Multi-Meter Series: What is amperage and how do you measure it?

What is amperage? For most people, it’s little more than a rating on a fuse. We know it takes very little to hurt someone and we know a battery system should offer a starter around 700 of them.

Amperage is the strength of electric current through a wire, a cable or anything else that can conduct electricity. Like the frame of a tractor or trailer often used as a ground for lights and other accessories. Amperage can be defined as ‘flow rate’. Any mechanic will know that their air tools take different CFM (air flow) just like any electric tool will take different amperage (current flow).

This is why Amperage is important. Restrict the air flow to an air tool, and the tool’s performance will suffer. Restrict amperage to an electrical motor, or light or anything else, and performance suffers. Therefore, in diagnosing the performance of, say, an electric starter motor, measuring amperage is important. Here is how.

How to read amperage

Measuring amperage is actually pretty straight forward, and with today’s tools, easier than ever. This is why it’s a shame, not more mechanics use amperage readings more often when going about their work. To be clear, however, it isn’t shameful that any given mechanic doesn’t know, or is uncomfortable with measuring amperage. It’s a shame that measuring amperage, and using those measurements has been made so complicated.

The main way, and the oldest way, of measuring amperage is by taking the multi-meter and making the meter a part of the series. It must be said that if ever there is a doubt of what you are doing, you are best to see a trained technician for help. If ever there is a risk of injury, which is often the case with electricity, you are best to see a trained technician for help. If ever you are working on something that can be damaged, you are best to see a trained technician for help. Especially if ruining any part of your test would be financially difficult. That being said, here are the basic, rough steps to measuring amperage with a multi-meter.

1- Choose the current type, alternating (A/C) or direct (D/C). IT almost always is D/C with vehicles, but A/c does happen at times. Either inform yourself of the sort of amperage that could be in the system or make the best estimate possible. For example, a light bulb will not take 50 amps. And choose the right setting on your multi-meter dial, as well as the plug you will use for your red (positive) test lead.

2- Once your meter is set, take apart, or cut one part of the circuit. An example would be to take the positive wire, cut it (this can lead to problems so be careful when and where you choose to do this) and use the test leads to complete the circuit.

3- Turn on the circuit and take a reading from your multi-meter. The higher the amperage, the quicker you should try to be about taking the reading and turning off the circuit. For your meters sake.

In short: setup your multi-meter. Make the meter, with the leads, a part of the circuit you want to measure. Turn on the circuit and read the meter.

What’s worse is that it has gotten even easier with today’s tools. You can easily, and with decent reliability, measure the amperage in a system without compromising the integrity of the system. The tool I’m referring to is the amp clamp. There are essentially two sorts of amp clamps. Both are pretty great.

First is the amp clamp you buy for maybe one or two hundred dollars and you have a great accessory for your multi-meter that you plug in like any other test lead. You push a button, the ‘mouth’ opens, and you put the clamp over the cable you are trying to measure. No taking the circuit apart. And no passing hundreds of amps through a thin wire and a sensitive electronic measuring device. This may not be as accurate as the old method. But for most purposes, it’s more than enough to give a clear image of what is happening with an electrical system.

The other is a standalone amp clamp meter. All they typically do is read amperage through the clamp. They are also much more expensive that the accessory type. So that’s the trade off with those two options. Often a safe middle ground for the majority of truck mechanics is a quality accessory type. There is no lack of great options as well from Fluke, to the Mac, and the Snap-On, there are many to choose from.

The amp clamp really doesn’t take much explaining. Find the wire or cable you want to measure. Put the clamp around it with the meter set to the closest expected range. And turn on the circuit.

**When in doubt of which amperage setting to use on your meter: Use the highest, and work your way down until you get a proper reading. This will severely lessen your chances of damaging your equipment.**

Conclusion

That is almost all you need to know about amperage and how to measure it. Of course, there is the question of what you are reading versus what you should be reading. You should guess as little as possible and when in doubt do the necessary research to find how much amperage a healthy circuit should be using. Be it a starter motor or a LED light, being armed with the right information is always the biggest advantage a mechanic can have.

I hope this helped and that in the future you or your team has one less doubt about attacking that electrical problem.

Drive safe,

Kevin Gauthier