Conduit Fill Calculator
Overstuffing conduit makes pulling wire nearly impossible and violates the NEC. This calculator uses the fill limits from NEC Chapter 9 to tell you whether your wire combination fits. Pick your conduit type and size, select the wire gauge and insulation type, and enter how many conductors you need. You get the fill percentage and a clear pass or fail result.
NEC Conduit Fill Limits
The National Electrical Code sets maximum fill percentages based on how many conductors are in the conduit:
- 1 conductor -- 53% fill maximum
- 2 conductors -- 31% fill maximum
- 3 or more conductors -- 40% fill maximum
These limits exist because wires generate heat, and tightly packed conductors can't dissipate that heat properly. Overfilling also makes pulling wire through the conduit physically difficult and risks damaging insulation.
Common Conduit Types
- EMT (Electrical Metallic Tubing) -- Thin-wall steel, most common in commercial and residential. Easy to bend, uses compression or set-screw fittings.
- PVC Schedule 40 -- Plastic conduit, popular for underground and wet locations. Lower cost, no corrosion.
- PVC Schedule 80 -- Thicker walls than Schedule 40, required for exposed outdoor installations.
- Rigid Metal (RMC/IMC) -- Heavy-duty steel or aluminum, used in industrial settings and where physical protection is critical.
Practical Tips
A good rule of thumb: if you can pull the wire bundle through by hand without excessive force, the fill is probably fine. If you need a pull rope, cable lube, and three people, you might be over the limit.
Always count ALL conductors, including ground wires. Equipment grounding conductors count toward fill even though they don't carry normal current.