Conduit Fill Calculator

How conduit fill works

Conduit fill compares two areas: the cross-section of all the wires and the open space inside the conduit. Wires are round and the conduit is round, so you can never pack it full. A share of the inside area has to stay empty so the wires slide in during the pull and can shed the heat they make while carrying current.

The calculator works it out in three steps, and prints each one with your numbers so you can check it by hand:

1. Inside area of the conduit. Area = π/4 × inside diameter². A 3/4 in EMT measures 0.824 in inside, which gives 0.533 sq in.
2. Area of the wires. Each wire’s area comes from its outside diameter, insulation and nylon jacket included. A 12 AWG THHN wire is 0.132 in across, about 0.0137 sq in. Multiply by the count of each size and add the sizes together.
3. Compare with the limit. The wires may take up to a set share of the conduit’s area, and that share depends on how many wires there are.

You can ask it two questions. How many fit takes one wire size and one conduit and gives the largest count that stays inside the limit. Conduit size takes a list of wires, mixed sizes allowed, and walks up the trade sizes of the conduit type you picked until it finds the smallest one that holds them all.

The fill limits

The limit depends on the total number of wires in the conduit: up to 53% of the inside area for 1 wire, 31% for 2 wires and 40% for 3 or more (NEC Chapter 9, Table 1). The 40% figure covers almost every branch circuit. Two round wires side by side can lie flat across the conduit and wedge during a pull, so 2 wires get the lowest limit, and two large wires can need a bigger conduit than three slightly smaller ones.

A nipple, a short piece of conduit 24 in or shorter between two boxes or enclosures, may be filled to 60% when it holds 3 or more wires (Note 4 to Table 1). Tick the nipple box and the calculator uses 60% for those jobs.

Grounds and mixed sizes count

Every wire in the conduit counts, including the equipment ground. A bare or green ground takes real space, and it also counts toward the number of wires that picks the limit. One hot, one neutral and one ground is three wires, so it uses 40%, not 31%.

For an insulated ground the calculator uses the same THHN size table as the other wires. For a bare ground it uses 1.1508 times the diameter of a solid wire of that size, the widest a bare stranded wire can measure across its strands. That covers both solid and stranded bare copper.

Mixed sizes are simple with the area method: add each size’s area times its count, then compare the total with the allowed area. Ready-made count charts only work when every wire is the same size, which is why this calculator never uses them.

Where the numbers come from

Inside diameters come from the conduit makers: Wheatland Tube and Allied Tube & Conduit for EMT, RMC and IMC. Where the two makers differ, as they do for IMC, we use the smaller inside diameter. Wire diameters are the larger of the Southwire and Cerrowire figures for THHN/THWN-2 copper, stranded by default, with solid wire as a switch for 14, 12 and 10 AWG.

Every rounding leans toward more room. Wire areas round up to 4 decimals, conduit areas round down to 3 decimals, counts round down, and the fill percentage rounds up to 1 decimal, so 40.04% shows as 40.1% and fails a 40% limit. The pass or fail check itself always uses the unrounded numbers. Textbooks and code-book charts use their own tables of areas, so their answer can differ from ours by a wire or so, usually with ours the more cautious of the two.

Conduit types

EMT is thin-walled tubing, the usual choice inside walls and ceilings. RMC is heavy threaded pipe, and IMC is a lighter threaded pipe between the two. The same trade size has a different inside diameter in each type: 3/4 in EMT is 0.824 in inside, RMC 0.836 in and IMC 0.864 in. The trade size is only a name, and so is the metric designator (21 for 3/4 in).

PVC conduit, flexible metal conduit and ENT are not covered yet. We could not find inside diameters for them from two makers, and guessing would put a wrong number on the page.

When to go up a size

Treat the fill limit as a ceiling. A long run, several bends or a tight pull through boxes all go easier with spare room, and a bigger conduit leaves space to add a circuit later. With exactly three wires of nearly equal size and a conduit about 3 times wider than the wire, the wires can jam during a pull; pick a bigger size or pull with a different method.

Conduit fill also does not answer how much current the wires may carry. When more than 3 current-carrying wires share one conduit, their current rating drops, and that is a separate check from fill. When in doubt, ask a licensed electrician.

Worked example

1. Inputs: how many 12 AWG THHN wires (stranded) fit in 3/4 in EMT, no ground wire.
2. 3/4 in EMT (metric 21): inside diameter 0.824 in, area π/4 × 0.824² = 0.533 sq in (343.9 mm²), rounded down
3. 12 AWG THHN, stranded: 0.132 in across, area π/4 × 0.132² = 0.0137 sq in, rounded up
4. Limit: 40% for 3 or more wires, grounds included: 0.40 × 0.533 = 0.2132 sq in
5. Most wires: 0.2132 ÷ 0.0137 = 15.56, rounded down to 15
6. Fill: 15 × 0.0137 = 0.2055 sq in ÷ 0.533 sq in = 38.6%

Questions people ask