How to use the conduit fill calculator
The method
Max wires = floor(conduit 40% fill area ÷ one wire's overall area). Each THHN conductor has a standard overall area (insulation included), and each EMT trade size has a standard usable area at the 40% limit for three or more wires.
How to use it
Choose your conductor size and the conduit trade size, then press Calculate. The result shows the maximum wire count and how much of the conduit those wires occupy.
Practical pulling advice
A fill that is legal on paper can still be a miserable pull on a long run with several bends. Many electricians stay a wire or two under the maximum on difficult runs — the conduit is cheaper than the labour of a stuck pull.
Example
12 AWG THHN (0.0133 in² each) in a 1/2″ EMT (0.122 in² allowed): 0.122 ÷ 0.0133 = 9.17, so the maximum is 9 wires. Try it above: 12 AWG and 1/2″ EMT.
Frequently asked questions
What is the 40% fill rule?
For three or more conductors, electrical codes limit the total cross-sectional area of the wires to 40% of the conduit's internal area. The rest is air space — wires need room to be pulled in without damaging their insulation.
Which tables does this use?
Overall THHN wire areas and EMT internal areas from the standard reference tables (NEC Chapter 9, Tables 4 and 5 style values), with the 40% column applied. Always confirm against the current code book for your area.
Does the count include the ground wire?
Yes — an equipment grounding conductor takes up space and counts toward fill. Enter it as one of the wires of its own size.
Can I mix wire sizes in one conduit?
Yes, and it is common. Add up each wire's area individually and compare the total against the conduit's allowed fill area. This calculator handles one size at a time; add the areas from two runs of it for a mixed fill.
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