Rigid Metal Conduit (RMC) Explained — NEC Article 344

The Heaviest Wall in the Metal Conduit Family

If you’ve been following along with the wiring methods series — PVC in Article 352, EMT in Article 358 — rigid metal conduit is the next stop, and it lives in Article 344. RMC is the heaviest, thickest-walled classification of metal conduit there is. Intermediate metal conduit (IMC) is still an excellent wiring method; it’s just not as thick and not as heavy.

The NEC defines RMC as a threadable raceway of circular cross section designed for the physical protection and routing of conductors and cables. That word “threadable” matters, and we’ll come back to it.

Article 344 follows the standard three-part layout you see across the wiring method articles:

• Part I — general, scope and definitions
• Part II — installation requirements
• Part III — construction specifications (essentially the manufacturer’s problem)

Clearing Up the Bushing Confusion First

Before touching Article 344, there’s a rule that trips up more electricians than almost anything else: 300.4(G).

Here’s what it actually says. Where raceways contain 4 AWG or larger insulated circuit conductors entering a cabinet, box, enclosure, or raceway, the conductors must be protected by an identified fitting providing a smooth, rounded insulating surface — and critically, that fitting has to be in place prior to the installation of the conductors.

“It is very clear that it says ‘where raceways.’ And I’m going to tell you, an MC cable is not a raceway. Go look up the definition.”

That’s the whole confusion in one sentence. SE cable is a cable assembly. MC cable is a cable assembly. Neither is a raceway, so 300.4(G) simply does not reach them. If you want to put an insulated bushing on SE cable anyway, nobody’s stopping you — just don’t tell people the code requires it.

The “prior to installation” language, clarified in the 2023 edition, has a real consequence: those split or “goof” bushings that snap on after the pull are hard to defend to an inspector. If a bushing comes apart to fit over conductors that are already pulled, the obvious inference is that it wasn’t there when it needed to be.

And there’s a good reason for the rule. Without that smooth surface, the edge of a fitting will scrape nylon off THHN or dig into the thermoplastic on a bad-angle pull. Once you’re down to bare copper or aluminium, no manufacturer is going to hand you a repair. Insulation only gets applied at the factory.

300.4(G) accepts any of the following:

1. An identified fitting with a smooth, rounded insulating surface (the common plastic screw-on bushing)
2. A listed metal fitting with a smooth, rounded edge — often doubling as a bonding bushing
3. Separation using identified insulating material securely fastened in place
4. Threaded hubs or bosses that are integral to the cabinet or box

One more detail: the insulating fitting or sleeve must have a temperature rating not less than the insulation rating of the installed conductor. Listed insulating bushings coloured black or brown indicate 150°C; any other colour indicates 90°C unless marked higher.

Note that 344.42(B) is a separate bushing rule. Where a conduit enters a box, fitting, or other enclosure, a bushing shall be provided to protect the wires from abrasion — no size limit at all — unless the box or fitting is designed to provide that protection (a threaded hub or a bell box, for instance). So 300.4(G) is size-specific and timing-specific; 344.42(B) is universal.

Where RMC Is Permitted — 344.10

This is where RMC earns its reputation:

• Galvanized steel, stainless steel, and red brass RMC are permitted under all atmospheric conditions and occupancies. That’s about as broad as it gets.
• Aluminium RMC is permitted where approved for the environment. “Approved” means the AHJ signs off — which may mean bringing manufacturer documentation to the table.
• Ferrous raceways and fittings protected from corrosion solely by enamel are permitted indoors only, and only in occupancies not subject to severe corrosive influences.

For corrosive environments, galvanized steel, stainless steel, and red brass — including elbows, couplings and fittings — are permitted in concrete, in direct earth contact, in direct burial, or in areas subject to severe corrosive influences where protected by corrosion protection approved for the condition. Galvanization itself is the protection; the zinc is the sacrificial lamb. If the environment is severe, that’s the one phone call worth making to the manufacturer.

Aluminium is stricter: aluminium RMC shall be provided with approved supplementary corrosion protection where encased in concrete or in direct contact with the earth, and shall be identified for the application for direct burial. Chloride additives in a concrete mix will attack non-ferrous conduit, which is exactly why that informational note points you at ANSI and Steel Tube Institute documents.

Cinder fill gets its own allowance: galvanized steel, stainless steel, and red brass may be installed in or under cinder fill subject to permanent moisture only when protected on all sides by at least 2 in of non-cinder concrete, or buried at least 18 in under the fill, or protected by approved corrosion protection. Cinder can contain sulphur, which combines with moisture to form sulphuric acid. You’ll almost never see it outside older commercial sites — but it’s in the code.

Finally, 344.10(E): RMC is permitted where subject to severe physical damage. Not just physical damage — severe.

Dissimilar Metals — 344.14

Where practicable, dissimilar metals in contact shall be avoided to eliminate galvanic action. But the section then grants specific permissions, packed into one long run-on sentence:

• Stainless steel and aluminium fittings and enclosures may be used with galvanized steel RMC
• Galvanized steel fittings and enclosures may be used with aluminium RMC, provided it’s not subject to severe corrosive influences
• Stainless steel RMC may be used with stainless steel fittings and enclosures, with steel (galvanized, painted, powder- or PVC-coated) boxes and enclosures where not subject to severe corrosive influences, with non-metallic fittings, or with other approved accessories

The teaching trick here is simple: highlight the segments in different colours. It’s one sentence doing three jobs, and on an exam it’s low-hanging fruit once you can dissect it. Testing shows galvanic corrosion at steel-and-aluminium interfaces is minor compared with natural corrosion on steel-to-steel or aluminium-to-aluminium — that’s why the code permits the mixing.

Sizing, Fill and Bends

344.20 sets minimum trade size 1/2 and maximum trade size 6. Notice the code says trade size, not inches.

344.22 sends you to Chapter 9, Table 1 for fill: over two conductors is 40%, exactly two is 31%, one conductor is 53%.

And yes — cables are permitted in RMC where not prohibited by the respective cable article. Nothing in Article 334 prohibits NM cable in a raceway. Two useful points:

• If the raceway is a complete run box-to-box or cabinet-to-box, you must do the fill calculation, and a cable counts as one conductor.
• If it’s a sleeve installed solely for physical protection, Chapter 9, Table 1, Note 2 lets you ignore fill.

On the informative annex question: Annex C isn’t enforceable, but it’s built on sound calculation. It only tells you how many conductors of the same size fit at the permitted fill. Use it as a quick sanity check — if the annex allows four 1/0 and you’ve only got three 1/0 plus a smaller equipment ground, you’re fine. But it says nothing about ampacity, adjustment, or correction. Nothing replaces knowing Chapter 9, Table 4 (raceway dimensions) and Table 5 (conductor dimensions).

344.24 requires bends be made so the conduit isn’t damaged and the internal diameter is not effectively reduced. Squash it oblong with the wrong bender and an inspector can reject it — even if it still functions, you’ve damaged it. Field bends follow Chapter 9, Table 2, measured as the radius from the start of the bend to the centre line.

Worth memorising which column you’re in: RMC, IMC and EMT use the one-shot and full-shoe bender column; FMC, LFMC, PVC, LFNC and ENT use the other column.

344.26 caps you at 360 degrees of bends between pull points. And here’s the gotcha — if you drop in a box purely to reset that count, that box still has to be sized per 314.28: eight times the largest raceway for straight pulls, six times for angle pulls, with 4 AWG and larger conductors. Solving one code rule introduces another.

344.28 requires field cuts to be reamed, and field threading to use a standard cutting die with a 1 in 16 taper (3/4 in per foot). That taper is why a conduit screws down into a meter hub and seals — a lot of straight-cut connectors don’t, and moisture finds the difference.

Securing vs Supporting — 344.30

People blur these two, but they’re different requirements.

Securing — 344.30(A):

• Securely fastened within 3 ft of each outlet box, junction box, device box, cabinet, conduit body, or other conduit termination. Note: a coupling is not a conduit termination. A termination is the end of the conduit’s intended path.
• Fasteners may be increased to 5 ft where structural members do not readily permit fastening within 3 ft.
• Where approved, conduit is not required to be secured within 3 ft of the service head for above-the-roof mast terminations.

Supporting — 344.30(B):

• Supported at intervals not exceeding 10 ft
• Straight runs may follow Table 344.30(B)(2) — up to 20 ft for trade size 3 and larger — provided the conduit is made up with threaded couplings and supported to prevent transmission of stresses to terminations
• Exposed vertical risers from industrial machinery or fixed equipment may be supported at intervals not exceeding 20 ft where made up with threaded couplings, securely fastened top and bottom, with no other intermediate support readily available
• Horizontal runs supported by openings through framing members at intervals not exceeding 10 ft are considered supported, provided the RMC is securely fastened within 3 ft of termination

“Within 3 ft” means anywhere within that span — 3 ft, 2 ft, 6 in, all fine. It does not mean 2 ft 11 in.

Couplings, Connectors and the Complete System

344.42 permits threadless couplings and connectors, made tight, and requires concretetight types where buried in masonry or concrete. Threadless fittings shall not be used on threaded conduit ends unless listed for the purpose — the fitting won’t seat, and a set screw driven onto threads lets moisture straight in. In practice you’ll rarely see threadless RMC; nearly everything in the field is threaded.

344.46 prohibits running threads on conduit for connection at couplings.

And 344.30 ties back to 300.18: RMC shall be installed as a complete system and securely fastened in place before conductors are pulled. Not one stick at a time, pull, add a coupling, pull again. Complete the raceway, install the fittings, then pull. That’s exactly why 300.4(G) demands the bushing be in place first — if you finish a pull and see curls of insulation on the floor, you’ve already lost insulation value on those conductors.

344.56 points splices and taps to 300.15. Realistically: you’re never making a splice or tap inside the conduit itself.

The Grounding Payoff — 344.60

344.60 permits RMC as an equipment grounding conductor, and it jives with 250.118, which lists RMC as an effective ground-fault current path when the conduit, couplings and fittings are listed.

RMC has an advantage over EMT here. EMT depends on set screws somebody might not have tightened. Rigid is threaded and made up wrench tight, so you get a genuinely low-impedance path. Most engineers still specify a wire-type EGC alongside it, and that’s fine.

The failure mode to watch for is interposition. Parking garages and decks with expansion joints are the classic case: the run transitions to FMC or an expansion fitting for movement, and nobody bonds across it. If you’re relying on the RMC as your fault path, you must maintain its integrity — buy the expansion fitting with the bonding strip, or run a jumper from rigid to rigid across the gap.

Construction and Marking — Part III

Part III is largely the manufacturer’s territory. RMC is made from steel with a protective coating (galvanizing — the zinc is the protection), aluminium, red brass, or stainless steel. 344.120 requires each length to be clearly and durably identified, and non-ferrous conduits of corrosion-resistant material to be suitably marked so you can select for the environment.

How NEC Mastery Fits Into This

Article 344 is a perfect example of why exam questions on wiring methods feel harder than they should. The rules aren’t difficult — they’re just distributed. Fill sends you to Chapter 9 Table 1, bends send you to Chapter 9 Table 2, bushings live in two different places (300.4(G) and 344.42(B)), and grounding is confirmed in both 344.60 and 250.118. That’s five lookups for one installation.

That’s the muscle NEC Mastery is built to train:

• 8,000+ exam-style questions covering wiring methods across Articles 344, 352, 358 and the rest — enough repetition that securing-versus-supporting stops being a coin flip
• Detailed explanations referencing specific NEC articles, so every question you review reinforces where the rule actually lives in the codebook, not just what the answer was
• Timed mock exams weighted to your exam type, so you practise chasing a 300.4(G) reference from a wiring-method question under the clock — which is exactly what exam day asks of you

Learn the structure of the article — Part I uses, Part II installation, Part III construction — and Article 344 stops being a wall of text and starts being a map.