Cable Size Calculator: 12 V, 24 V and 48 V DC
Work out the cable size and fuse for a 12 V, 24 V or 48 V circuit in a campervan, boat, caravan, shed or off-grid solar system. Enter the power and the cable length to see the voltage drop for every cable size, in mm² and AWG.
No sign-up required. Free to use, with every formula shown on this page. Last reviewed October 2026.
Work out your cable size
Cable Size Calculator
Find the cable size and fuse for a 12 V, 24 V or 48 V circuit in a campervan, boat, caravan, shed or off-grid solar system, so the cable stays cool and your kit gets the voltage it needs.
1 Your system
Battery voltage2 What the cable powers
I know theAdd up everything on the circuit that can run at the same time. The power is on the label or in the manual.
Is it an inverter (230 V from the battery)?An inverter loses some power as heat, so it draws more from the battery than it gives out. Size the cable for the inverter's full rating, not what you plan to plug in.
3 The cable run
Measure along the route the cable will take, from the battery (or fuse box) to the device. The tool doubles it for the return wire.
How much voltage can you lose? Where the cable runsUse cable of at least
4 mm²
about 10 AWG, copper
Current
0 A
Voltage drop
0 V
Fuse
0 A
Cable sizes for this circuit
| Size | Carries up to | Voltage drop | Verdict |
|---|
Before you wire it
- Fit the fuse as close to the battery as you can (within about 20 cm). The fuse protects the cable, so it must never be bigger than the cable can carry.
- Use the same size for the positive and the negative (return) cable.
- Use fine-stranded, tinned copper cable in vans and boats, with crimped lugs sized for the cable.
- This tool is for low-voltage DC only. Mains (230 V) wiring in a home, van or boat must follow the wiring rules where you live (BS 7671 in the UK); have an electrician do or check it.
How it's worked out: current = watts ÷ volts (÷ 0.9 for an inverter). Voltage drop = 2 × length × current × 0.0185 ÷ cable size in mm² (copper at about 40 °C, both wires counted). The fuse is the next standard size at or above 1.25 × the current. The cable must carry at least the fuse rating: current ratings are for single-core PVC cable at 30 °C from BS 7671 Table 4D1A (clipped direct, or in conduit and trunking). The recommended size is the smallest that meets both the voltage drop and the fuse. AWG is the American wire gauge, shown because many inverters and kits quote it.
Estimates only. Check the cable maker's current rating, keep cables away from heat, and follow your equipment's installation manual, which may ask for a bigger cable or a different fuse.
How it works
Two checks decide the cable size
A cable has to do two jobs: carry the current without getting hot, and deliver enough voltage to the far end. At 12 V, the second job usually decides the size. The sources and assumptions behind it are on our How we calculate page.
Voltage drop
Every cable has a little resistance, so some voltage is lost along it, out and back. Drop = 2 × length × current × 0.0185 ÷ cable size in mm². At 12 V, losing even 0.5 V can make a fridge or inverter cut out, so the limit is about 2–3%.
Current rating and fuse
The fuse goes next to the battery and must be at least 1.25 times the current. The cable must carry at least the fuse rating, or the fuse can't protect it. Cables in conduit or a bundle carry less, because they can't cool down.
The size to buy
The calculator picks the smallest standard size (1 to 120 mm²) that passes both checks, shows the nearest AWG size for kit that quotes it, and lists the sizes either side so you can see the margin.
Examples
What size cable for a 12 V fridge or an inverter?
Common circuits in a campervan, boat or off-grid cabin, worked out with the calculator above.
Copper cable in open air. Voltage drop limit 5% for lights, 3% for other circuits and 2% for inverters; inverters include 90% efficiency. A longer run or cables in conduit need more.
Tips
Getting the wiring right
The right size is only half the job. These make the biggest difference to a safe, reliable system.
Keep big cables short
Put the inverter close to the battery: every metre of cable counts twice. A 2,000 W inverter at 12 V draws about 185 A, which needs 95 mm² cable over 1.5 m but 120 mm² over 3 m.
Fuse at the battery
Fit a fuse or breaker on the positive cable within about 20 cm of the battery, sized for the cable, not the device. Use the same size cable for the negative.
Consider a higher voltage
Doubling the voltage halves the current. If you need more than about 2,000 W at 12 V, a 24 V or 48 V battery bank needs far thinner, cheaper cable. Plan the rest with the Off-Grid Planner.
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Common questions
Frequently asked questions about cable sizes and voltage drop
Common questions about sizing cables and fuses for 12 V, 24 V and 48 V systems.
What size cable do I need for 12 V?
It depends on the current and the length. A 60 W fridge 5 metres from the battery needs 4 mm² cable to keep the voltage drop under 3%; LED lights over 6 metres need 1 mm². Big loads such as inverters need 25 mm² and up, which is why they sit close to the battery.
How do I calculate voltage drop?
Voltage drop = 2 × one-way length (m) × current (A) × 0.0185 ÷ cable size (mm²). The 2 counts the positive and negative cables. Divide by the system voltage and multiply by 100 for the percentage.
How much voltage drop is acceptable?
About 2% between a battery and an inverter or charger, 3% for most other 12 V circuits, and up to 5% for lights. More than that and pumps run slowly, fridges struggle and inverters cut out on low voltage.
What size cable for a 2000 W inverter?
At 12 V a 2,000 W inverter draws about 185 A. With a 250 A fuse it needs 95 mm² cable over 1.5 m, or 120 mm² over 3 m. At 24 V the current halves and 35 mm² will do over 1.5 m. Always check the inverter's manual too.
What size fuse do I need?
A fuse at least 1.25 times the normal current, but no bigger than the cable can carry. For a fridge drawing 5 A that is a 7.5 A fuse; for a 1,000 W inverter at 12 V (about 93 A) it is a 125 A fuse.
What is mm² in AWG?
AWG is the American wire gauge: the smaller the number, the thicker the wire. Roughly, 1.5 mm² is 14 AWG (a little thinner), 4 mm² is 10 AWG, 10 mm² is 6 AWG, 25 mm² is 2 AWG and 50 mm² is 1/0 AWG. The calculator shows the nearest AWG at least as thick.
Can I use this for 230 V mains wiring?
No. It is for low-voltage DC circuits only. Mains wiring in a home, van or boat must follow the wiring rules where you live (BS 7671 in the UK) and should be done or checked by an electrician.