What size conduit do you need?

How do I calculate conduit fill?

Add up the cross-section area of every wire from NEC Chapter 9, Table 5, and compare it with the conduit's area from Table 4 times the fill limit from Table 1. Nine 12 AWG THHN wires take 9 × 0.0133 = 0.1197 square inches. A ½ inch EMT has 0.304 square inches, and 40% of that is 0.1216 — so they fit.

What is the 40 percent rule?

NEC Chapter 9, Table 1 limits the wires to 40% of the conduit's area when there are three or more. One wire may fill 53%, and two wires only 31% — two round wires side by side jam more easily when pulled.

How many 12 AWG wires fit in ½ inch conduit?

Nine 12 AWG THHN in ½ inch EMT, and eight in ½ inch PVC Schedule 40, which has a smaller inside. Twelve 14 AWG THHN fit in ½ inch EMT. These match the tables in NEC Annex C.

Does the ground wire count toward conduit fill?

Yes. Equipment grounding and bonding conductors count toward fill (NEC Chapter 9, Note 3). Add them to the list with their size.

Does this work for other wire types?

The areas here are for THHN, THWN and THWN-2, the most common building wire. XHHW, THW and other insulations are thicker or thinner — use their Table 5 areas. A short nipple, 24 inches or less, may be filled to 60% (Chapter 9, Note 4).

Which wire area table applies to my insulation?

Chapter 9, Table 5 lists dimensions for each insulation type, and Table 5A covers compact conductors. The numbers on this page are for THHN, THWN and THWN-2; for other insulation such as XHHW the areas differ, so a calculation that mixes the types needs each wire's own area. For bare conductors the areas are in Table 8.

What is not covered by conduit fill?

Fill is only one of the limits. The code also limits the bends between pull points to the equivalent of four quarter bends (360 degrees in total) for most raceways, and the ampacity of the conductors must be adjusted when more than three current-carrying conductors share a conduit. Passing the fill check does not mean the installation is complete.

How the fill is calculated

Each conductor's cross-section, with insulation, comes from NEC Chapter 9, Table 5. The areas are added, and the sum is compared with the permitted share of the conduit's internal area from Table 4: 53% for one conductor, 31% for two and 40% for more than two (Chapter 9, Table 1). The percentage is of the internal area, which differs by conduit type for the same trade size, so a ½ inch EMT and a ½ inch PVC Schedule 40 conduit hold different amounts. This calculator only handles round conductors of the same insulation family.

Worked example: wires for a small branch circuit run

Five 10 AWG THHN conductors (four circuit wires and a ground) have a combined area of 5 × 0.0211 = 0.1055 square inches. A ½ inch EMT has 0.304 square inches inside and the 40% limit is 0.1216, so the wires fit, filling 34.7% of the area; the smallest EMT that works is 1/2 inch. A bundle of six 12 AWG, three 10 AWG and one 8 AWG has an area of 0.1797 square inches; in EMT it fills 59.1% of a ½ inch size, so the smallest EMT is 3/4 inch, and in PVC Schedule 40 it is 3/4 inch.

Common mistakes

Forgetting to count the grounding conductor is the most common. Using the conductor's copper area instead of the area including insulation is the second; Table 5 gives the insulated area. Taking the 40% limit from the wrong row (a conduit with just two conductors can only be filled to 31%) is a third. Last, the trade size is not the inside diameter, and the same trade size in EMT, PVC or rigid has different internal areas, so choose the right type in the calculator.

Sources: NFPA 70, National Electrical Code, Chapter 9: Table 1 (percent of cross section of conduit and tubing for conductors), Table 4 (dimensions and percent area of conduit and tubing), Table 5 (dimensions of insulated conductors) and Annex C (maximum number of conductors). Always follow the edition adopted by your local authority. Related: box fill calculator, wire size calculator, EV charger circuit calculator and breaker size calculator.