On Monday, November 18, 2013 9:39:55 AM UTC-5, TimR wrote:
I hate to display ignorance, and I know I should know this.



But maybe somebody can explain.



The power lines are 3 phase, meaning they're 120 degrees out of phase to each other allowing 3 wires to carry what 6 wires should (since they're generated by 3 sets of coils at the power plant).


The 6 vs three makes no sense.





The house is fed by just one of these wires, through a center tap transformer. That transformer sends 3 wires to the house: Neutral, +120 volts, -120 volts. Connect the two hots and you get 240, connect either hot to the neutral and you get 120. Your safety ground is bonded to the neutral and so you should also have 120 from any hot wire to the ground. Neutral is the center tap.



Here's what I'm not getting at the moment. What are the connections to that transformer? If it is 3 phase leg to neutral on the primary side, then the center tap should be above neutral on the secondary (house) side, right? Which would mean that house neutral is NOT at ground level. But I'm pretty sure it is.

All the neutrals are tied to ground. Why would you expect
the neutral on the secondary to not be at ground level?
In the strictist sense, if you take a center tap transformer,
the secondary side doesn't have to have any relationship
to the potential on the primary side. It's can be completely
seperate, it's a seperate winding not connected to the primary.
What and how you hook it up
determines what level anything is referenced to. In the power
transformer case, the center tap (neutral) is tied to ground.