Conduit Fill Calculator
Check the NEC conduit fill percentage for EMT from the conductor size and count, and see how many wires of that size the conduit can legally hold.
How it works
fill % = (conductors × conductor area) ÷ conduit interior area × 100 · NEC limit: 53% for 1 wire, 31% for 2, 40% for 3 or more
Conduit fill limits keep wires from overheating and let you pull them without stripping insulation. The NEC caps total conductor cross-section at a fraction of the conduit’s interior area — 40% is the familiar figure once you have three or more wires. Both areas come from Chapter 9 tables; this uses EMT interior areas and THHN conductor areas, the most common combination in residential and light-commercial work.
Worked example
Three 12 AWG THHN in 3/4 in EMT: each conductor is 0.0133 in², so 3 × 0.0133 = 0.0399 in². The conduit’s interior is 0.533 in², giving 0.0399 ÷ 0.533 = 7.5% fill — far under the 40% limit. In fact 0.533 × 0.40 ÷ 0.0133 ≈ 16 such conductors could fit.
Sources: NEC Chapter 9 conduit fill (Wikipedia: Electrical conduit)
Frequently asked questions
Why is the limit lower for two wires than three?
With exactly two conductors the geometry leaves a narrow gap that jams during a pull, so the NEC drops the allowance to 31%. One wire gets 53% and three-plus settle at the familiar 40%.
Does the ground wire count toward fill?
Yes — every current-carrying and equipment-grounding conductor that occupies the raceway counts. Add the ground’s area (and any neutral) to your total before comparing to the limit.
Are PVC and rigid conduit the same as EMT?
The percentages are the same, but the interior areas differ by conduit type and even by schedule. This tool uses EMT areas; for PVC or rigid metal, substitute the interior area from the matching Chapter 9 table.
Related calculators
Educational estimates only — electrical work must follow the National Electrical Code (NEC) and local amendments, and be done or inspected by a licensed electrician.