In a 480/277 V system with a 600 V disconnect, what voltage is read between the line side of the B fuse and the load side of the C fuse when the disconnect is on?

In a 480/277 V system with a 600 V disconnect, what voltage is read between the line side of the B fuse and the load side of the C fuse when the disconnect is on?

• A. 277
• B. 480
• C. 208
• D. 0

Answer: (B)

In a 480Y/277 V system, the voltage between two different phases is the line-to-line voltage, which is 480 V. When the disconnect is on, the line side of one phase (B) and the load side of another phase (C) are connected through the energized circuit, so you’re effectively measuring the voltage between phase B and phase C. That difference is 480 V. The 277 V value is the phase-to-neutral voltage and isn’t what you’d read between two different phases. The 600 V rating of the disconnect is just its maximum switching voltage, not the actual voltage seen across the two points in this scenario.

In a 480Y/277 V system, the voltage between two different phases is the line-to-line voltage, which is 480 V. When the disconnect is on, the line side of one phase (B) and the load side of another phase (C) are connected through the energized circuit, so you’re effectively measuring the voltage between phase B and phase C. That difference is 480 V. The 277 V value is the phase-to-neutral voltage and isn’t what you’d read between two different phases. The 600 V rating of the disconnect is just its maximum switching voltage, not the actual voltage seen across the two points in this scenario.