DC voltage drop calculator
Calculate voltage drop, percent drop, voltage at the load and watts wasted as heat for a copper DC cable of any AWG, length and current.
Results
| Voltage drop | 4.99 % |
|---|---|
| Voltage lost | 0.599 V |
| Voltage at the load | 11.4 V |
| Power wasted as heat | 12 W |
| Round-trip resistance | 30 mΩ |
- More than 3% drop. Fine for some lighting, but inverters, fridges and battery charging work better below 3%. Try the DC wire size calculator.
About this calculator
Every cable has resistance, and in a low-voltage system that resistance is often the difference between an appliance that works and one that complains about low voltage. Voltage drop rises with current and length, falls with thicker wire, and turns straight into wasted heat.
Enter a gauge, length and current to see how many volts you lose, what the device actually receives and how many watts the cable burns off. That's useful for checking an existing install before blaming the battery.
How to use it
- Select the copper wire gauge you have or plan to use.
- Enter the one-way cable length. The return path is added automatically.
- Enter the current and source voltage. Use the battery's resting voltage under load, e.g. 13.0 V for LiFePO4 or 12.4 V for lead-acid.
- Look at percent drop and voltage at the load. Aim for 3% or less.
The formula
R = Ω/1000 ft × (1 + 0.00393 × (T − 20)) × 2 × length ÷ 1000.
Voltage drop = I × R. Heat loss = I × voltage drop = I² × R.
Resistance values come from the standard AWG table for annealed copper at 20 °C.
Worked example (default values)
Inputs
| Wire gauge | 10 AWG |
|---|---|
| One-way cable length | 15 ft |
| Current | 20 A |
| Source voltage | 12 V |
| Conductor temperature | 20 °C / 68 °F |
Results
| Voltage drop | 4.99 % |
|---|---|
| Voltage lost | 0.599 V |
| Voltage at the load | 11.4 V |
| Power wasted as heat | 12 W |
| Round-trip resistance | 30 mΩ |
Tips and common mistakes
Connections count too. A crimp lug that's loose or corroded can add as much resistance as several feet of cable. If you measure more drop than this calculator predicts, check every terminal, switch and fuse holder with a voltmeter while the circuit is under load.
Halving the current quarters the loss. Moving from 12 V to 24 V at the same power cuts cable heat loss by 75%.
Frequently asked questions
How much voltage drop is acceptable in 12V?
3% (about 0.36 V) is the common target for electronics, fridges and charging circuits. Up to 10% can be acceptable for non-critical lighting. Battery-to-inverter cables should aim for about 1-2%.
Does voltage drop matter on solar panel wires?
Yes. It wastes harvest, especially on low-voltage parallel arrays. Higher-voltage series strings reduce it a lot.
Is AC voltage drop the same?
For short residential runs it's close, but AC calculations also involve power factor and reactance. This tool is for DC.
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Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.