Voltage Drop Calculator

One source, one end load

The two conductors have equal length, size and temperature. The calculation doubles the one-way distance. Copper resistance is corrected from 20°C using a linear coefficient of 0.00393 per °C.

Rloop = R20 × [1 + 0.00393 × (T − 20)] × 2L / 1000
DC: Vdrop = I × Rloop · Vdevice = Vsupply − Vdrop
Cable power loss = I² × Rloop

R20 is the single-conductor resistance in Ω/km and L is the one-way distance in metres. AWG and mm² choices estimate ideal annealed copper resistance; stranding, plating, terminals and manufacturing tolerances are not included. Use a manufacturer's 20°C resistance for the actual cable.

AC and rated-power inputs

AC uses a sinusoidal fixed-current phasor calculation with the entered lagging power factor and cable reactance set to zero. AC voltage drop is the difference between source and device RMS magnitudes. At unity power factor it equals I × Rloop. Nonlinear loads and cable inductance require a more complete model.

W (DC) and VA (AC) inputs are converted to current using the entered supply voltage. That current is then held fixed; the tool does not solve a constant-power device whose current rises as its voltage falls. Supply regulation, inrush, connections and shared branches are excluded. This tool does not determine ampacity, fuse size, transformer VA or code compliance.

Resistance basis: NBS/NIST Copper Wire Tables, Handbook 100. Temperature correction: Cirris copper temperature coefficient. Full equations, numerical limits and verification notes.