The 40 Percent Fill Rule
The National Electrical Code limits how much of a conduit the wires can occupy, and for the usual case of three or more conductors that limit is 40 percent of the conduit interior. The limit exists so wire can be pulled without damage and so heat can escape. The percentage changes with conductor count: a single conductor may fill 53 percent, two conductors may fill 31 percent, and three or more are held to 40 percent.
THHN in EMT Quick Reference
The numbers below are a common reference for THHN conductors of the same size in EMT conduit. Always confirm against the NEC tables for your exact conduit type, since PVC and rigid metal have different interior areas than EMT.
| Wire Size (THHN) | 1/2 inch EMT | 3/4 inch EMT | 1 inch EMT |
|---|---|---|---|
| 14 AWG | 12 | 22 | 35 |
| 12 AWG | 9 | 16 | 26 |
| 10 AWG | 5 | 10 | 16 |
| 8 AWG | 3 | 6 | 9 |
Count the Ground Wire
A common mistake is leaving the equipment grounding conductor out of the fill count. The ground takes up space and must be included along with the hots and neutral. Inspectors check this, and forgetting it is a frequently cited violation.
Watch Out for Derating
Fitting wires in the conduit is only half the question. When more than three current-carrying conductors share a raceway, NEC 310.15(C) requires you to derate their ampacity because the bundled heat cannot escape as easily. Neutral conductors carrying unbalanced current count toward that total. On a long or heavily loaded run, derating can mean you need a larger wire even when the conduit has room.
Leave Room to Pull
Just because you can hit 40 percent does not mean you should. For long runs or pulls with several bends, many electricians target 30 percent or less so the wire pulls cleanly without nicking the insulation. A little extra conduit size saves a lot of effort on the pull.
Code and Safety Note
Conduit fill, derating, and bend counts all interact. Follow the National Electrical Code and local amendments, confirm with your authority having jurisdiction, and use a licensed electrician for conduit and circuit work.
IB Lighting stocks Southwire THHN and THWN-2 building wire by the spool in our electrical wire and cable collection, with free freight plus free freight insurance on every order. Call (800) 674-9019 for the gauges and footage your conduit run needs.