Electrical
NEC Conduit Sizing Calculator
Calculate the right conduit trade size for any wire mix. Runs the NEC Chapter 9 and Annex C fill tables so you don't have to.
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Conduit
Fill mode
Standard NEC Chapter 9 Table 1 limits: 53% for one conductor, 31% for two, 40% for three or more. Use this for typical raceway runs.
Wire groups
EGC: 1 x 10 AWG Copper THHN/THWN
Calculations use NEC Chapter 9 Table 4 for conduit dimensions and Table 5 for conductor cross-sectional areas. Fill limits in Auto mode are 53% for one conductor, 31% for two, and 40% for three or more (NEC Chapter 9 Table 1). Nipple = 60% (Note 4). Seal-off = 25% (NEC 501.15(C)(1)).
Conduit fill
27.8
%4 conductors in EMT 1/2"(auto)
NEC compliant
Comfortably within the 40% fill limit. Room for future conductors.
Heads up: with more than 3 current-carrying conductors in a raceway, NEC 310.15(C)(1) ampacity adjustment may apply. Check separately.
About this calculator
NEC-compliant. Free to use. No watermarks.
Conduit sizing comes down to one rule: the cross-sectional area of all the conductors inside can't exceed the NEC's allowable fill percentage for that conduit. 40% for three or more conductors, 53% for one, 31% for two.
The math is simple. The lookup is the painful part. You're cross-referencing wire size, insulation type, and conductor count against multiple NEC tables for every job. This calculator does all of that for you, with EMT, IMC, RMC, ENT, LFNC, LFMC, and PVC schedules supported.
NEC references
- NEC Chapter 9, Table 1: Conduit Fill Percentages
- NEC Chapter 9, Table 4: Internal Dimensions of Conduits
- NEC Chapter 9, Table 5: Cross-Sectional Areas of Conductors
- NEC Annex C: Conduit and Tubing Fill Tables
The same calculations, running live as you design.
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