Conduit Fill Calculator
Check NEC conduit fill compliance - select conduit and wire type for instant pass/fail results.
Conduit Fill Result
- Select conduit - choose the conduit type and trade size from the dropdowns.
- Select wire - choose insulation type and AWG/kcmil size. This determines the cross-sectional area per conductor.
- Enter conductor count - total number of individual insulated conductors (not cables). Include grounds if insulated.
- Check results - green means compliant, red means you need a larger conduit or fewer wires.
- Verify locally - this calculator follows NEC Chapter 9 tables but local codes and amendments may differ.
NEC caps conduit fill at 40 percent for three or more conductors, so a 3/4 inch EMT holds up to 16 #12 THHN wires and a 1/2 inch holds 9. The rule is area-based: add the cross-sectional area of every conductor, insulation included, and keep the total under 40 percent of the conduit inside area - 53 percent if there is a single conductor, 31 percent for exactly two. Pick your conduit, wire, and count above for the fill percentage and the maximum that fits.
The 40 Percent Fill Rule
NEC Conduit Fill Rules
The National Electrical Code (NEC) limits how many wires you can pull through a conduit to prevent overheating and make future wire pulling possible. Fill limits are based on the total cross-sectional area of all conductors compared to the internal area of the conduit.
The maximum fill percentage changes based on how many conductors are in the conduit. With one conductor you can use more of the conduit's area because heat dissipation is not a concern. With three or more conductors the limit drops to 40% to allow adequate airflow and prevent the wires from jamming during installation.
NEC Fill Limits
| Number of Conductors | Maximum Fill | NEC Reference |
|---|---|---|
| 1 conductor | 53% | Chapter 9, Table 1 |
| 2 conductors | 31% | Chapter 9, Table 1 |
| 3+ conductors | 40% | Chapter 9, Table 1 |
These limits apply to conduit and tubing only - not cable trays, wireways, or raceways, which have separate fill requirements.
Common Conduit Fill Scenarios
The table below shows maximum wire counts for frequently used combinations. All values assume same-size conductors of one insulation type per NEC Chapter 9.
| Conduit | Wire | Max Wires | Typical Use |
|---|---|---|---|
| ½" EMT | 12 THHN | 9 | Branch circuits, switches |
| ¾" EMT | 12 THHN | 16 | Multi-circuit home runs |
| 1" EMT | 10 THHN | 16 | 30A circuits, sub-panels |
| 1¼" EMT | 8 THHN | 16 | 40–50A feeders |
| 2" EMT | 6 THHN | 26 | 60A sub-feeders |
| 3" EMT | 3/0 THHN | 13 | 200A service entrance |
When mixing wire sizes in the same conduit, calculate total conductor area and compare against the conduit's total allowable fill area at 40%.
Formulas Used in This Calculator
All values are sourced from NEC Chapter 9, Tables 1, 4, and 5. The calculator performs a straightforward area-based comparison.
Fill % = (Total wire area ÷ Conduit internal area) × 100
Max fill = 53% (1 wire) | 31% (2 wires) | 40% (3+ wires)
Max wires = ⌊ (Conduit area × Max fill %) ÷ Wire area ⌋
Pass/Fail = Fill % ≤ Max fill %
Wire areas include insulation. Conduit areas are the internal cross-section per NEC Chapter 9, Table 4. All areas in square inches.
Code Notes & Sources
- Conduit fill limits come from NEC Chapter 9, Table 1: 53 percent for one conductor, 31 percent for two, and 40 percent for three or more. The conductor areas used here are the insulated-wire areas in Table 5, and the conduit areas are the internal cross-sections in Table 4.NFPA 70 (NEC) Chapter 9, Tables 1, 4, and 5
Next Steps
Fill is one check on the raceway. Confirm the rest:
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NEC limits conduit fill to 40% for three or more conductors. Browse the electrical calculator collection.