Conduit Fill Calculator
Check raceway fill against NEC Chapter 9 for THHN/THWN-2 and XHHW-2 conductors in EMT, IMC, Rigid, or PVC — and see the smallest conduit that fits, for every conduit type at once.
How it’s calculated. NEC Chapter 9, Table 1 maximum fill: 53% for one conductor, 31% for two, 40% for three or more — or 60% for a nipple ≤24″ (Note 4), selectable above. Conductor areas are from Table 5 for THHN/THWN-2 and XHHW-2 (XHHW-2 runs noticeably fatter on small sizes — a #12 is ~36% bigger than THHN, so the type matters); conduit internal areas are from Table 4 (EMT, IMC, Rigid, PVC Sch 40 & 80). The “smallest that fits” rows show the smallest trade size within the allowed fill — for the selected conduit and for every type at once. When every conductor is the same size, Chapter 9, Table 1, Note 7 lets a calculated count with a decimal of 0.8 or more round up to the next whole conductor, so a run can pass at slightly over 40% by area (26 × #12 THHN in 1″ EMT, the count Annex C tabulates, is 40.0%); the calculator applies that rounding and says so in the verdict.
More than three current-carrying conductors in one raceway also requires an ampacity adjustment per NEC 310.15(C)(1) — the calculator flags this, and the Wire Ampacity calculator applies it. Verify equipment-grounding and derating requirements against the full design.
Worked example. A 200 A feeder in four 3/0 AWG THHN copper (0.2679 in² each, Chapter 9 Table 5) with a 6 AWG THHN equipment ground (0.0507 in²) totals 1.1223 in² across five conductors, so the 40% column of Table 1 applies. A 1½″ EMT holds 2.036 × 0.40 = 0.814 in² — too small — while 2″ EMT (3.356 in²) allows 1.342 in², so 2″ is the smallest EMT that fits, at 33.4% fill. The same pull lands in 2″ for IMC, rigid, and Schedule 40 PVC as well; 2″ Schedule 80, with its thicker wall (2.874 in²), still fits but at 39.1%, with almost nothing to spare. Insulation type matters most on small conductors: nine 12 AWG THHN (0.1197 in²) fit a ½″ EMT at 39.4%, but the same nine in XHHW-2 (0.1629 in²) need ¾″. And nine current-carrying conductors in one raceway also pull every one of them down to 70% of its table ampacity under 310.15(C)(1).
Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.
Need it engineered and stamped?
Routing raceway and distribution on a bigger job? We’re a commercial electrical contractor in Tucson — we engineer it, pull the permits, and self-perform the install.
Request a BidKeep Going
Read the field guide
Fill is one input to a pull. The guides cover the rest — pulling tension and lubricant, bending EMT so the run you calculated still fits, and sizing the pull box where the run turns.