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Check NEC conduit fill for EMT, PVC Schedule 40, and rigid metal conduit. See how many THHN/THWN conductors fit under NEC Chapter 9, Table 1.
Ask questions about your calculation results
3 free questions per session
AI provides general information, not financial advice. Always consult a qualified professional.
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 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.
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.
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.
Yes. Equipment grounding conductors count in the fill calculation. Include every conductor in the raceway in the conductor count.
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.
EMT, PVC Schedule 40, and rigid metal conduit (RMC). Each has its own internal area from NEC Chapter 9, Table 4.
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.