Fit your wires under the NEC limit.
Pick your conduit, wire size and conductor count to see the fill percentage, whether it passes NEC Chapter 9, and exactly how much room is left before an inspector fails the run.
Your run
What this means
Fill for 1 to 10 conductors
| Conductors | Conductor area | Fill % | NEC max | Result |
|---|
How conduit fill works
How NEC Conduit Fill Is Calculated
Conduit fill is the percentage of a conduit’s internal cross-sectional area taken up by the conductors inside it. The National Electrical Code limits this so wires can be pulled without damage and so heat can dissipate. The calculator divides the total conductor area by the conduit area and compares the result to the NEC limit.
The conduit area comes from NEC Chapter 9, Table 4, and the conductor area comes from Table 5. This tool uses THHN/THWN-2 values, the most common building wire in conduit.
NEC Maximum Fill Percentages
NEC Chapter 9, Table 1 sets the maximum fill based on how many conductors are in the raceway. One conductor may fill up to 53 percent, two conductors up to 31 percent, and three or more conductors up to 40 percent. The lower limit for two wires reflects the difficulty of pulling a pair without jamming.
Why Conduit Fill Matters
Overfilled conduit makes conductors hard to pull, can strip insulation during installation, and traps heat that shortens wire life. An inspector will fail an overfilled run. Sizing the conduit correctly the first time avoids repulling and keeps the install code-compliant.
Common questions
What is the maximum conduit fill allowed by the NEC?
It depends on the number of conductors: 53 percent for one conductor, 31 percent for two, and 40 percent for three or more, per NEC Chapter 9, Table 1.
Does the ground wire count toward conduit fill?
Yes. Equipment grounding conductors count in the fill calculation. Include every conductor in the raceway in the conductor count.
What insulation type does this calculator use?
It uses THHN/THWN-2 conductor areas from NEC Chapter 9, Table 5, which is the most common building wire run in conduit. Other insulations have different areas.
Which conduit types are supported?
EMT, PVC Schedule 40, and rigid metal conduit (RMC). Each has its own internal area from NEC Chapter 9, Table 4.
My run fails the fill limit. What should I do?
Step up to the next conduit trade size, or split the conductors across two conduits. You can also use a smaller conductor if ampacity still allows it.
Estimates only, using THHN/THWN-2 areas from NEC Chapter 9, Tables 1, 4 and 5. Verify against your adopted code edition and local amendments.