GFCI protection is required for a branch circuit installed on solidly grounded wye electrical systems between which voltages?

• A. 150 volts to ground and 600 volts phase-to-phase
• B. 120 volts to ground and 240 volts phase-to-phase
• C. 208 volts to ground and 480 volts phase-to-phase
• D. 0 to 120 volts only

Answer: (A)

GFCI protection for a branch circuit on a solidly grounded wye electrical system is required when the system voltage qualifies as a ground-fault hazard, specifically within a defined voltage window: from 150 volts to ground up to 600 volts phase-to-phase. This range targets three-phase, four-wire systems where line-to-ground voltages are significant enough that a ground fault could create a dangerous shock or fire condition, while also staying within the practical limits where GFCI protection is intended and effective. The idea is that any branch circuit operating in this range should have GFCI protection to quickly trip on an imbalance between live conductors and the neutral/ground return path. Choices that fall below the 150-volt-to-ground lower bound or that don’t align with the phase-to-phase maximum (600 V) don’t fit the standard requirement for these solidly grounded wye systems.