Conduit fill calculator

Conduit fill is the share of a conduit's inside area taken up by the wires in it. The NEC limits it so wires can be pulled without stripping their insulation and so they have air around them to shed heat. The rules sit in Chapter 9 of the NEC, in three tables:

• Table 1 sets the limit: 53% for one wire, 31% for two, 40% for three or more.
• Table 4 gives the inside area of each conduit type and trade size.
• Table 5 gives the area of each insulated wire, insulation included.

The method is the same for every job: look up each wire's area, multiply by how many, add them up, and compare with the conduit's area times the limit. If the wires take up no more than the limit allows, the conduit is big enough for fill.

Fill % = total wire area ÷ conduit inside area × 100

Two wires get the lowest limit because two round wires side by side are as wide as two diameters, and they can wedge across a bend. Three or more settle into a bundle that pulls more easily.

Inside areas from NEC Chapter 9, Table 4, in square inches, with the 40% figure used for three or more wires:

Wire areas from Chapter 9, Table 5, in square inches:

Schedule 80 PVC has a thicker wall, so it holds noticeably less than Schedule 40 or EMT of the same trade size. Sizing Schedule 80 with EMT numbers is a common mistake.

A 3-wire feeder with a neutral and ground. Three 2 AWG THHN, one 4 AWG THHN neutral and one 8 AWG THHN ground in EMT:

3 × 0.1158 + 0.0824 + 0.0366 = 0.4664 sq in. Five wires, so the limit is 40%. 1 in. EMT allows 0.346, too small. 1-1/4 in. EMT allows 0.598, which works, at 31.2% fill.

Twelve 10 AWG THHN in PVC. 12 × 0.0211 = 0.2532 sq in. In a run of Schedule 40, 3/4 in. allows 0.203, too small, so it takes 1 in. (0.333). As a nipple of 24 in. or less, the 60% limit lets 3/4 in. carry it: 0.508 × 60% = 0.305.

Adding to a conduit that is already in. A 1-1/4 in. rigid conduit carries twelve 10 AWG THWN. How many 12 AWG THHN can be added? 1.526 × 40% = 0.610 sq in allowed, less 12 × 0.0211 = 0.253 already used, leaves 0.357. 0.357 ÷ 0.0133 = 26.8, so 26. The wires are not all one size, so the result is not rounded up.

A nipple of 8 AWG XHHW. An 18 in. length of 1 in. EMT at 60%: 0.864 × 60% = 0.519 ÷ 0.0437 = 11.9. All the wires are the same size and the decimal is over 0.8, so 12 are allowed.

When every wire is the same size and type, the question is simply how many fit. The calculator answers it for the size you pick. For THHN in common sizes, worked from the 40% areas above with the 0.8 rounding rule:

The rounding rule is Note 7 to Chapter 9, Table 1: when all the wires are the same size and the count comes to a decimal of 0.8 or more, the next whole number is allowed. 1 in. EMT gives 0.346 ÷ 0.0133 = 25.98 wires of 12 AWG THHN, so 26 are allowed. With mixed sizes you round down. The fill tables in the NEC's Informative Annex C follow the same rule, but only work when every wire is the same size and type.

Fitting the wires is not the same as being able to use them all at full ampacity. More than three current-carrying wires in one raceway lowers each wire's allowable current, so a conduit stuffed to its fill limit often needs bigger wire, or two conduits.

• Grounds count. Every equipment grounding conductor takes space. An insulated ground uses its Table 5 area. A bare ground uses the smaller bare-wire area from Chapter 9, Table 8; this calculator treats grounds as insulated, which is the same or more.
• Spares count. A wire pulled for later is a wire in the conduit.
• Insulation type matters. XHHW and THW are bigger than THHN in the small sizes, so the same count can need the next trade size up.
• Nipples. A length of 24 in. or less between boxes or enclosures may be filled to 60%. It is also exempt from the ampacity adjustment for more than three current-carrying wires.
• Cables in conduit. A multiconductor cable counts as one wire, using its outside diameter for the area.
• Bends and pulls. Fill is the minimum. A long run with several bends pulls far easier one size up.

The trade size printed on a conduit is a name, not a measurement. Inside areas differ by type, and the difference is big enough to change the answer:

• EMT has a thin wall, so its inside area is close to the largest for its trade size. 3/4 in. EMT has 0.533 sq in inside.
• Rigid metal conduit (RMC) has a heavier wall but a slightly larger bore in most sizes: 0.549 sq in at 3/4 in.
• IMC sits between the two in wall thickness and holds a little more than EMT in the small sizes: 0.586 sq in at 3/4 in.
• PVC Schedule 40 is a little smaller than EMT: 0.508 sq in at 3/4 in.
• PVC Schedule 80 has the thickest wall and the least room: 0.409 sq in at 3/4 in., about a fifth less than EMT.

Where a run changes type, say EMT inside the building and Schedule 80 PVC where it comes out of the ground, size each part for its own type. The smallest bore in the run is the one that sets the pull. The calculator's table under the answer lists every trade size of the type you picked, so you can see where the fill crosses the limit.

Conduit size drives the cost of a run more than the wire count suggests. Fittings, straps, connectors and bending time all go up with each trade size, so it pays to know the smallest size that works and then decide whether to go one up for an easier pull or room for a future circuit.

When the run is sized, build the estimate in Koira with the conduit, fittings, wire and labor as separate lines. For the wire size itself, use the voltage drop calculator; for the boxes at each end, the box fill calculator; and for the service the run feeds from, the electrical load calculator.