How to Use This Tool
Enter the load, the one-way run length and the gauge you were planning to use. The tool reports the drop and then finds the smallest conductor that stays inside the limit.
Two different questions, two different answers
Ampacity asks whether the conductor will overheat. That is a question about current alone, and it is what wire size charts answer: 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A.
Voltage drop asks whether enough voltage arrives at the far end. That depends on current times length, so a short 20 A run and a long 20 A run need different conductors. Both questions have to pass, and on runs over about 50 feet the second one usually decides.
VD = 2 x K x A x L / CM 14 AWG 2 x 12.9 x 20 x 100 / 4,110 = 12.55 V = 10.46% 12 AWG 2 x 12.9 x 20 x 100 / 6,530 = 7.90 V = 6.58% 10 AWG 2 x 12.9 x 20 x 100 / 10,380 = 4.97 V = 4.14% 8 AWG 2 x 12.9 x 20 x 100 / 16,510 = 3.13 V = 2.60% passes
Where the formula comes from
The 2 is the round trip: current flows out along one conductor and back along the other, so the resistive length is twice the run. K is the resistivity of the metal expressed in ohm-circular-mils per foot, 12.9 for copper and 21.2 for aluminium. CM is the conductor's cross-sectional area in circular mils, which is what AWG numbers actually encode.
Because CM roughly doubles every three gauge sizes, dropping three sizes halves the voltage drop. That is a more useful mental model than memorising a table.
Why undersizing kills motors rather than tripping breakers
A resistive load at reduced voltage simply produces less heat or light. A motor is different: it draws more current to deliver the same mechanical power, and that extra current heats the windings. A well pump, compressor or workshop tool at the end of a long undersized run runs hot for years and then fails, with no breaker trip to point at the cause.
Low voltage is where this bites hardest
The limit is a percentage, and on 12 V systems 3 per cent is only 0.36 V. Landscape lighting, solar and automotive runs therefore need conductors that look absurdly large for the current, which is why a 12 V LED path light 60 feet from the transformer often needs 10 AWG for a couple of amps.
Aluminium needs two sizes up
Aluminium's resistivity is about 1.64 times copper's, so for the same drop it needs roughly two AWG sizes larger. It also requires different terminations and antioxidant compound, so it is not a substitution to make casually on an existing circuit.
The 3 per cent figure is a recommendation
In the NEC, voltage drop appears in informational notes rather than as a mandatory limit: 3 per cent on a branch circuit and 5 per cent total including the feeder. Some jurisdictions adopt it as a requirement. Treat it as good practice and confirm locally.
