Conduit Fill Calculator

Conduit fill compares the insulated area of the conductors with the internal area of the raceway. A jam that will not pull, or a raceway packed so tight the wire cannot shed heat, is what the limit is there to prevent.

The usual caps, before any special rule, are 53% of the internal area for one conductor, 31% for two conductors, and 40% for three or more. This page looks up an approximate internal area for the raceway and trade size, an approximate area for one conductor of the size and insulation you pick, and multiplies that area by the count.

• Total conductor area = count × area of one conductor.
• Allowed area = raceway internal area × cap ÷ 100.
• Fill used = total conductor area ÷ raceway internal area × 100. The check passes when the total is within the allowed area.

Different raceways with the same trade size do not have the same internal area. EMT, PVC Schedule 40, PVC Schedule 80, and rigid metal are separate lookups. Insulation changes the conductor area too. THHN/THWN-2, XHHW-2, and TW/THW of the same AWG are not the same area. This version is one size and one insulation at a time.

The form opens on 3 × 12 AWG THHN / THWN-2 in 3/4" EMT. 3 conductors fall under the 40% cap.

Area each = 0.0133 in². Total conductor area = 3 × 0.0133 = 0.0399 in².

3/4" EMT internal area = 0.5330 in². Allowed area = 0.5330 × 40 ÷ 100 = 0.2132 in².

Fill used = 0.0399 ÷ 0.5330 × 100 = 7.49%. Remaining under the cap = 0.2132 − 0.0399 = 0.1733 in². The flag is within the cap.

Those areas are approximate Chapter 9–style field figures, not a reprint of the code book. If your phone shows different rounded digits, compare the substituted line in "Show the work." Areas are reported to four decimal places and the fill percent to two.

• The areas are approximate field figures. They are not a transcription of the code book, and a manufacturer's diameter can differ. Confirm the edition the authority having jurisdiction has adopted.
• One size and one insulation at a time. Add a second group by hand if the pull is mixed. Compact-strand and fixture wire are not listed.
• A nipple not longer than 24 inches is often allowed to be filled to 60%. This page does not apply that exception.
• Jam ratio, when three conductors can wedge instead of sit side by side, is not checked.
• Equipment grounding conductors count only if you include them in the conductor count. Bare equipment grounds are not given a separate area here.
• After you know the wire fits the pipe, it still has to carry the load and survive the voltage drop. See wire size and voltage drop.
• It is not a substitute for the NEC, the manufacturer's data, or a licensed electrician.