DC wire size calculator (12 V, 24 V, 48 V)
Find the right copper wire gauge (AWG) for a 12 V, 24 V or 48 V DC circuit from current, cable length and allowed voltage drop, checked against ampacity.
Results
| Recommended wire size | 6 AWG (13.3 mm²) |
|---|---|
| Voltage drop with that wire | 1.98 % |
| Voltage drop | 0.237 V |
| Minimum gauge for voltage drop | 6 AWG |
| Minimum gauge for ampacity | 8 AWG |
- Voltage drop, not heat, is setting the gauge here. That's typical for long 12 V runs.
- Ampacity check uses NEC Table 310.16 (copper, 75 °C) at 125% of the current. Bundled wires, hot engine bays and marine installs need further derating (see ABYC E-11 for boats).
About this calculator
Low-voltage DC wiring fails in a way household wiring rarely does. The wire can be thick enough not to overheat and still waste so much voltage over its length that the inverter cuts out, the fridge stalls or the battery never fully charges. At 12 V, a 0.5 V loss is already 4%.
This calculator checks both limits. It finds the smallest copper gauge that keeps voltage drop under your target over the round-trip length, and the smallest gauge that can carry the current at 125% without exceeding its ampacity. Then it recommends whichever is thicker.
How to use it
- Enter the current the circuit carries. For an inverter, use the DC current from the inverter size calculator.
- Measure the one-way cable length from the battery (or bus bar) to the device.
- Enter the system voltage and the voltage drop you can accept. Use 3% for most circuits and 1-2% for inverter and charger cables.
- Use the recommended gauge or thicker, and protect it with a fuse sized by the DC fuse calculator.
The formula
Voltage drop = current × (resistance per 1000 ft × temperature factor) × (2 × one-way length) ÷ 1000.
The temperature factor is 1 + 0.00393 × (T − 20 °C). Copper's resistance rises about 0.4% per °C.
The calculator picks the thinnest gauge where the drop is at or under the limit and the NEC 75 °C ampacity is at least 1.25 × the current.
Worked example (default values)
Inputs
| Current | 30 A |
|---|---|
| One-way cable length | 10 ft |
| System voltage | 12 V |
| Maximum voltage drop | 3 % |
| Conductor temperature | 20 °C / 68 °F (cool, light load) |
Results
| Recommended wire size | 6 AWG (13.3 mm²) |
|---|---|
| Voltage drop with that wire | 1.98 % |
| Voltage drop | 0.237 V |
| Minimum gauge for voltage drop | 6 AWG |
| Minimum gauge for ampacity | 8 AWG |
Tips and common mistakes
Use fine-stranded, tinned copper for vehicles and boats. It flexes without cracking and resists corrosion. Avoid CCA (copper-clad aluminum) cable: its resistance is roughly 60% higher than copper, so the gauge numbers in this calculator won't hold for it.
Metric sizes: the mm² shown is the closest equivalent cross-section. In the EU and Australia, buy the next size up if the exact size isn't sold.
Frequently asked questions
What size wire for 30 amps at 12 volts?
It depends on length. Over a 10 ft one-way run at 3% drop, you need 6 AWG. Over 3 ft, 12 AWG passes the voltage-drop check, but ampacity at 125% (37.5 A) still calls for 8 AWG.
Do I count the length of both wires?
Yes. Current flows out on the positive and back on the negative, so the calculator doubles your one-way length. If your negative returns through a vehicle chassis, results will vary with the chassis connection.
Why is 12 V wire so thick compared to house wiring?
At the same power, 12 V carries 10× the current of 120 V, and resistive loss rises with the square of current.
Related calculators
- 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.
- DC fuse & breaker size calculator
Pick a DC fuse or breaker size: at least 125% of continuous current, no larger than what your wire can safely carry. Covers 12 V, 24 V and 48 V circuits.
- Inverter size calculator
Size a power inverter from your appliance list: continuous watts, motor start-up surge, the next standard size and DC current from a 12, 24 or 48 V battery.
- Watts ↔ amps calculator
Convert watts to amps or amps to watts for DC, single-phase AC and three-phase AC circuits at any voltage and power factor.
Off-grid solar sizing, step by step
- Rough out the whole system · Off-grid solar system calculator
- Size the inverter from your loads · Inverter size calculator
- Plan the battery bank · Battery bank series & parallel calculator
- Check battery runtime · Battery bank runtime calculator
- Size the solar array · Solar panel size calculator
- Check panel string voltage · Solar string voltage calculator (series & parallel)
- Choose the charge controller · Solar charge controller size calculator
- Size the DC cables (you are here)
- Pick the fuses · DC fuse & breaker size calculator
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.