Voltage Drop Calculator

Why the wire eats your volts

Current through the finite resistance of a conductor drops voltage along the way. For a two-wire run the current goes out and comes back, so the drop uses twice the one-way length:

with resistivity $\rho$ (copper $1.724\times 10^{-8}$, aluminium $2.82\times 10^{-8}\Omega \cdot \text{m}$), length $L$ one-way and cross-section $A$ from the gauge. A common rule of thumb keeps drop under 3% of the supply for signal/logic rails and under 5% for power — beyond that, go up a gauge or shorten the run.