How does 3-phase power become 2-phase at the house?

Question

I read material about power grids that explained how 3-phase power at the power plant is eventually distributed as 7200-volt single-phase power, which is then stepped down to 240 volts right outside the houses that are served.

My question is how the single-phase power (which is 7200 V or 240 V to ground) becomes two 120 V-to-ground leads that are 180 degrees out of phase with each other in my electrical panel.

Thanks in advance for your explanation.

The first two responders apparently didn't understand what I was asking, and gave a lot of OTHER information that didn't answer the question.

However, the others explained the key point, which is:
The secondary winding on the transformer, which outputs 240 volts, is grounded in the middle, so that its two ends are each 120 volts to ground.

I didn't realize that you can ground the secondary winding at any point you choose. Thanks for the info.

Deep Thought was the first of several to provide the desired answer, so he get's a BEST ANSWER.

Answer 1

A (North American) 120/240 volt electric service is single-phase even though it uses three or four wires.

A 7200 to 120/240 volt distribution transformer has two 120 volt secondary windings. The secondary windings are connected in series (additive polarity) with the neutral connected between the windings to deliver 120/240 volt service. The voltage between the neutral and either end tap is 120 volts, and the voltage between the two end taps is 240 volts.

Answer 2

Actually, you are missing a few steps.

1. The "Power Grid" can have voltages as high as 3 million volts.

2. Typically, the "High Tension" lines you see running around town (or underground) are 32,000 to 16,000 volts.

3. All of these voltages are eventually fed into a local "Power Station" where they are "stepped down" then fed to the "local" grid where they eventually end up at the transformer near your home where they are "stepped down" to 240 volts (120 +120 "Single Phase" with a neutral wire) What you end up with in house is 120 volts on the Black Wire and 120 Volts on the red wire and the white wire is Neutral (return to the generator etc.) and a green ground wire.

From this you can get either 120 volts (single Phase) by using the black wire to the return (white wire) or 240 Volts (Single phase) by using the (out of phase) Red and Black wires to make 240 volts. 240 volts is generally used for BIG appliances to help reduce energy losses during use - Electric Dryers, Big Ovens, A/C Units etc..

The 7200 volts is the "typical" voltage that feeds your street side transformer.

3 Phase power is generally only available to commercial properties that use up to 50 KW per month.

Answer 3

Each one of the three phases can be stepped down by a transformer to 240 volts single phase. The output winding is FLOATING. When you ground one end of it, you can distribute 240 volts single phase from the other. When you ground the middle with a centre tap, you can distribute 120 volts single phase from one end, say to the houses on one side of a street, and from the other to the houses on the other side. Alternatively, you can distribute 120 volts from both ends of it to one house, to provide the supply arrangement which you describe in your question.

Answer 4

The three phases are 120 degrees apart.
Each phase is 240 volts to neutral - in England.
The inter-phase voltage is 440 volts.
I do not see how you get two phases 180 degrees apart.
RoyS

Answer 5

The step down transformer draws from a single phase and outputs three lines. Two lines are 240V apart and 120V from the third. That third line is the center tap of the transformer (secondary winding) which is run to earth ground. The 180 degree separation comes from transformer action of the moving current, in the windings.

Answer 6

A phase is not a wire exactly only in some contexs.
From memory it is Black White and Red in the US and Red White and Blue in Australia.

If you were to measure voltages you would place a lead on red and white; then white and blue; then blue and red.
You would have 3 measurements therefore 3 phases.
If you have 2 wires then you can only get one measurement therefore single phase.

I wii continue later