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.
Free templates and pricing strategy for solo tradespeople. Sent every Friday.
One email per week. Unsubscribe anytime.
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.