What a multimeter does and why you need the amp setting
A multimeter is a handheld tool that measures three things: voltage (electrical pressure), resistance (how much a material blocks current), and amperage or amps (the amount of electrical current flowing through a wire or device). When you need to know how much current something is actually drawing, you switch the dial to the amp or "A" setting and connect the meter in a specific way — not across the wires like you would for voltage, but directly in the path of the current itself.
Most household circuits run on either 15 or 20 amps, and knowing whether a device is pulling 2 amps or 12 amps tells you whether it is safe to run on that circuit or whether you risk tripping the breaker. A space heater might draw 12 amps alone; plug it in alongside a microwave that also draws 12 amps, and you have exceeded the circuit's limit.
The reason you measure amps differently than voltage is physical: voltage is the pressure pushing the current, so you measure it by touching the meter's probes to two points and reading the pressure between them. Amps is the flow itself, so you have to break the circuit and let the current flow through the meter to measure it.
Key Takeaways
- To measure amps, you must break the circuit and insert the meter into the path of the current — the current flows through the meter itself, not around it.
- Set the dial to the amp setting (usually marked "A" or "mA" for milliamps) before you connect the meter, and choose the range that matches what you expect to measure.
- The red probe goes into the amp jack (often labeled "A" or "10A") and the black probe stays in the common jack, opposite to how you measure voltage.
- If you measure across the wires instead of in series, you will short-circuit the device or blow the meter's internal fuse.
Setting up the meter and choosing the right range
Before you touch any wires, turn the dial on the front of the meter to the amp setting. Most multimeters have two amp ranges: milliamps (mA), which measure very small currents in the range of 0 to 200 or 0 to 500 milliamps, and amps (A), which measure larger currents in the range of 0 to 10 or 0 to 20 amps depending on the meter. If you are measuring a wall outlet or a device plugged into standard household power, use the amps (A) setting, not milliamps.
Next, move the red probe from the voltage jack to the amp jack. On most meters, the amp jack is labeled "A" or "10A" and sits separate from the other jacks. The black probe stays in the common jack (usually labeled "COM"). This is the opposite of how you set up for voltage, where both probes come from different jacks but the red one goes to the voltage jack.
If you are not sure what range of current to expect, start with the highest range (often 10 or 20 amps). You can always measure again on a lower range if the needle barely moves or the display shows a very small number. Starting too low can damage the meter's internal fuse.
Breaking the circuit and inserting the meter
Turn off the device or circuit you are measuring. This is the critical safety step: you cannot safely insert a meter into a live circuit without risking a shock or a spark. Flip the switch off, unplug the device, or turn off the breaker at the panel.
Locate the wire you want to measure current through. If you are testing a device that is plugged in, you will need to cut or disconnect one of the wires between the plug and the device — usually the hot wire (the one carrying current from the outlet). If you are testing a circuit at the breaker panel, you will disconnect the wire from the breaker itself. If you are testing a battery-powered device, you will disconnect one of the battery terminals.
Connect the red probe to the end of the wire that is coming from the power source (the outlet, the breaker, or the positive battery terminal). Connect the black probe to the end of the wire that is going to the device (the device's input, or the negative battery terminal). The current now flows from the power source, through the red probe, through the meter's internal circuit, through the black probe, and into the device. The meter reads the amps flowing through it.
Reading the display and understanding what the number means
Once the circuit is complete and current is flowing, the meter will display a number. If you are using an analog meter (one with a needle), read where the needle points on the amp scale. If you are using a digital meter, read the number on the screen. The number is the current in amps (or milliamps, depending on which range you selected).
A typical household device might draw 1 to 15 amps. A phone charger draws less than 1 amp. A microwave draws 10 to 15 amps. A space heater draws 12 to 15 amps. A refrigerator draws 3 to 6 amps when the compressor is running. If the number seems unusually high or unusually low compared to what the device's label says, the device may be faulty or the meter may not be set to the right range.
Once you have the reading, turn off the circuit again, disconnect the meter, and reconnect the wire to its original position. Do not leave the circuit broken while the meter is removed.
Common mistakes that damage the meter or give wrong readings
The most common mistake is measuring amps the same way you measure voltage — by touching the probes to two points without breaking the circuit. This creates a short circuit and either blows the meter's internal fuse or damages the device. The meter's fuse is designed to protect against this, but once it blows, the amp function stops working until you replace it.
The second mistake is leaving the red probe in the amp jack when you switch to measuring voltage or resistance. If you then touch the probes across a live circuit to measure voltage, you will short it out. Always move the red probe back to the voltage jack (usually labeled "V" or "Ω") when you are done measuring amps.
A third mistake is choosing a range that is too low. If you select the milliamp range (0 to 200 mA) and measure a device drawing 5 amps, the meter will either show an error or blow the fuse. Start high and work down if needed.
Finally, some people forget to turn off the circuit before inserting the meter. This is a shock hazard and can damage the meter. Always kill the power first.
When you should and should not measure amps yourself
Measuring amps is safe when the circuit is off and you are working with low-voltage devices like battery-powered tools, phone chargers, or small appliances. It is also reasonably safe when you are measuring the current draw of a device that is plugged into a standard outlet, as long as you disconnect the plug or the wire before inserting the meter.
You should not measure amps directly at a breaker panel or on live wires carrying household current unless you have training in electrical work. The risk of shock is real, and the consequences are serious. If you need to know how much current a circuit is drawing, hire a licensed electrician to measure it for you.
For most household troubleshooting — figuring out whether a device is drawing too much power, testing a battery, or checking whether a charger is working — measuring amps with the device off is the safe approach and gives you the information you need.
Frequently Asked Questions
What is the difference between amps and milliamps?
One amp equals 1,000 milliamps. Use the milliamp range for very small currents like those in phone chargers, LED lights, or sensor circuits. Use the amp range for larger devices like appliances, power tools, or anything plugged into a wall outlet.
Can I measure amps on a live circuit without turning it off?
No. Inserting a meter into a live circuit is a shock hazard and can damage the meter. Always turn off the device or circuit before breaking the connection to insert the meter.
What does it mean if the meter shows zero amps?
Zero amps usually means the circuit is broken or the device is not drawing power. Check that the device is turned on, that the battery is good, and that all connections are find. If the device should be drawing power but the meter shows zero, the device may be faulty.
How do I know which wire to break to measure amps?
Break the hot wire — the one carrying current from the power source to the device. For a plug, this is usually the smaller prong or the wire marked with a stripe. For a battery, break the positive terminal. If you are not sure, ask an electrician rather than guessing.
Will measuring amps damage my multimeter?
Not if you set it up correctly. The meter has an internal fuse that protects it if you accidentally measure amps across a live circuit like you would voltage. Once the fuse blows, you need to replace it, but the meter itself is usually fine.