Conduit Fill Calculator (EMT)
NEC Chapter 9 conduit fill for THHN/THWN-2 conductors in EMT — add each wire size and quantity, get the minimum trade size.
How this is calculated
NEC Chapter 9, Table 1 caps how full a conduit may be packed with conductors, based on conductor count:
| Conductors | Max fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
Table 5 gives each conductor's cross-sectional area (including insulation) — this calculator uses THHN/THWN-2, the most common building wire. Table 4 gives each conduit trade size's total internal area. The minimum conduit size is the smallest trade size where total conductor area ÷ fill % ≤ conduit's total internal area.
EMT internal area reference
| Trade size | Total internal area |
|---|---|
| 1/2" | 0.304 in² |
| 3/4" | 0.533 in² |
| 1" | 0.864 in² |
| 1-1/4" | 1.496 in² |
| 1-1/2" | 2.036 in² |
| 2" | 3.356 in² |
| 2-1/2" | 5.858 in² |
| 3" | 8.846 in² |
| 3-1/2" | 11.545 in² |
| 4" | 14.753 in² |
In-depth guide: Conduit fill rules: how NEC Chapter 9 sizes a raceway
How this calculator works
NEC Chapter 9: Table 1 fill limits (53 / 31 / 40%), Table 4 conduit areas, Table 5 conductor areas.
Worked example
Nine 12 AWG THHN conductors in EMT.
Each 12 AWG THHN takes 0.0133 sq in, so the total is 9 × 0.0133 = 0.1197 sq in. With three or more conductors the fill limit is 40%, so the conduit needs at least 0.1197 ÷ 0.40 = 0.299 sq in of internal area.
1/2" EMT has 0.304 sq in, so 1/2" works — just barely. If you might add circuits later, 3/4" gives real room to pull.
Frequently asked questions
How many wires fit in 3/4" EMT conduit?
With THHN/THWN-2 at 40% fill, 3/4" EMT (0.533 sq in) holds up to 22 × 14 AWG, 16 × 12 AWG, 10 × 10 AWG, 6 × 8 AWG or 4 × 6 AWG. These match NEC Annex C, Table C.1. Count the equipment ground as one of the wires.
How many 12 AWG wires can go in 1/2" EMT?
Nine 12 AWG THHN conductors fit in 1/2" EMT (0.304 sq in × 40% = 0.1216 sq in; 9 × 0.0133 = 0.1197 sq in). For 14 AWG the limit is 12, for 10 AWG it is 5. In 1" EMT the limits are 35, 26 and 16.
Why is the fill limit 40%?
NEC Chapter 9, Table 1 sets 53% for one conductor, 31% for two, and 40% for three or more. The limits leave room for heat dissipation and keep wires from being damaged when they are pulled.
Does the ground wire count toward conduit fill?
Yes. Equipment grounding conductors count toward fill just like any other conductor (they do not count for ampacity derating, but they do take up space).
Can I use these numbers for PVC or rigid conduit?
No — internal areas differ by conduit type. This calculator uses EMT areas from Chapter 9, Table 4. PVC Schedule 40 and 80, rigid and IMC each have their own column.
What about XHHW or other insulation types?
Conductor areas here are for THHN/THWN-2, the most common building wire. XHHW, RHW and others have different cross-sectional areas in Chapter 9, Table 5, so the result will change.
Common mistakes
- Leaving out the ground. Equipment grounding conductors, insulated or bare, are included in the fill calculation at their actual area.
- Applying 40% to two wires. Two conductors may fill only 31% of the raceway; one conductor 53%; three or more 40%.
- Using THHN areas for other insulation. XHHW, RHW and other insulations have larger areas in Chapter 9, Table 5, so the same count may not fit.
- Using EMT areas for other raceways. Rigid, IMC and PVC Schedule 40 and 80 each have their own internal areas in Chapter 9, Table 4.
What it does not cover
Conduit types and insulation types not in the menus, and nipples under 24".
Sources
Sources, checks and limits for this calculator →
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