Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC)

What it is

Rigid metal conduit (RMC) is the heaviest metal raceway. It is made of galvanized steel, aluminum, stainless steel or red brass, and joined with threaded couplings. Intermediate metal conduit (IMC) is a steel conduit with a thinner wall than steel RMC. Its outside diameter is close enough that it takes the same threaded fittings. RMC is covered by NEC Article 344 and IMC by Article 342.

How to identify

Both come in 10 ft lengths with a coupling on one end and thread protectors on the other. Lengths are marked with the type, trade size and maker. Steel RMC is heavier than IMC in the same trade size. IMC is usually marked "IMC". Aluminum RMC is much lighter and non-magnetic. Stainless is bright and usually specified for corrosive or food areas.

RMC vs IMC

Key points

• Conduit threads use the same 3/4 in per foot taper as pipe threads. Field threads must be cut with conduit dies and protected after cutting.
• Running threads are not allowed as couplings. Use a three-piece union or listed coupling where pieces cannot be turned.
• Support and securing intervals are in 344.30 and 342.30. The longer spans for straight runs are in Table 344.30(B)(2). Use the table, not a general rule.
• Dissimilar metals cause corrosion. Avoid direct contact of aluminum conduit with steel fittings in wet areas, and follow code for aluminum in concrete.
• RMC and IMC are listed as equipment grounding conductors under 250.118 when joints are tight.
• Conductor fill uses the RMC or IMC rows in NEC Chapter 9, Table 4. IMC has a slightly larger inside diameter than steel RMC.

Common mistakes

• Leaving freshly cut threads unprotected outdoors.
• Using EMT fittings on rigid or IMC.
• Assuming IMC is allowed in sizes above 4.
• Using aluminum conduit in concrete without the protection the code calls for.

Governing standards: NEC Articles 344 and 342; NEC 250.118; UL 6, UL 6A, UL 1242; ANSI C80.1, C80.5 and C80.6. See the conduit trade size chart.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.