What is RMC (Rigid Metal Conduit)?
RMC is heavy-wall threaded galvanized steel raceway, the most robust metallic conduit, permitted in hazardous locations under NEC Article 344.
RMC (Rigid Metal Conduit) is the thickest-walled steel raceway recognized by the NEC. It is hot-dip galvanized inside and out, threaded at every coupling, and rated for hazardous locations, direct burial, and severe physical damage. NEC Article 344 governs its installation. RMC is the benchmark against which all other metal raceways are measured.
Construction
- Material: Mild steel, hot-dip galvanized (or aluminum, red brass, or stainless variants)
- Wall thickness: Approximately 0.133 inch at 1" trade size — roughly 3× thicker than EMT
- Length: 10 ft standard, threaded each end with one coupling included
- Fittings: Tapered NPT threads (3/4-inch taper per foot)
The thread on RMC provides both mechanical strength and reliable electrical bond — critical in hazardous areas and for use as an equipment grounding conductor.
Interior cross-section (NEC Chapter 9 Table 4)
| Trade size | Interior area in² |
|---|---|
| 1/2" | 0.314 |
| 3/4" | 0.549 |
| 1" | 0.887 |
| 1-1/4" | 1.526 |
| 1-1/2" | 2.071 |
| 2" | 3.408 |
| 2-1/2" | 4.866 |
| 3" | 7.499 |
| 3-1/2" | 10.010 |
| 4" | 12.882 |
| 5" | 20.212 |
| 6" | 29.158 |
Use these values on the rigid conduit fill chart — multiply by 0.40 for three or more conductors per NEC Chapter 9 Table 1.
NEC code reference (Article 344)
- NEC 344.10 — permitted uses: all atmospheric conditions, occupancies, hazardous locations, direct burial, cinder fill
- NEC 344.14 — dissimilar metal protection (aluminum RMC vs steel fittings)
- NEC 344.22 — number of conductors limited by Chapter 9 Table 1
- NEC 344.24 — bends limited to NEC Table 2 radii; max 360° total between pulls
- NEC 344.30 — supports within 3 ft of each box, then every 10–20 ft
Common applications
- Class I Division 1 and 2 hazardous locations (NEC 501.10)
- Service masts and weatherheads
- Risers exposed to physical damage (parking garages, loading docks)
- Direct-buried feeders without supplementary protection
- Industrial process areas with corrosive atmospheres (PVC-coated RMC)
Trade-offs
| Property | RMC | IMC | EMT |
|---|---|---|---|
| Wall thickness | 0.133" | 0.080" | 0.042" |
| Weight per 10 ft (1") | ~16 lb | ~12 lb | ~6.5 lb |
| Cost per ft | Highest | Middle | Lowest |
| Threaded? | Yes | Yes | No |
| Hazardous locations | Yes | Yes | No |
| Physical damage | Best | Good | Limited |
IMC is the most common substitute — same threaded fittings, lighter, and slightly more interior space at most trade sizes. Many contractors install IMC wherever RMC is not specifically required, since both meet the same code uses per NEC 342.10.
Bonding and grounding
Per NEC 250.118(2), threaded RMC is recognized as an equipment grounding conductor. Couplings and connectors must be wrenched tight; running threads (long threads) are not allowed for grounding paths. For service raceways and hazardous locations, bonding jumpers around concentric/eccentric knockouts are required per NEC 250.92(B).
Quick reference
- Article: NEC 344
- Material: galvanized steel (or aluminum)
- Wall: ~0.133" at 1"
- Threaded: yes, NPT
- Hazardous-rated: yes
- EGC qualifier: yes (NEC 250.118(2))