Wire size & voltage drop

The calculator that explains why your pump 300 feet away doesn't start — low-voltage DC runs punish distance brutally.

Result

System voltage12 V

Current10 A

One-way distance50 ft

Acceptable voltage drop3 %

Minimum copper wire size

4 AWG

Needs 35,833 circular mils; 4 AWG has 41,740.

Actual drop at that size

0.31 V (2.6%)

Volts arriving at the load

11.69 V

Power lost as heat in the wire

3.1 W

Ampacity check

OK — 4 AWG carries ~85 A

What this will not do

This sizes for voltage drop, which at low voltage is usually the binding constraint — but ampacity, breaker sizing, conduit fill, insulation type and burial depth are code matters. For anything that touches your house wiring, an electrician signs off. This calculator informs the conversation; it does not replace it.
Doubling the voltage quarters the loss. If a 12 V run is forcing you into enormous cable, the real answer is a 24 V or 48 V system, not thicker wire.
Copper assumed. Aluminum needs roughly 1.6× the circular mils and different terminations.

The full build and the reasoning are in Chapters 40, 41 of the guide.