Conduit Fill Calculator
Too many conductors in a conduit overheat, jam during pulling, and fail inspection. This calculator checks the NEC Chapter 9 fill rule for EMT conduit with THHN/THWN-2 conductors: one conductor may fill 53% of the conduit, two conductors 31%, and three or more 40%. Conduit areas follow NEC Chapter 9 Table 4 and conductor areas Table 5.
Method: fill % = (conductors × conductor area) ÷ conduit area × 100 · Limits: 53% (1 conductor), 31% (2), 40% (3+)
Worked example: four 12 AWG THHN conductors in 3/4-inch EMT: total conductor area = 4 × 0.0133 = 0.0532 in², conduit area = 0.533 in², so fill = 10.0% — comfortably inside the 40% limit. The same four conductors in 1/2-inch EMT give 17.5%, also legal, but the pull will be much harder — fill percentage is a maximum, not a target, and long or bendy runs deserve spare room.
For the conductor size itself, start with the wire size calculator; for the voltage lost over the run, use the voltage drop calculator.
Frequently asked questions
Why are the limits different for 1, 2 and 3+ conductors?
A single conductor can use 53% because it cannot jam against others. Two conductors are limited to 31% because two round conductors side by side waste the space between them. Three or more get 40%, the practical balance between usable space and safe pulling.
Does this cover all conduit types?
No — this calculator covers EMT with THHN/THWN-2 conductors, the most common combination. PVC, rigid and flexible conduits have different Table 4 areas, and other wire types have different Table 5 areas; check the tables for your exact combination.
Is passing the fill test enough?
No. Fill is one of several checks: ampacity derating for bundled conductors, voltage drop, and the physical pull (bends, length, lubricant) all still apply.