Add your conductors, get the total fill area, and find the minimum conduit trade size—EMT, IMC, or rigid. No more guessing on the job site or arguing with inspectors.
When 3 or more conductors are in a conduit, maximum fill is 40% of the conduit's interior cross-sectional area. 2 conductors = 31%. 1 conductor = 53%. These limits exist for heat dissipation and future wire pull capacity. Overfilling causes insulation damage and failed inspections—size up.
Each wire gauge and insulation type has a specific cross-sectional area defined in NEC Annex C. THHN 12 AWG is 0.0133 in². XHHW 12 AWG is 0.0153 in²—15% larger. The insulation type matters. Use actual NEC table values—this calculator pulls directly from those tables.
The NEC allows 40% fill, but experienced electricians size for 25–30% fill on any conduit that might need additional circuits later. Upsizing from ¾″ to 1″ EMT adds $0.30/lf in materials and saves $8–$12/lf if you need to pull additional wire in 5 years. The conversation with the customer takes 30 seconds.
The Electrician's Business Starter Kit includes NEC quick-reference cards for conduit, ampacity, and box fill, plus a flat-rate price book and service agreement template. Guide from $19 — Complete Kit with all 19 templates, $49.
Get the Electrician's Kit →NEC conduit fill limits exist to prevent overheating, and exceeding them fails inspection. This calculator determines the maximum number of conductors allowed in a given conduit size and type based on wire gauge and insulation type. Use it during takeoff to spec the correct conduit and avoid costly rework when the inspector shows up.