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Select AWG wire size from ampacity and circuit load. Calculate conductor size for copper and aluminum per NEC ampacity tables with temperature rating.
Calculate total electrical load for residential service sizing. Sum appliance watts and apply demand factors for panel and service entrance capacity.
Size circuit breaker from wire gauge and load type. Match overcurrent protection to conductor ampacity per NEC 240.4 for residential circuits.
K=12.9 copper, I=amps, L=one-way feet, CM=circular mils of wire. Factor 2 for round-trip length.
VD = 2 × K × I × L ÷ CM (single phase)NEC informational note recommends max 3% on branch circuits, 5% total from service to load.
VD% = (VD ÷ System Voltage) × 100Upsize conductor when drop exceeds limit even if ampacity is adequate on short calc.
Increase AWG until VD% ≤ 3% (or design limit)Updated: July 2026
VD ≈ 3.9V (3.25%) — borderline for 3% branch limit. Consider 10 AWG for long shed circuit.
Long 240V run at 20A may need 8 AWG despite 20A ampacity on 12 AWG. Voltage drop drives sizing.
Lower voltage DC systems tolerate less drop — often design for 1–2% on PV source circuits.
Determine voltage drop on long wire runs to ensure NEC-recommended limits (3% branch, 5% total). Enter load current, wire AWG, one-way length, and system voltage to calculate drop in volts and percent, and size up wire if needed.