A bank of three 240x480-120/240 V transformers connected together can produce which 3-phase outputs?

• A. 240 V 3Ø, 3-wire
• B. 120/240 V 3Ø, 4-wire
• C. Both I and II
• D. Neither I nor II

Answer: (C)

A bank of three identical transformers with a 240 V secondary that is center-tapped (giving 120/240 V) can be wired in two ways to yield different 3-phase outputs. The wiring choice is what determines the 3Ø configuration.

In a delta connection, wire the three secondary windings end-to-end to form a closed triangle and connect each line to the outer end of a winding. The center taps are not used, so there is no neutral. This gives a 240 V 3Ø, 3-wire supply.

In a wye connection, tie all three center taps together to form a neutral, and connect the three outer ends to the three supply lines. This provides a neutral (4th conductor) and 120 V from each line to neutral, producing a 120/240 V 3Ø, 4-wire system. The neutral allows 120 V loads on each phase, while the line-to-line arrangement serves 240 V loads.

So both outputs are achievable with the same bank, simply by choosing delta (no neutral) or wye with a neutral.