Rigid metal conduit (RMC) and intermediate metal conduit (IMC) are required for certain hazardous locations, outdoor exposures, and areas subject to physical damage where EMT would not be suitable. RMC has the thickest wall of any metallic conduit type, which means its internal diameter is slightly smaller than IMC at the same trade size. IMC has a thinner wall and a larger internal bore than RMC, allowing slightly more fill. Both RMC and IMC use the 40% fill limit from NEC Chapter 9, Table 1. The fill calculation uses the conductor cross-sectional areas from NEC Chapter 9, Table 5 for each insulation type.
1-1/4" RMC (1.496 sq in internal area): 40% fill = 0.598 sq in. Five 4 AWG THHN conductors: 5 × 0.0824 sq in = 0.412 sq in. Fill = 27.5%. Within 40% limit. Could add one more 4 AWG THHN (6 × 0.0824 = 0.494 sq in = 33%).
This is a variant of the full Conduit Fill Calculator — which supports EMT, PVC, RMC, IMC, and flexible conduit with any conductor type and size.
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