How this calculator works
For a resistive two-wire circuit, loop resistance is twice the one-way conductor resistance. A balanced three-phase resistive load uses the square root of three multiplier with line-to-line voltage. Solving the drop relation for area gives the minimum from this criterion alone. Resistivity is corrected linearly from 20 C using representative metal coefficients. The AWG comparison rounds upward in area. Ampacity, insulation, fault protection, grouping, installation rules and reactive voltage drop are separate requirements.
Formula
DeltaV = k rho(T) l I/A; A = k rho(T) l I/DeltaV; k = 2 for two wires or sqrt(3) for three phase; rho(T) = rho20[1+alpha(T-20 C)].
Worked example
At 20 C, copper rho = 1.68 × 10^-8 ohm m. A 10 A two-wire circuit over 10 m one-way with a 0.3 V drop limit needs 11.2 mm² by voltage drop. The inverse current calculation gives 10 A for that area.
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Frequently asked questions
Is the calculated current an ampacity rating?
No. It is only the current meeting the entered voltage-drop limit; thermal and code limits can be lower.
Is length one-way or round trip?
Enter one-way distance. The two-wire multiplier already includes the return path.
Can I use this for a low-power-factor motor?
The simple resistive voltage-drop relation omits cable reactance and load phase angle. Use a full AC cable model when those effects matter.