possible 5 ft), then interval (10 ft or table). Finally ask whether the raceway is also answering an EGC question under 250.118. That sequence keeps rigid-conduit items fast on a 3-hour, 60-question Arkansas Prov exam.