What this calculator does
Choosing a wire that is too thin for the current and distance it needs to carry causes the voltage to drop along the cable, which can leave a device underpowered, dim lights, or in the worst case overheat the wire itself. This calculator recommends a standard AWG (American Wire Gauge) copper wire size from the current draw, the one-way cable length and how much voltage drop is acceptable, which is the standard approach for sizing 12V DC circuits such as solar, automotive and marine wiring.
The calculation works from published resistance-per-length values for standard AWG copper wire sizes. It works out the maximum resistance the circuit can tolerate for the voltage drop you specify, then finds the thinnest wire gauge on the standard AWG table whose resistance comes in at or under that limit, accounting for the full round-trip length of both the outgoing and return conductor.
The formula
The maximum allowable resistance is (system voltage × allowed drop %) ÷ current. That resistance is compared against a standard table of AWG wire resistance per 1,000 ft, scaled to the round-trip cable length (twice the one-way run, since current has to travel out and back). The thinnest gauge whose resistance is at or below the limit is recommended, along with the actual voltage drop it would produce.
| Term | Meaning |
|---|---|
| AWG | American Wire Gauge, the standard sizing system for round wire; a smaller AWG number means a thicker wire. |
| Voltage drop | The voltage lost along a cable due to its resistance, equal to current × wire resistance. |
| Round-trip length | Twice the one-way cable distance, since a DC circuit needs a conductor running out to the load and another returning to the source. |
The inputs explained
| Field | What to enter |
|---|---|
| Current draw (A) | The current the circuit will actually draw, in amps. |
| One-way cable run length (ft) | The one-way distance from the power source to the device, in feet. The calculator doubles this for the round-trip resistance. |
| System voltage (V) | The nominal system voltage, such as 12V for a typical automotive, marine or solar DC circuit. |
| Maximum acceptable voltage drop (%) | The maximum voltage drop you are willing to accept, as a percentage of system voltage. 3% is a common target for 12V DC circuits; some sensitive electronics call for less. |
When to use it
Wiring a 12V solar or battery system
A cable run from a battery or charge controller to a load draws current over a real distance, and an undersized wire on a low-voltage system loses a much larger share of the available voltage than the same drop would on a higher-voltage circuit.
Running power to accessories in a vehicle or boat
Auxiliary lighting, winches and other 12V accessories are often mounted well away from the battery, and this calculator sizes the wire for the actual run length rather than guessing.
Checking an existing installation
Entering the wire's actual gauge alongside the current and length (by comparing its resistance per 1,000 ft against the calculator's recommendation) shows whether an existing cable run is adequate or under-sized.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What gauge is needed for common 12V loads at a 15 ft run?
A range of current draws at a fixed cable length and drop tolerance.
| Current | Recommended wire size | Actual voltage drop at this gauge |
|---|---|---|
| 5 A | 12 AWG | 0.24 V |
| 10 A | 10 AWG | 0.30 V |
| 15 A | 8 AWG | 0.28 V |
| 20 A | 6 AWG | 0.24 V |
| 30 A | 6 AWG | 0.36 V |
| 50 A | 2 AWG | 0.23 V |
How does cable length change the recommended gauge?
A fixed 20A load at a range of one-way cable lengths.
| One-way cable length | Recommended wire size | Actual voltage drop at this gauge |
|---|---|---|
| 5 ft | 12 AWG | 0.32 V |
| 10 ft | 8 AWG | 0.25 V |
| 20 ft | 6 AWG | 0.32 V |
| 30 ft | 4 AWG | 0.30 V |
| 50 ft | 2 AWG | 0.31 V |
| 75 ft | 1/0 AWG | 0.29 V |
Questions
Why does the calculator double my cable length?
A DC circuit needs a complete loop: current flows out to the load along one conductor and returns along another. Both conductors add resistance, so the round-trip length, not just the one-way distance, is what determines total voltage drop.
What voltage drop percentage should I use?
A commonly used target for 12V DC circuits is 3%, and some sensitive electronics or long runs call for 1 to 2%. There is no universal rule; check the requirements of the specific device or the wiring standard you are working to.
Are these resistance values accurate for any wire?
They are standard published resistance-per-length figures for solid copper wire at each AWG size. Actual resistance varies slightly with temperature, and stranded or aluminium wire has different figures, so treat the recommendation as a solid starting point rather than a substitute for the wire manufacturer's own spec.
Does this apply to AC wiring too?
The voltage-drop physics is the same for AC and DC, but AC circuits are also governed by local electrical codes covering ampacity, insulation ratings and installation method, which this calculator does not check. For household AC wiring, follow the applicable electrical code or use a licensed electrician.
For the underlying voltage, current and resistance relationship this calculator is built on, see the Ohm's law calculator.