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Conduit Fill Calculator — NEC Chapter 9
Find the minimum trade-size raceway for a THHN/THWN pull — EMT, rigid metal conduit, or PVC Schedule 40 — at the 40% fill NEC Chapter 9 allows when there are more than two conductors.
Conductor size (THHN/THWN)
Conductor count
Conduit type
How conduit fill works
Three NEC Chapter 9 tables drive this calculation. Table 5 gives the cross-section area of each THHN/THWN conductor (0.0133 in² for 12 AWG, up to 0.7073 in² for 500 kcmil). Table 1 sets the fill limit: 40% of the conduit's internal area when more than two conductors are installed — one conductor may fill 53%, exactly two may fill 31%. Table 4 gives each conduit type's usable area per trade size. The calculator multiplies conductor area by count, then walks up the Table 4 column for your conduit type until the total fits inside the 40% allowance.
Conduit type genuinely changes the answer at the margins, because EMT, RMC, and PVC Schedule 40 have different internal diameters at the same trade size. Sixteen 12 AWG THHN conductors total 0.2128 in² — that squeaks into 3/4" EMT (0.213 in² at 40%) but not 3/4" PVC Schedule 40 (0.203 in²), which forces a 1" raceway.
Worked example
Three 12 AWG THHN conductors in EMT: 3 × 0.0133 in² = 0.0399 in². A 1/2" EMT allows 0.122 in² at 40%, so the pull fits with room to spare — about 13% actual fill. If nothing through 4" fits your pull, the answer is not a bigger number: split the conductors into multiple raceways.
Frequently asked questions
How many 12 AWG THHN wires fit in 3/4 inch EMT?
Sixteen. Each 12 AWG THHN is 0.0133 in² (NEC Chapter 9, Table 5), and 3/4" EMT allows 0.213 in² at 40% fill — 16 × 0.0133 = 0.2128 in², just inside the limit.
Why is conduit fill limited to 40%?
NEC Chapter 9, Table 1 sets 40% as the maximum fill when more than two conductors share a raceway — it protects the insulation during the pull and limits heat buildup. One conductor alone may fill 53%, and exactly two may fill 31%.
Does the conduit type change the answer?
Yes. EMT, rigid metal conduit, and PVC Schedule 40 have different internal areas at the same trade size, so a pull that fits one type can fail another. The classic case: 16 × 12 AWG THHN fits 3/4" EMT but needs 1" in PVC Schedule 40.
What if my conductors don't fit the largest conduit?
This calculator tops out at 4" trade size. If the pull doesn't fit, split it into multiple raceways rather than overfilling — an over-40% pull is a code violation and a damaged-insulation risk.
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