What you're measuring when you check amperage

Amperage is the amount of electrical current flowing through a wire or device, measured in amps (A). When you check amperage with a multimeter, you're finding out how much electricity a device is actually drawing while it runs. This is different from voltage, which measures the electrical pressure pushing through the circuit.

Knowing the amperage tells you whether a device is working normally, drawing too much power (which can indicate a problem), or drawing too little. A phone charger might draw 2 amps, while a microwave draws 15 amps. If your charger suddenly draws 10 amps, something is wrong.

Multimeters measure amperage in two ways: AC (alternating current, what comes from your wall outlets) and DC (direct current, what batteries provide). Most household devices use AC, but phones, laptops, and battery-powered tools use DC.

Key Takeaways

  • You must break the circuit and insert the multimeter in series (in line with the current flow) to measure amperage safely and accurately.
  • Set your multimeter to the correct amperage range before connecting it — using the wrong range can damage the meter or give a false reading.
  • AC current (from wall outlets) and DC current (from batteries) require different meter settings, and using the wrong one will not give you a reading.
  • If the meter shows 0.00 or OL (overload), you either have the range set wrong, the circuit is broken, or the meter's fuse is blown.

Setting up your multimeter for amperage measurement

Before you touch any wires, turn off the device you're testing and unplug it from the wall. Set your multimeter to the correct mode. Look at the dial and find the amperage settings — they are usually marked with an A symbol. You will see separate settings for AC (marked with a wavy line) and DC (marked with a straight line with dots).

Choose the range that matches what you're testing. If you're measuring a wall outlet device, use AC. If you're measuring a battery-powered device or something with a DC power adapter, use DC. Start with the highest amperage range on your meter (often 10A or 20A) if you don't know how much current the device draws. You can always switch to a lower range if the reading is very small.

Look at the input jacks on the front of your multimeter. You will see a black jack (labeled COM or common ground) and a red jack. For amperage measurement, move the red probe cable from the voltage jack to the amperage jack — this is usually labeled with an A or mA symbol. The black probe stays in the COM jack.

Breaking the circuit and inserting the multimeter

To measure amperage, you must insert the multimeter directly into the path of the current — this is called measuring in series. You cannot just touch the probes to the outside of a wire like you do with voltage. The current has to flow through the meter itself.

Locate the point where you want to measure. For a simple test, this might be the power cord of a device. Carefully strip about half an inch of insulation from both the hot wire (usually black or red) and the neutral wire (usually white) using a wire stripper. Do not strip the ground wire (usually green or bare copper).

Touch the red probe to the stripped hot wire and the black probe to the stripped neutral wire. The current will flow from the hot wire, through the meter, and out through the neutral wire. Watch the display — it will show the amperage in real time. If you're testing a battery-powered device, connect the red probe to the positive terminal and the black probe to the negative terminal.

Once you have your reading, disconnect the probes immediately. If you were testing a wall-powered device by stripping wires, twist the bare wires back together securely and wrap them with electrical tape, or use a wire connector to rejoin them safely.

Choosing the right amperage range

Multimeters usually have two or three amperage ranges: milliamps (mA), 10 amps, and sometimes 20 amps. Milliamps measure very small currents — a typical LED light might draw 20 mA. Household devices usually draw between 1 and 15 amps.

If you set the range too high, the meter will show a reading but it will be less precise. If you set the range too low, the meter will either show OL (overload) or blow an internal fuse. Always start with the highest range. Once you see a reading, you can switch to a lower range for more detail if needed.

Some multimeters have an auto-ranging feature that picks the right range for you. If your meter has this, look for a button labeled AUTO or RANGE. Press it before you connect the probes, and the meter will adjust itself as you measure.

Reading the display and understanding the numbers

The display will show a number followed by the unit — usually A (amps) or mA (milliamps). A reading of 2.5A means the device is drawing 2.5 amps. A reading of 500mA is the same as 0.5A. If the display shows 0.00, either the circuit is broken, the range is set too high, or the meter's fuse is blown.

If the display shows OL, the current is higher than the range you selected. Switch to a higher range and try again. If you are already on the highest range and still see OL, the device is drawing more current than your meter can measure — this is common with large appliances like air conditioners or electric ovens.

Compare your reading to what the device should draw. Most devices have a label on the back or bottom that lists the amperage — look for "Max A" or "I" (the symbol for current). If your reading is much higher or lower than the label says, the device may have a problem.

Common mistakes that give wrong readings

The most common mistake is measuring voltage instead of amperage. Voltage is measured by touching the probes to two points in the circuit without breaking it. Amperage requires breaking the circuit and inserting the meter in series. If you touch the probes to a wire without breaking it and expect an amperage reading, you will get nothing.

Another mistake is leaving the red probe in the voltage jack instead of moving it to the amperage jack. The meter will not give you an amperage reading, and you may damage the meter if you try to measure high current through the voltage input. Always check the probe position before you start.

Using the wrong AC or DC setting is also common. If you set the meter to AC and measure a DC device, the display will show 0.00. If you set it to DC and measure an AC device, you may get a reading but it will be incorrect. Check the device label or power adapter to confirm whether it uses AC or DC.

When the meter's fuse blows

If you measure amperage on a high range and then switch to a low range without turning off the device, or if you accidentally measure a very high current, the internal fuse in the amperage circuit will blow. When this happens, the meter will show 0.00 on every amperage measurement, even on devices that are definitely drawing current.

To replace the fuse, open the battery compartment on the back of the meter. You will see a small cylindrical fuse near the batteries — it is usually labeled with the amperage rating (often 10A or 15A). Unscrew or unclip it and replace it with an identical fuse from an electronics store. Use the exact same amperage rating, or the meter may not work correctly.

If you do not have a replacement fuse, you can still use the meter for voltage and resistance measurements. Just do not attempt to measure amperage again until the fuse is replaced.

Frequently Asked Questions

Can I measure amperage without breaking the circuit?

No. Amperage must be measured in series, which means the current has to flow through the meter itself. If you just touch the probes to the outside of a wire, you are measuring voltage, not amperage. Breaking the circuit is the only safe and accurate way.

What's the difference between measuring AC and DC amperage?

AC (alternating current) comes from wall outlets and reverses direction many times per second. DC (direct current) comes from batteries and flows in one direction. Your meter must be set to the correct type, or it will not give you a reading. Check the device label or power adapter to see which one you need.

Why does my meter show OL when I try to measure?

OL means overload — the current is higher than the range you selected. Switch to a higher amperage range and try again. If you are already on the highest range, the device draws more current than your meter can measure, which is normal for large appliances.

Is it safe to measure amperage on a live circuit?

Yes, as long as you follow the steps correctly. However, if you are not comfortable working with electricity, have someone experienced help you. Always double-check your probe positions and meter settings before connecting to any circuit. Never measure amperage on a circuit you do not understand.

How do I know if a device is drawing too much amperage?

Check the label on the device — it will list the maximum amperage it should draw. If your meter reading is significantly higher, the device may have an internal short or other problem. If the reading is much lower than expected, the device may not be working correctly. Either way, stop using the device and have it inspected.