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Five example circuits, calculated
All with a 3% voltage drop and standard copper cable, except the inverters, which use 90–105 °C cable. Lengths are one way.
| Circuit | Current | Length | Fuse | Cable (metric) | Cable (AWG) |
|---|---|---|---|---|---|
| 12 V fridge | 5 A | 3 m (10 ft) | 7.5 A | 1.5 mm² | 14 AWG |
| Long run to a water pump | 8 A | 8 m (26 ft) | 10 A | 10 mm² | 8 AWG |
| Solar charge controller to battery | 40 A | 1.5 m (5 ft) | 50 A | 16 mm² | 6 AWG |
| 1,500 W inverter, 12 V | 147 A | 1.5 m (5 ft) | 200 A | 95 mm² | 3/0 AWG |
| 1,500 W inverter, 24 V | 74 A | 1.5 m (5 ft) | 100 A | 35 mm² | 2 AWG |
- The pump run carries little current but is long, so voltage drop sets the size.
- The solar controller run is short, but the 50 A fuse needs a cable that can carry it, so current rating sets the size.
- The 24 V inverter draws half the current of the 12 V one and needs a much thinner cable. That is why large systems use 24 V or 48 V.
How the sizes are calculated
- Minimum area (mm²) = 2 × 0.0175 × length (m) × current (A) ÷ allowed voltage drop (V). The 0.0175 Ω·mm²/m is the resistivity of copper at about 25 °C.
- Load current = the amps you enter, or watts ÷ volts. For an inverter it is AC watts ÷ 0.85 ÷ volts, to include conversion losses.
- Fuse = the smallest standard rating at or above 125% of the load current. Ratings considered: 1, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 100, 125, 150, 175, 200, 250, 300, 400 and 500 A.
- Cable current rating = copper values from NEC Table 310.16, using the 60 °C column for standard insulation and the 90 °C column for high-temperature cable. Small sizes are capped at 7 A (18 AWG), 10 A (16 AWG), 15 A (14 AWG), 20 A (12 AWG) and 30 A (10 AWG). Metric sizes use the next smaller AWG row, which is conservative. Bundled or hot cable is derated to 70%.
- The result is the smallest size that meets both the voltage-drop limit and a current rating at least equal to the fuse.
This is a sizing aid, not a code compliance check. It does not cover household AC wiring, and the results do not replace your local rules, such as the NEC, ABYC, ISO 10133, IEC 60364 or AS/NZS 3001.
Using it anywhere in the world
- Enter the length in meters or feet. The page chooses one from your browser language and you can change it.
- Both metric (mm²) and American (AWG) sizes are always shown. Australian “B&S” gauges are often the same numbers as AWG, but check the area on the product.
- Buy by cross-section. A cable’s printed area matters more than its name or color.
- Check the rules that apply where you live. Marine, vehicle and building codes set different limits.
Fuses: where they go and what to buy
- Put the fuse on the positive cable as close to the battery or source as practical. A common guideline is within about 18 cm (7 in).
- Use a fuse rated for DC at your system voltage. AC-rated fuses and breakers are not safe substitutes.
- Lithium batteries can deliver very high fault currents, so choose a fuse with a high interrupt rating, such as a Class T or marine-rated type.
- Do not fit a fuse larger than the cable’s current rating, even if the appliance asks for more.
Common mistakes
- Entering the round-trip length. The calculator doubles the length itself.
- Sizing by current only on long runs. Low current over a long distance can still lose too much voltage.
- Choosing the fuse for the appliance. It must protect the cable first.
- Poor crimps and undersized lugs. A good cable with a loose terminal still overheats.
- Bundling many cables tightly. They heat each other, so use the bundled setting.
Common questions
What size wire do I need for a 12 V, 30 A load?
It depends on the distance. At 3 m (10 ft) one way and a 3% voltage drop, a 30 A load needs at least 8.8 mm², so a 10 mm² or 6 AWG cable with a 40 A fuse. Double the distance and you need the next size up or two. Enter your own length in the calculator.
Why does the calculator count the length twice?
Current flows from the battery to the load and back again, so a 3 m run is 6 m of copper in the circuit. Enter the one-way distance, from the battery or busbar to the load, and the calculator doubles it for you.
How much voltage drop is acceptable?
A common guideline for boats and vehicles is 3% for sensitive circuits such as electronics, navigation and lighting, and up to 10% for less critical loads. At 12 V, 3% is only 0.36 V, and a larger drop can trip the low-voltage alarm on an inverter or charger. The calculator defaults to 3% and lets you choose 2%, 5% or 10%.
What size fuse protects my cable?
The fuse protects the cable, not the device. Choose a fuse of about 125% of the continuous load current, and never larger than the current the cable can safely carry. The results show the largest fuse each cable size can take, so you can check both limits.
How do I convert AWG to mm²?
Each AWG size has a fixed copper area: 10 AWG is about 5.3 mm², 8 AWG 8.4 mm², 6 AWG 13.3 mm², 4 AWG 21.2 mm², 2 AWG 33.6 mm², 1/0 AWG 53.5 mm², 2/0 AWG 67.4 mm² and 4/0 AWG 107 mm². AWG and metric sizes do not line up exactly, so the calculator picks the nearest size that is large enough in each system. Use whichever your local shop sells.
Can I use copper-clad aluminum (CCA) cable?
The calculator assumes pure copper. CCA cable has higher resistance and carries less current than copper of the same size, so it needs a larger size, and some standards do not accept it for battery cabling. Check the product description for “pure copper” or “OFC”.
Related calculators
Not sure what load to enter? Use the van and RV solar and battery calculator for your total power use, or the power station runtime calculator to see how long a battery will last.
You can also check a battery bank with the battery runtime calculator.
For an inverter circuit, find the right size first with the inverter size calculator.
See the 12 V and 24 V wire gauge chart for quick lookups, and the off-grid power glossary for the terms.