In working with wiring and related in your vehicle you will need to know a bit about Ohm's Law. This is a very simple set of formulas that help you calculate Power, Voltage, Current and Resistance. It's often seen in Wheel with the various formula embedded in it.

Why do I need to know about this stuff? Well if you are doing any sort of wiring you need to know the current capability of your circuit to, for example, determine the proper wire size. Also note that this is for Direct Current (DC) as that is primarily what you use in your vehicle. These formula apply only to DC, and will be different for Alternating Current (AC) that you might find in your home. As mentioned elsewhere check out this Toyota Training Document on elementary automotive electrical systems.

Let's say you want to connect up

What do you know about this problem?

Car Battery Voltage is approx.

Each Bulb is

So how much Current is being Drawn by all 2 bulbs at 12 Volts? That's Easy, each bulb draws 8.34 Amps at 12 Volts, and I have 2 bulbs each running at 12 Volts, so that's right at 16.7Amps. Wait, how is the magic accomplished. By this simple formula -

Where :

I = Current in

P = Power in

V = Force in

So each Bulb's current is calculated as follows -

Current in Amps =

8.33 = 100/12

Now that I have

Their are a few terms that you need to know about and then you are on your way to becoming an electrical engineer. Here they are the important symbols and what they mean.

You can find out any of the OTHER 2 values if you know the other 2. This makes it easy to solve for most any common electrical problem. Here are a few that are common and helpful with you Automotive work. The ENGLISH SPEAK is given to help understand what each formula is trying to find given what known values.

Sqrt(X) means take the Square Root of X (Your Calculator will have this button)

V

R = P / I

V = Sqrt(P x R)

Finding The Current

I = P / V

I = Sqrt(P / R)

Finding The Power

P = I