What voltage should a meter indicate when measuring phase A and phase B on a 208/120, 3-phase, 4-wire system?

What voltage should a meter indicate when measuring phase A and phase B on a 208/120, 3-phase, 4-wire system?

• A. 0
• B. 208
• C. 240
• D. 120

Answer: (B)

In a 120/208 V three-phase system that is wired in a Wye (star) configuration, the voltage from line to neutral is 120 V, while the voltage between any two different lines (line-to-line) is 208 V. This line-to-line voltage is the result of the math for a balanced three-phase system: V_line-to-line = V_line-to-neutral × sqrt(3). So 120 V × sqrt(3) ≈ 207.8 V, typically rounded to 208 V. Therefore, measuring phase A to phase B yields about 208 volts. If you measured phase to neutral, you’d read 120 V. The other numbers don’t apply here because 240 V is typical for a 240 V delta or a different system, and 0 V would only occur if you were measuring the same conductor or a perfect short.

In a 120/208 V three-phase system that is wired in a Wye (star) configuration, the voltage from line to neutral is 120 V, while the voltage between any two different lines (line-to-line) is 208 V. This line-to-line voltage is the result of the math for a balanced three-phase system: V_line-to-line = V_line-to-neutral × sqrt(3). So 120 V × sqrt(3) ≈ 207.8 V, typically rounded to 208 V. Therefore, measuring phase A to phase B yields about 208 volts. If you measured phase to neutral, you’d read 120 V. The other numbers don’t apply here because 240 V is typical for a 240 V delta or a different system, and 0 V would only occur if you were measuring the same conductor or a perfect short.