Free Wire Voltage Drop calculator with clear step-by-step results.
Voltage drop is current times the resistance of the whole circuit path, which for a single-phase or DC run means twice the cable length because current returns through the second conductor. Three-phase circuits use a root-three factor instead. Aluminium's higher resistivity increases drop by about 64% for the same cross-section.
Voltage drop
Drop = k x resistivity x length / CSA x current, where k = 2 single phase or root 3 three phase
Power lost
Watts = voltage drop x current
Cable sizing must satisfy voltage drop, current-carrying capacity after derating, and fault-loop impedance requirements simultaneously. Designs must comply with the applicable wiring regulations and be verified by a qualified electrician.
Commonly 3% for lighting and 5% for other circuits measured from the origin of the installation, though specific limits vary by code and by circuit type.
Low voltage at the load causes motors to draw more current and overheat, dims lighting, and can prevent contactors from holding in. The dissipated power also heats the cable.