Where do standard AC voltages come from?

Thread Starter


Joined Apr 3, 2009
i know what 3 phase and delta and wye transformers are. im wondering if someone could explain where the standard AC voltages(in the states) come from? i know that some come phase to phase and some come phase to neutral. the voltages in the states are:110 to 120v, 208v, 220 to 240v, 277v and 480v. TY


Joined Feb 19, 2009
The different standard voltages come about when the three phase power (3 hot wires, 120 degrees out of phase from each other) high voltage is stepped down at your substation.

When they wire the transformers as delta or wye, different current and voltages appear on each leg.

Since each house gets "one leg" of 3 wires (2 hot and 1 neutral) it will be 110/208 or 120/220V single split phase (220V into house, but 120V between each hot wire and neutral)

To keep the current draw between all 3 phases balanced, they may use a mix of delta connected transformers and wye connected, others use all delta or all wye. This causes the different (but very close) voltage ranges.

--ETA: Link that explains a bit more
Nicer graphics on this link
Last edited: