Conduit Fill Calculator

The NEC 40% Fill Rule

Conduit fill is governed by NEC Chapter 9, Table 1. When a conduit contains three or more conductors, the combined cross-sectional area of all the wires may not exceed 40% of the conduit's internal cross-sectional area. The remaining 60% is not wasted space — it is what makes it physically possible to pull the wires through without damaging insulation, and it gives the conductors room to dissipate heat.

There are two lesser-known exceptions. A conduit containing exactly one conductor may be filled to 53%, because a single wire has no neighbors to bind against during the pull. A conduit with exactly two conductors is limited to just 31% — counterintuitively lower than the three-plus limit — because two wires tend to lie side by side and jam against the conduit wall when pulled around bends. This calculator automatically applies the correct percentage (53%, 31%, or 40%) based on the conductor count you enter.

The math itself is simple: look up the cross-sectional area of your wire type and gauge in NEC Chapter 9, Table 5, multiply by the number of conductors, and compare the total against the allowable fill area from Table 4 for your conduit type and trade size. NEC Chapter 9, Note 7 adds one wrinkle: when all the conductors are the same size, if the division works out to a decimal of 0.8 or higher, you may round up to the next whole conductor. That is why 3/4-inch EMT is listed for six 8 AWG THHN conductors even though 6 × 0.0366 slightly exceeds 40% on paper.

Max THHN Conductors in EMT (NEC Annex C)

The table below shows the maximum number of same-size THHN/THWN conductors allowed in common EMT trade sizes at 40% fill, computed from the NEC conductor and conduit areas used by this calculator.

Derating: The Rule That Bites After Fill Passes

Passing the fill check is only half the story. NEC 310.15 requires you to derate conductor ampacity when more than three current-carrying conductors share a conduit, because bundled wires cannot shed heat as easily. Fill tells you what physically fits; derating tells you what can safely carry current. A 3/4-inch EMT may legally hold 16 12 AWG conductors, but at that count each wire's ampacity is cut in half, which usually makes the circuit unusable.

Note that only current-carrying conductors count for derating — equipment grounding conductors are excluded, and neutrals of typical 120/240V multiwire circuits often are too. But every conductor, including grounds, counts toward the 40% fill limit. In practice, most residential conduit runs are planned around derating, not fill: keeping a run to six or fewer current-carrying conductors preserves 80% ampacity, which THHN's 90°C column usually absorbs without upsizing wire.

Fill Limit vs. Pullable Reality

A conduit filled right to the 40% limit is legal but often miserable to work with. Pulling tension rises quickly with fill percentage, run length, and the number of bends. The NEC caps total bends at 360 degrees between pull points, but even a code-compliant run with four 90s and maximum fill can require serious force, wire-pulling lubricant, and a helper. Nicked or stretched insulation from a hard pull is a latent failure you will not see until it faults.

Experienced electricians usually plan around 25–30% actual fill on long runs or anything with three or more bends, and they upsize the conduit when in doubt. Conduit is cheap compared to a re-pull. Oversizing also leaves room for future circuits — pulling one extra conductor into a half-empty pipe later is trivial; replacing a maxed-out run is a project.

EMT vs. PVC Schedule 40

EMT (electrical metallic tubing) is thin-wall steel conduit joined with set-screw or compression fittings. It is the default for exposed interior work — garages, basements, workshops — because it is rigid, bends cleanly with a hand bender, and can serve as the equipment grounding path when fittings are made up properly (most installers still pull a ground wire). PVC Schedule 40 is glued plastic conduit, the standard for underground and wet locations since it cannot corrode. PVC always requires a pulled ground wire, and its slightly thicker wall means marginally less internal area than EMT at the same trade size — this calculator uses the correct area table for each. PVC also expands and contracts significantly with temperature, so long outdoor runs need expansion fittings.