How this calculator works
Use 24 V as the nominal source and enter current at that voltage. Select the balanced three-phase option only for a resistive load with 24 V line-to-line; otherwise the model uses two equal conductors. The cross-voltage table always compares two-wire supplies at the same total source power. At 24 V, a 3% drop leaves 23.28 V at the load. Temperature changes resistance through a rounded coefficient of 0.0039/K for either metal. The calculation omits connection losses, cable reactance and any temperature rise calculation. Ampacity and protection remain separate from the voltage-drop area.
Formula
DeltaV = k rho(T) L I/A; A = k rho(T) L I/DeltaV; L = DeltaV A/(k rho(T) I); I = DeltaV A/(k rho(T) L). For two wires k = 2; for balanced three-phase k = sqrt(3). rho(T) = rho20[1 + alpha(T - 20 C)].
Worked example
At 24 V, 20 A over 10 m one-way with a 3% limit requires 9.3333 mm² of copper at 20 C. The equivalent 480 W source load at 12 V draws 40 A and requires 37.3333 mm² over the same two-wire run.
Advertisement
Frequently asked questions
Is 24 V always the actual battery voltage?
No. State of charge and charging equipment change terminal voltage. Use the general wire size page when a measured source voltage matters.
Can I use this for a 24 V control circuit?
It estimates conductor drop for the entered steady current. Check device minimum voltage and any switching or starting current separately.
Does the comparison include converter efficiency?
No. It holds electrical power supplied to the cable constant. Conversion losses and differences between actual devices are excluded.