Residential homes and most small businesses use single-phase electrical current, but this isn't the form electricity takes as it moves across the electrical power grid. Electric utilities generate high-voltage, three-phase electric current that is transmitted and changed into dual-phase and single-phase currents through transformer boxes. Three-phase current is reserved for use in factories and similar settings, where it powers large motors, electric furnaces and other heavy machinery. You can check three-phase voltage by examining a three-phase transformer.
TL;DR (Too Long; Didn't Read)
To check three-phase voltage, use an electrical multimeter to test all six of the wires in the transformer box, starting with the wires labeled line and ending with those labeled load.
Be incredibly careful while performing the voltage check and be aware of your movements at all times. Testing three-phase voltage means exposing yourself to potentially life-threatening electric currents. Ground yourself and note that the motor disconnect switch on certain motors also functions as the stop-start switch. If this is the case, moving the disconnect switch to the "on" position will start the motor.
Before You Test
Before testing three-phase voltage, it is critically important that you be careful and take appropriate safety precautions. Wearing a grounding strap is advised. When ready, move the high-voltage transformer's motor disconnect switch to the "off" position. Remove the screws holding the cover on the disconnect switch and remove the cover. Set the multimeter to detect AC or DC voltage depending on what the box specifies, connect the probe leads to the "common" and "volts" connections, and select a voltage range somewhat higher than the voltage you intend to check.
With your multimeter set and calibrated, examine the inside of the transformer. In high-voltage transmissions, three wires are most often used: you should see six wires in total, with three on each side of the box. The terminals these wires are attached to should be labeled L1, L2 and L3 on one side, and T1, T2 and T3 on the other – the L wires are the incoming, or line wires, each carrying one phase of the three-phase current. To test the incoming voltage, place one of the multimeter's probes on L1 and the other on L2. Allow the multimeter to display the voltage and then repeat the tests while probing L1 and L3, then L2 and L3. If the transformer is working properly, the voltage readings should be the same after each test.
After you test the incoming voltage, you need to test the outgoing voltage. With the box still off, test the T1 and T2 leads with the multimeter, as you did with the line wires. Test T2 and T3, then T1 and T3. The voltage reading for each test should be zero volts. When you are ready, carefully turn the box back on and repeat this test of load wires to determine the outgoing three-phase voltage. There should be little variation in voltage between each test.