Current Basics: Volts, Amps and Ohms

In this article we are getting nerdy about electrons. To be more precise, we are discussing why choosing the right wire gauge is important and why an incorrectly sized wire gets hot. We also go through the governing rules that dictate how much current a circuit is going to draw.

The Basics

Before we even get into the volts, amps, and resistance let’s look at some of the basic types of electrical circuits we deal with in an automotive system.

Within the engine bay and from an engine control perspective there are two major types of circuits we deal with. We have sensors, which relay information into the control system, and actuators that are being controlled by a control system – either an ECU or PDM. The actuator is just a technical name for the things in the engine bay that actually do something, like fuel injectors, ignition coils, fans, pumps, solenoids etc. These are the components that the control system is actually turning on and off to make something happen in the physical world.

Today’s discussion of circuits is going to concentrate mainly on the actuators because these are the components that have the highest potential for things to go wrong. For example, if you size the wiring to the fuel pump inadequately, you may end up creating enough heat in the wiring to turn a small electrical problem into a serious fire risk.

Ohm’s Law

Why is getting the gauge (or thickness) of the wire important for an electrical circuit? The gauge of a wire required is proportional to the amount of current the circuit is drawing and the current being drawn is proportional to the resistance. This is where our old friend Georg Ohm steps in with Ohm’s law.

Ohms law is what we use to relate current, voltage and resistance. And Ohms Law states that:
Voltage = Current x Resistance (or V=IR)

What does that mean for powering up a light globe, a fuel pump, a nitrous solenoid or any other device in our engine bay? Traditionally we’d use a 12V electrical system, or when the engine is running 13.8V. ( That’s what the alternator is regulating the voltage to, in the real world that could be 14.2, 14.4 or even 16V depending on your particular application. If you are not sure, just grab a multimeter, set it to DC Voltage and measure what the voltage is across your battery with the engine running.)

So let’s say we’re using 13.8V, and, according to Ohm’s Law (voltage = current x resistance), all we have to do is measure the resistance of the device we are trying to run to calculate the amount of current that device will draw.

Now that we know the theoretical amount of current the device should draw we can go about sizing up not only the wiring but also ensuring we select an appropriate set of plug and pins to use for each device.

The simplest and most practical way of sizing your wiring appropriately is to use an online wire gauge calculator like this one by wirebarn.com. You simply input the voltage, current and length of wire and the calculator will provide you with a minimum wire gauge for your application.

Why girth is important

Anyone who has used a wire too small for the intended application knows that too thin a wire gets hot, really hot – but why? Why does this heat get generated in a thin wire but not a thicker one?

The answer to that one gets even nerdier and goes all the way down to the sub-atomic level of electrons passing down from one molecule of copper to the next molecule of copper in length a wire.

Amps (or current flow), is a measure of the actual number of electrons that are being moved through your circuit. Imagine having to move the same number of electrons through a very thin wire, as opposed to a very thick wire. The best way to illustrate it is with a water hose analogy. Say we have to move the same volume of water through a regular garden hose (that’s our thin wire) and a commercial-size water pipe that supplies water to an entire suburb (that’s our thick wire).

The water in the commercial-size pipe is moving much slower down the pipe to deliver the same flow rate as thinner the garden hose. 

That’s because the speed at which the water moves along the hose is directly proportional to its diameter for any given flow rate. This example is exactly the same as electrons moving through a wire – the electrons need to move much faster through the thinner wire to provide the same volume out the other side.

Why does this matter? The faster the electrons have to move in the wire, the more heat they generate. So when we are pushing a very high volume of electrons through a very thin wire, they’ll still go through but they’ll generate a lot of heat because they are having to move very fast. If you ask for a lot of current and for a long period of time, that heat can get too much for the wire covering, and that is where you let the smoke out!

If you haven’t watched the video at the start of this article, now would be a good time to do it as it shows a practical example of this using a simple light globe.

A light globe like the one used in our video is a perfect example of leveraging the fact that when you draw enough current through a wire, it’ll heat up.

The only difference here is that we use this to our advantage in a light globe, by intentionally getting the wire red hot so that it provides us with light.

Real-World Practical Applications

Using Ohm’s Law we can calculate the expected current draw on any circuit. Armed with this information we can appropriately size the wiring to high-current devices. We also know why and how things can go pear-shaped when we undersize the wiring.

We also need to remember that just measuring the resistance of a component may not always tell us the full story about the amount of current the device will draw in real-world applications. Fuel injectors and ignition coils are switched on and off at high frequencies, so while you can measure the individual component resistance and calculate the current draw – the total current draw for the fuel or ignition system varies significantly with things like RPM and engine load.

The real-world practical application of Ohm’s Law actually requires a little more thought because it’s nuanced. The good news is, devices like the Haltech PD16 can give you real-time measurements of the actual current being drawn by a circuit and the ability to either shut it down or leave it active for a predetermined period of time. Using a device like the Haltech PD16 gives the user full control and diagnostics over the entire vehicle’s electrical system.