Calculating the size of electrical cables and wires may not be the most fun part of your campervan conversion, but it is a critical part of your build.
If cables are are too thin, heat buildup within the wires can get too high. You can compare the use of wires that are too thin to trying to push a golf ball through a hose. It may go through, but it will cause a lot of friction!
Wires that cannot handle the current can melt. In the worst case scenario, they’ll cause short circuit or they can even catch on fire. If you want for you and your family to be safe in your campervan, it is important to pay close attention to the electrical cable sizes you use during your build.
To make things easier for you, we’ve created two calculators that help you calculate the size of your electrical wires and cables. The first electrical wire size calculator is based on the USCS system used in the United States. It calculates the size of wires in American wire gauge (AWG) and uses feet as measure of length. The second calculator is based on the metric system. It calculates the wire size in mm² and uses meters as measure of length.
For distance, enter the distance between your battery and the appliance. For ampere enter the peak ampere of your appliance. Lastly, the voltage drop most times is set to maximally 5%. Which results for 12V in 0.6V. But going lower is of course never bad.
You probably won’t end up with an exact size that is available in stores. For AWG, it is best to round down and for mm² to round up to stay on the safe side.
Background information on the calculators
To create the calculators, we have used the general equation that is normally used to calculate the resistance of a given object. The resistance (R) of a given object is equal to:
ρ represents the specific electrical resistance of a given material. In our equation, we will be using that of copper. This is generally the material that makes up the inside of electrical wires and it conducts the electricity. The specific resistance of a copper wire of 1 meter in length and a cross section of 1 square millimeter is 0.0175.
L is the length of the object. It represents the length of the cable you are using. Note that electrical appliances use two cables; a min and plus. The distance from your leisure battery to the appliance for example, has to be doubled to account for this.
Lastly, a represents what we actually want to calculate: the surface area of your cable.
What we thus have to do now, is rewrite the equation given above. To simplify things, it is good to know that:
Where V is volt, and A is ampere. For some background information on volt, ampere, and watt, read this blog post.
We can thus rewrite our first equation to:
In order to calculate the size of our wires, all that is left to do is rewrite this as a function of a. This results in the following equation that we have used in our calculators:
We have replaced ρ with the conductivity of copper which was equal to 0.0175. The length L is multiplied by 2 to account for both the plus and minus wires.
Let us now express our final equation in terms relatable to electrical wires. For A, ampere, one has to fill out the amount of amps the appliance will use. L is the physical distance in meters that the wire has to bridge.
And lastly, V represents the loss of electricity. Transporting electricity through cables always results in a loss through the emission of heat. This loss is here expressed in volts. Here one has to fill out the allowable loss of volts; which generally is said to be 5% at maximum. So for 12V appliances, you have an allowable loss of 0.6V, and 230V has an allowable loss of 11.5V. But less is of course always better.
From this equation we can easily see that, if we increase either the length of the cable or the Amps of the appliance, the thickness of the wire has to increase. It also shows us that if we want to minimize the loss, we also have to increase the thickness of the wire.
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