pdf chart How to calculate the right size wire: Step 1: Determine the current rating (i.e. amps) for every appliance you expect to use in your campervan. Some appliances will state their rating as watts rather than amps. For 12 volt components like your fridge, campervan lights and USB phone charger for example the calculation is: Amps = Watts / 12 volts (if running a 24 volt system then divide it by 24.) Step 2: Determine the length of the longest circuit you plan to install. To do this, measure the total distance you’ll lay your cable from the battery to the appliance and back to the battery again. Step 3: For a quick estimate to determine the exact minimum wire gauge needed, use the table below. Choose the maximum cable length on the left based on your measurements in step 2 above, and the amps based on the highest number calculated in step 1 above.