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AWG Wire Gauge & Voltage Drop
Calculate physical properties of AWG solid copper wire and predict voltage drop and loop resistance.
AWG Diameter Formula (ASTM B258): d = 0.127 × 92^((36-AWG)/39) mm. The cross-sectional area is A = (π/4) × d².
Copper Temperature Correction: Copper resistivity increases with temperature. The formula applied is ρ_T = ρ_20 × (1 + α(T - 20)) where ρ_20 = 1.7241 × 10^-8 Ω·m and α = 0.00393 / °C.
Voltage Drop: V_drop = I × R_total where R_total is calculated for the round-trip wire length (2 × One-Way Length).
Safety & Drop Rating grades the drop %: <3% PASS, 3–5% WARNING, >5% FAIL. A FAIL means the drop is too large (load under-voltage & wire heating) — pick a thicker wire (lower AWG number), shorten the run, or raise the supply voltage.
When you need it: Choosing a wire gauge for a current and run length, then checking the resulting voltage drop and self-heating before you commit to a harness.
Worked example: 18 AWG is about 0.82 mm² and ~2.3 mΩ/ft. Carrying 5 A over a 10 ft round trip (20 ft of copper) drops 5 × 0.046 Ω ≈ 0.23 V.
Tips & gotchas:
- Three AWG numbers lower doubles the copper area and roughly halves the resistance.
- Size for both ampacity (heating) and voltage drop — on long runs the drop is usually the tighter limit.
- Bundling wires and higher ambient temperature both derate the safe current.
- Count the full loop length (out and back) when computing drop, not just the one-way distance.