What it is the correct size cable for my 12 volt electrical system?
This post is also available in: Español
This is probably the question I get most frequently. Sooner or later, building our beloved camper van, we always get to the point of wondering about the most suitable wire size for each power line. And that’s a good sign that we are doing things right! 😉
Cables not properly dimensioned can lead to voltage loss, excessive heating, and high risk of fire. So let’s go into the details. I will have to put some electrical formulas but I will try to explain everything in the simplest way possible.
If you are not interested in the entire technical explanation, you may go directly to the Ultimate Wire Sizing Formula and the Practical Examples below
Each meter of cable provides a certain electrical resistance, which increases by increasing cable length (positive + negative) and decreases by increasing cable size.
Ohm’s Law says that V [volt] = R [ohm] * I [ampere]
So, simply, the greater the resistance, the greater the voltage drop at the ends of the cable. Therefore, we have to look for cables that do not make much resistance (R) to avoid an excessive voltage drop.
In 12 V electrical systems the maximum acceptable voltage drop is 0.5V.
But how to calculate cable resistance?
The formula is R = K * L / S
Where K is the specific resistance of the cable (0.025 Ohm / meter), L the length of the cable (positive + negative) in meters and S the section in mm2
It is clear that the resistance of the cable is a function of length and size.
That is why it is always necessary to prefer cables as short and thicker as possible, to minimize resistance and consequently the voltage drop.
It makes sense, right?
But we are not engineers, we are just DIYers, and we are trying to make with our hands a safe and simple electrical installation in our camper, then we cannot play much on the length of the cables, as there are objective difficulties in vans that do not always allow very short wiring as would be ideal. For example, many times it is not possible to connect the water pump near the fuse box, or it may be convenient to install a 12V plug far away from the battery.
Consequently, the only thing where we have total freedom of choice is the wire size! 🙂
Calculating wire section in 12 volt installations – ULTIMATE FORMULA
The simplest formula to calculate the most suitable section of the electric cable, taking into account the resistance and the voltage drop is the following:
S = 0.025 * I * L / V >> S = 0.025 * I (Ampere) * L (Meters) / 0.5
S: cable size in mm2
I: current in Ampere
L = cable length in meter (total of the two cables, positive + negative)
V = maximum accepted voltage drop (we put 0.5 Volt)
I hope this formula is useful to you, anyway you may use the table below to quickly find out the sections, but always better to use the formula.
Here below I am going to give you some practical examples that will help you clarify everything 😉
Example 1 – 200W Compressor Fridge Installation
What size cable should you use to supply a compressor fridge with a power of 200W?
First we calculate the length of the cable, positive plus negative. In this example we say it is 3 meters in total.
Thanks to the classic formula P = V * I we calculate the current that the fridge will need to function properly
I = P / V = 200/12 = about 17 Ampere
Now we apply the formula S = 0.025 * I * L / V >> S = 0.025 * 17 Ampere * 3 Meters / 0.5 = 2.5 mm2
We can then use a cable with a size of 2.5mm2 or thicker, and we will not have problems with voltage drop or excessive heating of the cable.
Remember, the commercial sections of electric cables are:
In the previous example we have been lucky that the size was a commercial one, but in case it is not, you always have to take the next largest commercial size available for obvious security reasons. 😉
Example 2 – Installation of 20W Water Pump
In this case the length of the cable, positive plus negative, is 6,5 meters in total.
We calculate the current that the pump will need to function properly:
I = P / V = 20/12 = about 1.6 Ampere
Now we apply the formula to finally calculate the cross-sectional area of the wire:
S = 0.025 * 1.6 Ampere * 6.5 Meters / 0.5 = 0.52 mm2
Since 0.52 mm2 is not a commercial size, we will use a 0.75 mm2 cable. In case it would be hard to find that sive on the market, you may just take a 1.5 mm2 wire that is much more common in DIY stores 🙂
Well, I hope you found this guide useful and that you feel motivated to continue with the electrical installation of your Volkswagen, Mercedes, Iveco, Fiat, Toyota or 4×4 van, whatever it may be. If you have any questions please contact me here in the comments and I will be happy to help you as I can! And now let’s go to work! 😉