How this calculator works
The area and distance modes hold load current at 100 A. A run-length table shows how the area requirement changes from 1 to 100 metres at your source voltage. The current mode checks a known conductor against the same drop limit. Results use one conductor area and one-way distance; the return path is included. Copper and aluminium use rounded textbook resistivity at 20 C and alpha = 0.0039/K. These estimates do not establish a conductor ampacity or a compliant 100 A service size. Cable reactance, terminals, insulation, installation method and protective devices need separate checks.
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
For 100 A at 120 V, a 3% drop permits 3.6 V. Copper at 20 C over 10 m one-way requires 9.3333 mm² by resistance alone. At 240 V, a 25 mm² copper conductor reaches the 3% limit at 53.5714 m one-way.
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Frequently asked questions
Does this tell me the code size for a 100 A panel?
No. A voltage-drop calculation cannot determine service or feeder ampacity. Use the applicable installation rules and equipment ratings for that decision.
Why does changing 120 V to 240 V change area?
At the same 100 A and percentage limit, twice the voltage allows twice the voltage drop and therefore half the resistance-based conductor area.
What does the run table hold constant?
It holds current at 100 A and retains your voltage, material, temperature, circuit geometry and percentage drop. Only one-way distance changes.