Conduit Fill Calculator
Calculate NEC conduit fill percentage and check code compliance for EMT, IMC, RMC, and PVC conduit.
Wires in Conduit
Conduit Fill Results
NEC Fill Limits & Wire Areas
About NEC Conduit Fill
The National Electrical Code (NEC Article 358 and Table 1 of Chapter 9) limits how much of a conduit's internal area can be occupied by wires. These limits exist to prevent heat buildup and ensure wires can be pulled without damage.
Fill percentages: When a single wire occupies a conduit, up to 53% fill is allowed because the wire can move freely. Two wires are limited to 31% because they can wedge against each other. Three or more wires are limited to 40% — the standard most installers work with.
Conduit types: EMT (thin-wall) is the most common for commercial and residential work. IMC and RMC (rigid) offer more protection in outdoor or industrial settings. PVC is popular for underground runs and corrosive environments but cannot be used where physical protection is needed.
Always use the internal diameter (ID) of the conduit for fill calculations. Nominal conduit sizes refer to trade size, not actual dimensions. When in doubt, go up one conduit size — it costs little and makes future wire pulls much easier.