Voltage Verification
3. Measuring the Juice
If you're comfortable using a multimeter and have some basic electrical knowledge, you can measure the voltage at various points in your electrical system to determine the phase. If you're not comfortable with this, seriously, call a qualified electrician. Electricity is not something to mess around with if you're unsure.
First, turn off the main breaker. Then, carefully remove the panel cover, exposing the wiring. Before you even THINK of touching anything, double-check that the main breaker is OFF. Use a non-contact voltage tester to ensure there is no live voltage near the area you'll be working in. Seriously, be careful.
Once you are certain it's safe, turn the main breaker back ON. Use your multimeter to measure the voltage between different points. In a single-phase system, you'll typically find a single voltage (e.g., 120V or 240V). In a three-phase system, you'll find multiple voltage readings, often including a higher voltage between some of the phases (e.g., 208V or 480V depending on the configuration).
If you're seeing multiple different voltage readings, especially if one of them is significantly higher than the others, it's a strong indication that you have a three-phase system. Remember to carefully document your findings and double-check your measurements to ensure accuracy. And, for the love of all that is electric, turn the main breaker OFF again before replacing the panel cover!