Conduit Fill Calculator
NEC 40% fill (3+ wires). Find conduit size that holds N wires of given gauge.
Result
Loading calculator…
—
How to use this calculator
- Enter wire count + gauge.
- Read smallest conduit that meets 40% fill.
About this calculator
NEC chapter 9 table 1: when 3+ wires share conduit, total wire area cannot exceed 40% of conduit cross-section. Why: heat dissipation, pulling without damaging insulation. THHN wire areas (cross section): 14 AWG 0.0097 in², 12 AWG 0.0133 in², 10 AWG 0.0211 in². EMT conduit ½" allows 0.122 in² fill; ¾" allows 0.213; 1" allows 0.346.
Frequently asked
Why 40% fill?+
NEC 9-1: for 3+ wires. 1 wire allows 53% fill, 2 wires allow 31%. Heat + pulling friction set the limit.
THHN vs THWN?+
Almost identical insulation areas. Use THHN-specific table (most common). Slight differences for XHHW.
Bonding/grounding wire counts?+
Small equipment grounding conductors don't count toward fill. Larger ones do — check NEC 250.
EMT vs PVC vs flex?+
Slightly different inner diameters. Most calculators (this one too) approximate from EMT. Verify with NEC for exact.
Bend penalty?+
Each 90° bend reduces effective fill capacity. Many electricians limit to 360° total in a single run.
Related calculators
Breaker Size for Load
Required breaker amps = load watts × 1.25 / volts. NEC 80% derate rule.
kWh Cost Per Month
Cost = W × hr/day × 30 / 1000 × $/kWh.
Solar Panel Count for Energy Needs
Panels = daily kWh need / (panel-W × sun-hours × derate / 1000).
Wire Gauge for Amperage Calculator
NEC ampacity: typical residential AWG sizes for current load.
Voltage Drop Calculator
Vd = (2 × K × I × L) / CM for single-phase circuits. NEC recommends ≤ 3% drop.
Ohm's Law Calculator (Residential)
V = IR. Pick which to solve for. Residential circuit framing.