Wire Size Calculator

Wire gauge selection balances safety, voltage drop, and cost. Undersized conductors overheat under sustained current; oversized wire wastes copper and complicates terminations. National electrical codes set minimum ampacity for branch circuits, but long runs — solar arrays, shed subpanels, EV charger feeders — also require voltage-drop checks so equipment at the far end receives adequate voltage.

Voltage drop on a round-trip (out-and-back) DC or single-phase AC run is:

V_drop = I × 2 × L × R_wire

Where I is load current in amperes, L is one-way length in meters, and R_wire is resistance per meter for the chosen AWG size. Select the smallest gauge whose drop stays below your limit — commonly 3% for branch circuits or 2% for sensitive loads.

AWG numbers run inverse to diameter: lower gauge means thicker wire and lower resistance. AWG 10 copper handles roughly 30 A in conduit; AWG 14 suits 15 A lighting circuits. Ampacity tables assume specific insulation temperature ratings and bundling — always verify against local NEC, IEC, or BS 7671 requirements for your installation type.

Worked example: A 12 V solar pump draws 8 A, 25 m one-way (50 m round trip). Allow 3% drop: max V_drop = 0.36 V. AWG 14 (2.08 mm²) has ~0.0083 Ω/m → drop = 8 × 50 × 0.0083 = 3.32 V — too much. AWG 10 (5.26 mm²) at ~0.0033 Ω/m → drop = 1.32 V (11%) — still high. AWG 6 (13.3 mm²) at ~0.0013 Ω/m → drop = 0.52 V (4.3%). AWG 4 (21.2 mm²) → drop ≈ 0.33 V (2.7%) — acceptable. The calculator automates this search across standard AWG sizes.

Pair with the AWG-to-mm² converter for metric projects and the PCB trace width calculator for board-level current paths.