Electrical Conduit Fill Calculator

NEC Chapter 9, Table 1 caps the total cross-section of conductors at 53% of the conduit's internal area for a single wire, 31% for exactly two wires, and 40% for three or more wires. The single-wire limit is highest because there is no bundle to bind, while the two-wire limit is lowest because two parallel conductors can wedge against each other during the pull. The calculator applies the correct percentage automatically based on how many conductors you list, including the equipment ground if you include it.

An overfilled conduit can make a pull difficult and raises the risk of insulation damage. Fill is only one electrical-design check. Ampacity also depends on current-carrying conductor count, ambient temperature, terminals, insulation, load, and the adopted code. An overfill result does not by itself calculate operating temperature or establish a safe installation. Have a qualified electrician verify the design before work begins.

All three are metal raceways, but the wall thickness and the application change. EMT (Electrical Metallic Tubing) is thin wall steel with set-screw or compression fittings, used indoors above ground for most commercial branch wiring. IMC (Intermediate Metal Conduit) is thicker and threaded, allowed almost anywhere, and weighs about a third less than RMC. RMC (Rigid Metal Conduit) is the heaviest, full-thread, schedule 40 steel pipe, used in hazardous locations, outdoor service masts, and anywhere it needs to take a hit. The internal area of each (NEC Table 4) drops in the same order, so the same wire pull may pass in IMC and fail in EMT of the same trade size.

THHN is a dry-location marking. Building wire is often dual marked THHN/THWN or THWN-2, which changes its wet-location rating. XHHW and XHHW-2 also have different wet ratings. Insulation area varies by the exact marking and gauge, so do not assume one material is always a fixed percentage larger. The calculator uses a limited planning lookup. Confirm the actual conductor marking, listing, and adopted code before installation.

Every conductor in the raceway occupies area, including an equipment grounding conductor. The calculator estimates EGC size from the largest entered hot conductor, but NEC Table 250.122 sizes an EGC from the rating or setting of the upstream overcurrent device, with additional rules for some installations. Verify conductor size and fill against the adopted code and project design.