Short answer: this campervan cable size calculator takes your system voltage, what you're powering, its watts or amps and the one-way cable run, and gives the cable size in mm², the fuse size and type, and the voltage drop. As a check, a 45 W fridge on 12 V with a 4 m run needs 1.5 mm² cable and a 5 A blade fuse.
More options
Cable size
-
Guidance only, not a substitute for a qualified installer. Use tinned copper cable and check your parts' manuals: if a manufacturer specifies a cable or fuse size, follow theirs.
How the campervan cable size calculator works
This is the same maths as the Cable size tool in the Roam app, moved onto a web page. It does three things in order.
1. Works out the current. Amps are watts divided by volts. A 45 W fridge on 12 V draws 3.75 A. For an inverter, it also divides by efficiency (90% unless you change it), because the inverter draws more from the battery than it delivers: a 1,000 W inverter at 12 V pulls about 93 A.
2. Picks the fuse. The fuse is the next standard size at or above 1.25 times the current, so it doesn't blow in normal use. For 3.75 A that's a 5 A fuse. The type follows the size: blade fuses up to 40 A, MIDI up to 200 A, MEGA up to 300 A and ANL above that.
3. Finds the smallest cable that passes every check. The cable has to keep voltage drop within the allowed share, has to safely carry the fuse's rating (so the fuse protects the cable, not the other way round), and can't be thinner than 1 mm². Voltage drop is worked out over the whole loop, the positive cable out and the negative back, which is why you enter the one-way length and the tool doubles it. This is the method Victron's Wiring Unlimited uses too.
Why what you're powering changes the answer
You don't need to know what voltage drop is to use this. Picking what you're powering sets it. Fridges, pumps, heaters, chargers and electronics get 3%, because they misbehave on low voltage: a compressor fridge may cut out, a charger may charge slowly. Lights and USB get 10%, because they work fine with a small dip. Anything we're unsure of gets the safe 3%. Victron, for comparison, aims for under 2.5% on its own systems, so if you want to be stricter, set your own figure under More options.
The options there change how much current a cable can carry. Higher temperature insulation carries more. Cables bundled together can't shed heat as well, so the tool takes 30% off what they can carry. Engine bays are hot, so their figures drop further, and 60 °C cable isn't allowed there at all.
Two worked examples
| Fridge | Inverter | |
|---|---|---|
| Input | 12 V, 45 W, 4 m one way | 12 V, 1,000 W, 90%, 1 m one way |
| Current | 3.75 A | 92.6 A |
| Fuse | 5 A blade | 125 A MIDI |
| Cable | 1.5 mm² | 16 mm² |
| What decided it | Voltage drop | Fuse rating |
The inverter example shows why a short run can still need thick cable. Over 1 m, 10 mm² would keep the drop low enough, but 10 mm² can't safely carry the 125 A fuse, so it steps up to 16 mm².
Doing a whole build? Roam's Build your electrics tool sizes every cable in your system at once, draws a live wiring diagram and turns it into a shopping list of cable, fuses, holders and lugs. It's in the app now, in early access.
For where each fuse physically goes, see the campervan wiring diagram. For inverter and charger runs in more depth, see inverter size and DC-DC charger size, or start from the campervan electrics overview.
Questions
What size cable do I need for a 12 V fridge in a campervan?
For a typical 45 W compressor fridge on a 4 m run, 1.5 mm² with a 5 A fuse, allowing 3% drop. Longer runs need thicker cable, so put your own length in the calculator, and check the fridge's manual in case it specifies a size.
What size cable for a 1000 W inverter on 12 V?
On a 1 m run, 16 mm² with a 125 A MIDI fuse by this calculator. Many inverter manuals give their own recommended cable and fuse, so follow those where they're given, and keep the run as short as you can.
Should I measure cable length one way or both ways?
Enter the one-way distance from the battery or busbar to the device. The calculator doubles it for the out-and-back loop.
Where does the fuse go?
On the positive cable, as close as practical to the battery or busbar it's fed from, so the whole length of cable is protected. Every circuit gets its own fuse.