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Conduit Fill Calculator — NEC Wire Capacity Estimator

Calculate wire capacities and conduit sizing ratios. Ensures safe, code-compliant spacing within standard electrical raceways.

Under NEC Chapter 9, Table 1, the maximum allowable fill factor for three or more conductors inside a conduit is exactly 40%. For a single wire, it is 53%, and for two wires it is 31%. Exceeding these thresholds leads to wire heating and insulation failure.

NEC Conduit Fill Calculator
THHN / THWN Insulation Standards
Conduit Spatial Output
Conduit Dimension
Estimated Result
NEC Target Limit
Spacing Status
Total Conduit Area
0.304 in²
Total Wires Area
0.053 in²
Calculated Fill Percentage
17.5 %
Allowable Fill Limit
40.0 %
Conduit Fill Status: Passes! Plenty of space remaining. ✔ NEC COMPLIANT

Standard Conduit Fill Rules

The NEC regulates the maximum fill ratio of conductors to prevent overheating due to current flow and to allow cables to be safely pulled without tearing insulation:

NEC Chapter 9, Table 1 Fill Limits:
  • 1 Wire inside conduit: Max 53% fill ratio.
  • 2 Wires inside conduit: Max 31% fill ratio.
  • 3+ Wires inside conduit: Max 40% fill ratio.

EMT Conduit Wire Capacity Chart (40% Fill Rule)

Conduit Size (EMT) Allowable Area (40% sq. in.) Max 14 AWG Max 12 AWG Max 10 AWG Max 8 AWG Max 6 AWG
1/2" EMT 0.122 in² 12 wires 9 wires 5 wires 3 wires 2 wires
3/4" EMT 0.213 in² 21 wires 16 wires 10 wires 5 wires 4 wires
1" EMT 0.346 in² 35 wires 26 wires 16 wires 9 wires 6 wires
1-1/4" EMT 0.598 in² 61 wires 45 wires 28 wires 16 wires 11 wires
NEC Chapter 9 EMT conduit wire limits for standard THHN copper.

Real World NEC Sizing Example: 8 THHN in 3/4 EMT

A frequent job-site question when using a conduit sizing calculator is determining exact wire counts for common runs. For instance, exactly how many 8 thhn in 3 4 emt can you safely pull?

  • 1. Find the Conduit Area: According to NEC Chapter 9, a 3/4" EMT conduit has a total internal area of 0.533 square inches. Under the 40% fill rule, the maximum allowable fill is 0.213 square inches.
  • 2. Find the Wire Area: A single #8 AWG THHN wire has a cross-sectional area of 0.0366 square inches.
  • 3. The Math: Divide the allowable area by the wire area (0.213 ÷ 0.0366 = 5.82).
  • Conclusion: You can legally and safely pull a maximum of 5 wires of #8 THHN in a 3/4" EMT conduit.

This same mathematical process applies regardless of what wire size in conduit you are calculating. Always use the strict 40% limit for three or more conductors to prevent overheating and insulation damage during the pull.