Voltkit US · NEC 2023
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NEC 2023NEC Ch. 9

Conduit Fill Calculator

Add each conductor size, insulation and count, pick the raceway and trade size, and see the fill percentage against the Chapter 9 limit.

Conduit fill FILL OK
13.1%
Used
0.040 in²
Max
0.122 in²
Wires
3

NEC Ch. 9 · 40% max fill · NEC 2023

Values checked against the printed NEC 2023 tables, using the same engine as the Voltkit app.

Show calculation
  1. Conduit areaEMT ½" (Table 4)0.3040 in²
  2. Conductor area3 × 12 AWG THHN/THWN/THWN-2 (Table 5)0.0399 in²
  3. Allowable fill0.3040 × 40%0.1216 in²
  4. Fill percent0.0399 ÷ 0.304013.1%
  5. Same-size max (Note 7)0.1216 ÷ one conductor = 9.149
  6. ResultWithin the 40% limitOK

NEC Chapter 9, Tables 1, 4 & 5 (Table 8 for bare)

Nipple ≤ 24 in (60% fill)

Conductors

Count every conductor in the raceway, including the equipment grounding conductor (Chapter 9, Note 3).

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How conduit fill is calculated

Add up the cross-sectional area of the conductors and compare it with the inside area of the raceway. The parts of NEC Chapter 9 that do the work:

  • Table 1: the maximum fill, 53% for one conductor, 31% for two, 40% for three or more.
  • Table 4: the inside area of each raceway type and trade size. ¾" EMT, ¾" PVC 40 and ¾" rigid are three different numbers.
  • Table 5: the area of each insulated conductor by size and insulation. Bare conductors use Table 8.
  • Notes 3, 4 and 7: grounding conductors count, a nipple of 24 in or less may be filled to 60%, and a same-size count rounds up at 0.8.

fill % = Σ conductor areas ÷ raceway area × 100

Worked examples

  1. How many 12 AWG THHN fit in ½" and ¾" EMT? 9 in ½" and 16 in ¾", the same counts as Annex C. With XHHW, ¾" EMT takes 11: the thicker insulation needs more room.
  2. Three 8 AWG THHN plus a 10 AWG THHN ground in ¾" EMT. The conductors total 0.1309 in² against 0.5330 in² of raceway: 24.6%, within the 40% limit.
  3. The nipple rule. Six 10 AWG THHN fill ½" EMT to 41.6%. As a run that's over the 40% limit; as a nipple of 24 in or less the limit is 60%, so it's allowed. It only helps on short connections between enclosures, such as a panel to a wireway.

More of these, with a table by wire size: How many 12 gauge wires in 3/4 conduit?

Common mistakes

  • Leaving out the equipment grounding conductor (Note 3).
  • Using EMT areas for PVC. At the common trade sizes, Schedule 40 and 80 PVC are smaller inside than EMT.
  • Stopping at fill. A ¾" EMT that legally holds 16 current-carrying 12s has to derate them under 310.15(C)(1); check that with the Wire Size calculator.

Worked examples on this page are computed by the same engine as the Voltkit app. The page cites NEC 2023 sections but doesn't reproduce NFPA text or tables; use your copy of the code for the exact wording.

Worked examples

Frequently asked questions

Why is 22 × 14 AWG in 3/4" EMT OK at 40.04%, over the 40% line?

Chapter 9, Note 7. When all the conductors are the same size and the count works out to a decimal of 0.8 or more, you round up. The same-size count for 14 AWG THHN in 3/4" EMT is 21.98, so 22 is allowed even though 22 of them fill 40.04% of the area. The calculator reads that as OK, marked "Note 7", and shows the division in the steps. Note 7 doesn't apply to a mix of sizes or insulations, and the calculator doesn't extend it to the 60% nipple rule.

Why is two conductors only 31%?

Table 1 sets 53% for one conductor, 31% for two and 40% for three or more. Two round conductors side by side span most of the raceway's inside diameter, which leaves the least room to pull. The calculator picks the percentage from the number of conductors you enter.

What is the 60% nipple rule?

A nipple is a raceway no longer than 24 in between enclosures. Chapter 9, Note 4 lets it be filled to 60% of its area. Turn on "Nipple ≤ 24 in" to apply it. The ampacity adjustment of 310.15(C)(1) doesn't apply to a raceway that short either.

Does insulation type change how many wires fit?

Yes. Fill uses each conductor's actual area from Chapter 9, Table 5, and that depends on the insulation. THHN/THWN-2 has a thin nylon jacket, so the same size takes less room than XHHW, RHW or TW. Each row in the calculator carries its own insulation, so a mixed pull is added up exactly.

Can I use this for NM, MC or other cable in conduit?

Not directly. Chapter 9, Note 9 treats a multiconductor cable as a single conductor, using its outside diameter (the major diameter, for a flat cable like NM). The calculator works with single conductors from Table 5. For cable, get the diameter from the manufacturer, work out its area, and check it against Table 1's one-, two- or three-conductor percentage yourself.

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