When one voltage in a 3 phase system is at its maximum, what are the other two phases at?

In a three-phase system, when one voltage reaches its maximum value, the other two phases are at a specific relationship defined by their phase angles. In a balanced three-phase system operating with a 120-degree phase separation, while one phase is at its maximum (let's say the peak positive voltage), the other two phases will be at a 120-degree and a 240-degree phase angle relative to the first.

At the moment one phase is at its maximum, the other two phases will not be at their maximum (which would be a completely different scenario) but instead will be at a negative voltage value, essentially corresponding to 50% of their maximum value but in the opposite direction. This means that they are experiencing negative voltages, which mathematically can be represented as being at half of their amplitude but in phase opposition to one another.

Thus, when one voltage is at its peak, the other two are indeed at reduced values that reflect this negative relationship in a balanced three-phase system, which aligns with the choice that indicates they are at 50% of their maximum value in the opposite direction.