<tangent>
The quirky thing about 3
phase center tapped delta
power (
http://www.controlbooth.com/forums/attachment.php?attachmentid=480 ) is that when you eliminate the wild
leg (pictured "
Phase B") you end up with standard domestic single
phase power! (120-0-120 h-h 240) Three
phase transformers are actually groupings of 3 single
phase transformers. (
Wye or Delta depending on how their outputs are wired together.) On Icewolf's Delta reference, One
transformer (center tapped) is red, another is black, and the third is blue.) As we are talking about only the points attached to the red winding (120-0-120) we effectively have single
phase power, as the hots are 180 degrees out of
phase with reference to
neutral. This configuration was widely used in the 60's and 70's as you could feed domestic outlets as well as household 240* volt appliances, but you still had 3
phase power available to run large
HVAC units. (Plenty still around!)
In my opinion, it fell out of favor because of concerns over the wild
leg. Most boxes back then had no mechanical difference on this
leg, so you could accidentally
plug a two pole
breaker into them and catch the wild
leg. If this
breaker then
fed a sub box that distributed to outlets, well... you get the picture.
Three
phase Wye service eliminated the wild
leg, yet still provided 120* volts while maintaining 3
phase power to larger equipment. The problem is when you have to run something that requires 240* volts, you were out of luck!
* ok, you will see some people saying 110 / 220 volts, and others saying 120 / 240 volts. For those not in the US, we are actually talking about the same thing! I am not sure when it happened, but as a child, it was always called 110 volts. These days, most people
call it 120 volts, and in general if you put a tester on it, your wall
outlet will probably be somewhere between 118 and 127 volts depending on where you live! Maybe one of the power-gods could post the answer to that one!
EDIT:
Ok, found the answer (maybe) here! (Couldn't resist, had to google!)
http://en.allexperts.com/q/Electrical-Wiring-Home-1734/difference-110-volt-120.htm
From that site:
Robert Wilber
Licensed Philadelphia Electrician
Philadelphia License # 3516 - 16765
*
For simplicity's sake:
110 volts, 115 volts, 120 volts, 125 volts - all the same thing.
You get what you get, and have no say about it.
... and here's why ...
Thomas Edison [no kidding] invented the electric light bulb, and needed to distribute electricity to make it usable. He built a system based on 110 volts DC. It was uneconomical and had large conductors and big voltage drops.
George Westinghouse came up with the idea of using alternating current to transmit electricity. This was valuable, in that the voltage could be changed by the use of a transformer, which allowed the transmission of electricity at a higher voltage, thus reducing line losses due to conductor resistance.
The initial voltage level delivered to homes using the Westinghouse method was nominally 100 volts. Over the years, the secondary voltage has been incrementally raised to 110, then 115, and 120 until it is common today to find 125, 127 and 130 or 132 volts in homes.
The reason for this is the upward ratcheting of transmission voltages to allow for higher demand. Every time your neighbor buys a new device that requires electricity to run, it increases the demand for electrical current on your utility's service conductors. Since these wires are not changed to bigger cables [with lowered resistance due to larger conductor cross-section area] every time demand increases, the only variable under the utility control is voltage levels.
The only way to push a certain variable demanded current [which consumers define] through a fixed resistance [unchanging wire] is to increase the voltage [electrical pressure].
</tangent>
