Tools you might need next
Select AWG wire size from ampacity and circuit load. Calculate conductor size for copper and aluminum per NEC ampacity tables with temperature rating.
Calculate voltage drop on wire runs for NEC compliance. Enter wire size, length, amperage, and voltage to find drop percent and minimum wire size.
Calculate total electrical load for residential service sizing. Sum appliance watts and apply demand factors for panel and service entrance capacity.
Sum cross-sectional areas of all conductors including equipment grounding conductor.
Total Area = Σ (Number of Wires × Area per Wire from Ch. 9 Table 5)NEC Chapter 9 Table 1 fill percentages based on conductor count in raceway.
1 conductor: 53%; 2 conductors: 31%; 3+ conductors: 40%Internal area from Chapter 9 Table 4 for conduit type. Fill must not exceed maximum for conductor count.
Fill % = (Total Conductor Area ÷ Conduit Internal Area) × 100Updated: July 2026
Three conductors → 40% max fill. 12 AWG THHN area 0.0133 sq in × 3 = 0.040. 1/2″ EMT 40% = 0.122 sq in — 33% fill, OK.
Six 10 AWG in 3/4″ EMT may exceed 40%. Calculator flags overfill — upsize to 1″ conduit.
All conductors occupying same raceway count toward fill regardless of circuit type unless separated by partition.
Verify NEC conduit fill limits before pulling wire. Enter conduit trade size, conductor AWG, insulation type, and number of conductors to calculate fill percentage and confirm compliance with Chapter 9 tables.