On Monday, November 18, 2013 10:08:23 AM UTC-5, TimR wrote:
On Monday, November 18, 2013 9:48:27 AM UTC-5, wrote:

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.



Well, you're not seeing the problem. I wish I could draw here.



Your transformer has two wires going in, and three going out.



I don't know what primary is in my neighbor hood, let's say 2400 vac for an example. Which two wires do we use? You only have the choice of leg to leg, or leg to neutral.



It's one of the 3 primary phases to primary neutral.





Choose leg to neutral: use a 10 to 1 turns ratio, your secondary will be 240 volts line to line. Then center tap it, and you get 120 from each line to center.



BUT: why would center tap be anywhere near neutral?


The center tap has no relation in terms of potential
difference to anything other than the two ends of the
secondary, until you connect it to something. The
center is called the neutral and as part of the
installation, it's run to the house and also tied
to ground. Now the neutral of the transformer
is at the same potential as ground.







Choose leg to leg: does the same problem exist?



Once it gets into the house I understand how it works, but on the pole I'm hazy.



By the way you are dead wrong on the 6 wires thing. That part I do remember.

You would go from 6 wires to 3 wires in a distributions system
if and only if you were to consider the 6 wire system as the only
other alternative to deliver power. That is that you have fixed
in stone that you're to have a generator with 3 windings, delivering
3 phases via 6 wires to the load. Of course if you start with that,
then a balanced 3 phase load reduces the wires in half.

But that isn't the real world. No one would do it that way to begin
with. There are other alternatives and even a single phase system can
deliver the same power without 2X the wires. 3 phase does use less
copper and is better for other reasons as well. But it's not a miracle.
The current carrying capability of the conductors is what it is and
3 wires can't suddenly carry the same current as 6. As i recall,
the reduction in copper you get with 3 phase is around 25%.