Testing your 240v outlet is necessary if the outlet is not delivering enough power, 240V appliances are not operating normally, or you’re having issues with the outlet. You must conduct a voltage test using a voltage tester or voltmeter. It’s a straightforward procedure.
Insert the probes of the voltage tester or voltmeter into the slots of the 240V outlet after setting it to measure VAC (AC Volts) and check the reading. It should be within the range of 220 to 260 volts. If it is, then you’ve confirmed it’s a 240V outlet and is receiving the full voltage.
I will go into more detail below.
240V Outlets in the USA
240V outlets normally look different from regular 120V ones.
A 3-prong 240V outlet may look like the one on the left in the picture below. A 4-prong 240V outlet will have an extra hot connection to double the voltage. This type of outlet may look like the one on the right.
The two identical vertical slots on the 4-pronger outlet are two separate 120V connections, and the third slot is for the neutral connection. So the total (of the two separate hot connections) will add up to 240V. Make sure you know which type you have when testing. The semicircular slot is for the ground connection.
A 240V outlet will normally have a double-pole circuit breaker on its circuit.
Testing a 240V Outlet
Requirements
To test a 240V outlet, you only need a voltmeter or a multimeter. But for safety, you can use safety (rubber-insulated) gloves.
Step 1: Set the Multimeter to Measure VAC ~240V
The first step is to set the voltmeter or multimeter to measure VAC (AV Voltage), which covers 220 to 260 volts. This will measure AC voltage instead of DC voltage.
Taking a closer look at how the probes are connected to the voltmeter/multimeter, the two connectors are connected as follows: the red connector is to be inserted into the port marked ‘V,’ ‘V~,’ ‘Volts,’ ‘+,’ or ‘Ω,’ and the black connector goes into the port marked ‘COM’ (see the picture below).
On some multimeters, you will see a ‘V’ with a wavy line on top or adjacent to it [V~].
If there is a single voltage selector and a ‘FUNC’ (function) button, select voltage and press the extra button to choose AC.
The range should cover up to 260V, so it should be at least 300V if rounded to the nearest higher multiple of 100V.
Step 2: Insert the Probes
With the multimeter set to measure VAC, insert the probes into the outlet slots you will test.
As it’s AC voltage, it doesn’t matter which probe goes where (unless you need to determine the polarity), but otherwise, touch the black probe with the metallic junction box or the bare ground wire. Then insert the red probe into the smaller live slot of the hot wire.
If the meter has probe holders at the top, like the ones in the picture below, you can use them to safely snap the probes into the slots without touching them.
Please make sure the probes make good contact with the metal parts they’re touching.
Step 3: Check the Reading
When both probes are inserted or touch the points mentioned in Step 2, check the reading on the meter.
You should obtain a reading close to 240V. It might be a little less or a little more. That’s acceptable, but it shouldn’t be too low or too high. If it’s within the range of 220 to 260 volts at most, it confirms that you have around 240V.
If the reading is too low, like ~210V in the picture below, the voltage level is short of what it should be. The outlet is ‘under voltage.’ If, however, you get a reading that is too high, like above 260V, the supply voltage is too high and may damage your appliances. You should call an electrician to check your main supply and electrical outlets.
A reading of 120V would confirm that the outlet is a regular 120V outlet, not a 240V outlet.
If you get an ‘OL’ (overload) reading instead, you probably set a short range, perhaps 200V. It should be higher to cover up to 260V. Check the setting and try again.
How 120V and 240V Supplies Differ
While most of the world uses 220-240V, 110-120V is the normal household voltage in the United States.
Regular outlets operate on 120V in a single-phase system. 240V provides more power because it is based on a three-phase supply. 240V outlets are more suitable for high-powered appliances and industrial machinery. Incidentally, they are not necessarily dual power, as this depends on the current draw.
Examples of appliances that may require 240V are air conditioners and washing machines.
Another advantage of providing 240V is that you can use foreign appliances where 240V is normal. You won’t need an adapter to convert the voltage level.
References
Editors of Cool Springs Press. Black & Decker Advanced Home Wiring, Updated 4th Edition. Cool Springs Press, 2015.
Video References
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