Estimate the voltage drop over a run of copper wire from its gauge, length and current.
How voltage drop is worked out
Long wire runs lose voltage to resistance. Enter the wire gauge, one-way length and current, and this calculator estimates the voltage drop and the voltage that reaches the load.
Keeping drop under about 3% is a common target.
Using the calculator
- Choose the wire gauge (AWG).
- Enter the one-way length and current.
- Read the voltage drop and end voltage.
Formula reference
drop = 2 × length × current × resistance ÷ 1000
(resistance in ohms per 1000 ft for copper)
A worked example
On the values this calculator opens with, the voltage drop is 4.76 V. Underneath, % drop comes out at 3.97% and Voltage at load at 115.2 V. Change any field and every figure updates as you type.
Frequently asked questions
What causes voltage drop?
Wire resistance. Longer runs, higher current and thinner wire all increase the drop.
How much drop is acceptable?
A common guideline is under 3% for branch circuits, so devices get enough voltage to run properly.
How do I reduce it?
Use thicker wire (lower AWG number), shorten the run, or reduce the current.