Conduit fill calculator

How many #14, #12 and #10 THHN wires fit in 1/2, 3/4 and 1 inch conduit?

The most THHN/THWN-2 conductors of one size that stay within 40% fill. For any other mix, including a ground of a different size, use the calculator above.

Counts compare areas directly. Note 7 lets a same-size count of x.8 or more round up, and the Annex C tables are built that way, so they sometimes allow one more conductor.

Chapter 1

Fill is a share of the area

Slice the conduit across and look at the cut end. The inside of the pipe is a circle with a known area. Each conductor, measured over its insulation, is a smaller circle with its own area. Add up the conductor areas, divide by the conduit's inside area, and you have the fill percentage.

Area is the right thing to measure because round wires can never tile a round pipe. There are always gaps between them, and you need those gaps. Wire gets pulled through long runs and around bends, rubbing against the pipe and against other wires the whole way. Packed too tight, the pull tension climbs, the insulation gets scraped or crushed, and you end up with a fault that may not show until years later.

Chapter 2

Pull wire into a conduit

The bench below holds the same raceway as the calculator. Click it or press + to pull another wire, step the trade size and wire size, or switch between EMT and PVC. The dial reads the fill against the orange limit index, and the lamp lights when the set is over. Changes here go back to the calculator above.

Each trade size is drawn true to life inside its own scale, and the ruler on the bench reads real inches at that scale. Other rows and the ground from the calculator ride along in the pipe; the bench changes only the first row.

Chapter 3

Why the limit jumps: 53%, then 31%, then 40%

The limits come from Chapter 9, Table 1. The share you're allowed depends only on how many conductors are in the raceway:

A pair is the worst case: two wires across a diameter leave little side clearance, and in a bend they can stack and bind. So two wires get less total area than one large wire, and a third wire raises the limit again. One 4/0 THHN in 1″ EMT is 37% and passes. Add a second and it's 75% against a 31% limit.

Chapter 4

Every trade size at once

Turn the question around and ask which raceway this set of conductors needs. Here is the same set in every modeled size of both raceway types, with the smallest that passes marked in each column.

Chapter 5

Where the numbers come from: NEC Chapter 9 Table 4 and Table 5

Two tables supply every number on this page. Table 4 gives the inside area of each conduit type and trade size. Table 5 gives the area of each insulated conductor. Neither is as simple as the nominal size suggests.

• Trade size isn't the inside diameter. A "1-inch" EMT has an inside diameter of about 1.05″. Each raceway type has its own wall thickness, so the same trade size gives a different area. At 2-1/2″, EMT has about 5.86 in² inside, while Schedule 40 PVC has about 4.70 in², roughly 20% less.
• Wire size isn't the conductor area. Fill counts the insulation, and insulation types differ. A #12 THHN, with its thin nylon jacket, is 0.0133 in². A #12 XHHW-2 is 0.0181 in², about 36% larger. That's the difference between 16 and 11 conductors in a 3/4″ EMT. At 4/0 the two are nearly equal, so the insulation choice matters most on branch-circuit sizes.

Chapter 6

Three wires can jam

An informational note to Table 1 adds a subtler effect. With exactly three conductors, if the pipe's inside diameter is 2.8 to 3.2 times the wire's outside diameter, the wires can line up three abreast and jam in a bend. Area alone won't catch that, which is one reason fill numbers are a starting point for judgment rather than a replacement for it.

A worked example

Take eleven #10 THHN in 3/4″ EMT. Table 5 lists #10 THHN at 0.0211 in², so eleven of them are 0.2321 in². Table 4 allows 3/4″ EMT 0.213 in² at 40% fill, so eleven wires are over, at about 43.6% of the pipe. Ten wires come to 0.211 in² and pass. Step up to 1″ EMT, which allows 0.346 in², and the same eleven wires are about 26.8%, with room for sixteen conductors in total (16 × 0.0211 = 0.338 in²).

Mixed sizes and grounds

Real raceways rarely hold one size. For a mixed set, find the Table 5 area for each insulation and size, multiply each by its quantity, and add the results. The limit comes from the total number of conductors, so a feeder of three 1/0 with a #6 ground is judged at 40% for four conductors.

1. List every conductor. Group identical size and insulation combinations, one row each in the calculator.
2. Count the ground. Note 3 requires equipment grounding and bonding conductors to be included. Circuits that share a raceway can share one wire ground sized for the largest overcurrent device under 250.122(C), and properly installed EMT can serve as the equipment grounding conductor under 250.118.
3. Add the areas. Multiply each conductor's area by its quantity and sum the groups.
4. Check the raceway. Compare the total with the inside area at the percentage for the total count.

What this calculator leaves out

• Modeled scope. EMT and PVC Schedule 40 from 1/2″ through 4″, with THHN/THWN-2 and XHHW-2 insulated conductors from #14 through 600 kcmil, at their NEC 2023 Chapter 9 Table 4 and Table 5 areas.
• Other raceway types. Flexible metal conduit (FMC), liquidtight flexible conduit, rigid metal conduit, IMC, ENT and PVC Schedule 80 each have their own Table 4 areas and are not modeled.
• Bare and compact conductors. A bare ground counts at its Table 8 area and compact conductors use Table 5A. Neither is modeled; enter an insulated ground to stay conservative.
• Cables count as one conductor. Under Note 9, a multiconductor cable such as NM or MC pulled into a raceway counts as a single conductor at its overall outside area. Cables are not modeled.
• Short nipples get 60%. Note 4 allows a nipple up to 24″ long between enclosures to be filled to 60%.
• The 0.8 rounding rule. When every conductor is the same size, Note 7 lets a calculated count of, say, 9.8 round up to 10. The Annex C tables are built that way, so they sometimes allow one more conductor than this direct area comparison.
• Bends have their own limit. EMT (358.26) and PVC (352.26) allow no more than 360° of bends between pull points, whatever the fill.
• Heat is a separate rule. More than three current-carrying conductors together reduces each one's ampacity under 310.15(C)(1). A raceway can pass fill and still need larger wire.
• The limit isn't a target. Maximum fill is what the code permits, not what pulls easily. Long runs, several bends and pulling-tension limits can justify a larger raceway even when the fill passes.

This page and ZapSketch's checks use NEC 2023 Chapter 9. Count every conductor and use actual cable dimensions. Follow all applicable codes and regulations.