NEC Conduit Fill Reference
A plain-language overview of the raceway-fill framework used by the calculator. This page is not a substitute for the adopted edition of NFPA 70.
Chapter 9 percentage framework
The public NFPA development record for Chapter 9, Table 1 documents a 53% maximum for one conductor or cable, 31% for two, and 40% for more than two in a complete conduit or tubing system. The calculator also offers a fixed 40% low-voltage planning mode, which is distinct from automatically changing the percentage by cable count.
Use actual cable dimensions
Cable fill is an area calculation. For a round cable, the calculator converts outside diameter to cross-sectional area, multiplies by quantity, and compares the combined occupied area with the permitted raceway area. Manufacturer outside diameter should be confirmed for the exact product revision.
Conditions beyond percentage fill
A percentage-only result does not evaluate the complete physical pull. Pull length, bend configuration, cable alignment, jamming, pulling tension, sidewall pressure, and manufacturer limits can require a larger raceway or lower fill.
Authoritative source
Consult the project’s adopted edition of NFPA 70, National Electrical Code. A public NFPA Chapter 9 committee record provides supporting context for the percentages summarized here. Complete copyrighted NEC tables are intentionally not reproduced.