Electrical Conduit Fill Calculator

NEC 300.17, implemented through Chapter 9 Note 4 to the tables, 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 causes two problems at once. During installation, the friction of pulling jams the wires together and can shred the insulation, leaving bare copper that arcs to the conduit. After installation, the bundle traps heat from the I squared R losses of every conductor, so the insulation runs hotter than its rated temperature and degrades faster. NEC ampacity tables assume the heat can escape, so an overfilled conduit effectively forces conductors above their rated current even when you stay within the breaker size.

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 the most common building wire in the US. It uses a thin nylon jacket over thermoplastic insulation and is rated 90 degrees C in dry locations, 75 degrees C in wet (when dual rated as THWN). XHHW uses a thicker cross-linked polyethylene jacket, gives up the nylon, and is rated 90 degrees C in both wet and dry locations. The trade off is size: XHHW takes up roughly 35 percent more area than THHN at the same gauge, so a fill that passes in THHN may force the next trade size up in XHHW. The calculator uses Table 5 areas for each insulation type and shows the difference per row.

NEC Chapter 9 Note 3 says every conductor in the raceway counts, current carrying or not. The equipment grounding conductor sits in the same bundle, takes up real cross-section, and contributes to the heat envelope during a fault, even though it normally carries no current. The calculator pulls the EGC size from NEC Table 250.122 based on the largest hot conductor you have entered, then uses the bare conductor area from Table 8 to add it to the total. Leaving the ground out gives a fill number lower than what an inspector would calculate, so the default is to include it.