What is IMC (Intermediate Metal Conduit)?
IMC is intermediate-wall threaded steel conduit, lighter than RMC with more interior fill area, permitted in all RMC uses under NEC Article 342.
IMC (Intermediate Metal Conduit) is a threaded galvanized steel raceway with a wall thickness between EMT and RMC. It accepts the same NPT fittings as Rigid Metal Conduit but weighs about 25 percent less and offers more interior cross-sectional area at most trade sizes. NEC Article 342 governs IMC, and NEC 342.10 explicitly permits IMC anywhere RMC is permitted.
Construction
- Material: Steel, hot-dip galvanized inside and out
- Wall thickness: Approximately 0.080 inch at 1" trade size (vs RMC's 0.133")
- Length: 10 ft, threaded each end with a coupling
- Identification: Green or orange stripe (manufacturer-dependent) plus "IMC" marking every 5 ft
IMC was developed as a lighter alternative to RMC for non-corrosive, non-hazardous use cases — though code now permits it in nearly all the same applications as RMC.
Interior cross-section (NEC Chapter 9 Table 4)
| Trade size | Interior area in² |
|---|---|
| 1/2" | 0.342 |
| 3/4" | 0.586 |
| 1" | 0.959 |
| 1-1/4" | 1.647 |
| 1-1/2" | 2.225 |
| 2" | 3.630 |
| 2-1/2" | 5.135 |
| 3" | 7.922 |
| 3-1/2" | 10.584 |
| 4" | 13.631 |
Note that 1" IMC has more interior area than 1" RMC (0.959 vs 0.887 in²). This means IMC fits more conductors at the 40% fill limit — useful when running THHN or THWN-2 feeders. Cross-check on the rigid conduit fill chart.
NEC code reference (Article 342)
- NEC 342.10(A) — permitted in all atmospheric conditions and occupancies
- NEC 342.10(B) — permitted in corrosive environments with supplementary protection
- NEC 342.10(D) — permitted in Class I, II, III hazardous locations
- NEC 342.20 — minimum trade size 1/2", maximum 4"
- NEC 342.22 — fill per Chapter 9 Table 1
- NEC 342.30 — securely supported within 3 ft of boxes, every 10 ft thereafter
Common applications
- Commercial service masts and risers
- Feeder runs in industrial plants where lighter weight matters
- Hospital and institutional branch wiring needing threaded raceway
- Outdoor runs subject to physical damage
- Direct-buried feeders (with appropriate fittings)
Trade-offs vs RMC and EMT
| Property | IMC | RMC | EMT |
|---|---|---|---|
| Wall (1") | 0.080" | 0.133" | 0.042" |
| Weight/10 ft (1") | ~12 lb | ~16 lb | ~6.5 lb |
| Threaded | Yes | Yes | No |
| Interior area (1") | 0.959 in² | 0.887 in² | 0.864 in² |
| Hazardous-rated | Yes | Yes | No |
| Cost per ft | Middle | Highest | Lowest |
IMC is often the "best of both worlds": all the code coverage of RMC, but easier to handle, threaded ends already in place, and slightly better fill. The main reason to specify RMC over IMC is corrosive environments where the extra wall thickness extends service life, or where a project specification explicitly calls for rigid.
Bonding and grounding
IMC is recognized as an equipment grounding conductor per NEC 250.118(3). All threaded couplings and connectors must be wrench-tight. Compression-type fittings (not threaded) require listing for grounding.
Quick reference
- Article: NEC 342
- Material: galvanized steel
- Wall: ~0.080" at 1"
- Threaded: yes, NPT
- Hazardous-rated: yes
- EGC qualifier: yes (NEC 250.118(3))