How it works
NEC Chapter 9 limits how much of a conduit's cross-section wires may fill so they can be pulled and can shed heat. One wire may fill 53%, two wires 31%, and three or more wires 40%. A short nipple (24 inches or less) may go to 60%.
Look up each conductor's area (Table 5), multiply by how many you have, and add them up, including the equipment ground. Then find the smallest conduit whose internal area (Table 4) times the fill percentage is at least that total. A multiconductor cable counts as a single conductor at its overall diameter.
Worked example
Three 10 AWG THHN conductors in EMT (the published answer is 1/2 inch for the wires alone):
- 3 × 0.0211 in² (10 AWG THHN/THWN) = 0.0633 in²
- Total conductor area = 0.0633 in² (3 conductors, 40% fill limit)
- Smallest EMT whose internal area × 40% covers 0.0633 in² = 1/2 in
| Input | Value |
|---|---|
| Conduit type | EMT |
| Installation | Conduit run (Chapter 9 Table 1) |
| Conductor size and type, group 1 | 10 AWG THHN/THWN |
| Quantity, group 1 | 3 |
| Conductor size and type, group 2 | 14 AWG THHN/THWN |
| Quantity, group 2 | 0 |
| Conductor size and type, group 3 | 14 AWG THHN/THWN |
| Quantity, group 3 | 0 |
| Conductor size and type, group 4 | 14 AWG THHN/THWN |
| Quantity, group 4 | 0 |
| Multiconductor cable outside diameter (cable is counted at its major diameter) | 0.39 in |
| Number of multiconductor cables | 0 |
| Result | Value |
|---|---|
| Minimum conduit trade size | 1/2 in |
| Total conductor area | 0.0633 in² |
| Fill limit applied | 40 % |
| Fill in that conduit | 20.8 % |
Assumptions and limits
- Areas are the approximate conductor areas in Chapter 9 Table 5 for THHN/THWN and XHHW, and the conduit areas in Table 4. THHW and compact conductors are not included.
- When every conductor has the same area, Note 7 lets a count with a remainder of 0.8 or more round up. This tool applies that rule.
- Includes only the areas you enter. Do not forget the equipment grounding conductor.
- Conduit bodies, bends, and pull length do not change the fill limit, but they change how hard the pull is. Check the adopted NEC edition and any local amendments.
Common questions
How many wires can I put in a 1/2 inch EMT?
Using the 40% limit, 1/2 inch EMT holds up to twelve 14 AWG THHN, nine 12 AWG THHN, five 10 AWG THHN, three 8 AWG THHN, or two 6 AWG THHN conductors, counting the ground.
Why is the fill limit lower for two wires?
Two conductors of a size that fills 40% or more of a conduit can jam against each other, so Chapter 9 Table 1 holds two-wire fill to 31%.
Do I count the ground wire?
Yes. Chapter 9 Note 3 says equipment grounding conductors and bonding jumpers count in conduit fill.
What is the 60% nipple rule?
A conduit nipple that is 24 inches or less between boxes or cabinets may be filled to 60% instead of 40% (Chapter 9 Note 4).
Sources
- NEC (NFPA 70) Chapter 9: Table 1 (percent of conduit fill), Table 4 (conduit dimensions), Table 5 (conductor dimensions), Notes 3, 4, 5, 7 and 9.
- Table values transcribed from PDHonline Course E276, Conduit Fill Calculations, which reproduces those tables. Confirm against the NEC edition adopted where you work.
- Read them: PDHonline E276, Conduit Fill Calculations (NEC Chapter 9 tables)
Updated 2026-09-30