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I have sort of a dumbass question . . .


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I checked out a commercial building today that had a three-phase incoming electrical service. No big deals. Nothing out of the ordinary. But a question occurred to me . . .

Three-phase electrical services have three hot SE cables coming into the house, hence its name.

But the residential stuff we normally see has two hot SE cables servicing the house. So . . . drum roll . . . why is it called single phase? Why not double phase?

It is what it is, to invoke a current--pun intended--cliche, but "single phase" seems like a misnomer.

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I checked out a commercial building today that had a three-phase incoming electrical service. No big deals. Nothing out of the ordinary. But a question occurred to me . . .

Three-phase electrical services have three hot SE cables coming into the house, hence its name.

But the residential stuff we normally see has two hot SE cables servicing the house. So . . . drum roll . . . why is it called single phase? Why not double phase?

It is what it is, to invoke a current--pun intended--cliche, but "single phase" seems like a misnomer.

Actually, it's quite accurate. There's only one phase. Think of it as an oscillating wave. It has two poles.

Three-phase systems actually have three separate phases. They can have many different wire configurations entering the service, not just three.

There are some two-phase systems, but you're unlikely to every run into one. They were used, I believe, on railroads and a few large manufacturing facilities.

- Jim Katen, Oregon

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I checked out a commercial building today that had a three-phase incoming electrical service. No big deals. Nothing out of the ordinary. But a question occurred to me . . .

Three-phase electrical services have three hot SE cables coming into the house, hence its name.

But the residential stuff we normally see has two hot SE cables servicing the house. So . . . drum roll . . . why is it called single phase? Why not double phase?

It is what it is, to invoke a current--pun intended--cliche, but "single phase" seems like a misnomer.

Actually, it's quite accurate. There's only one phase. Think of it as an oscillating wave. It has two poles.

Three-phase systems actually have three separate phases. They can have many different wire configurations entering the service, not just three.

There are some two-phase systems, but you're unlikely to every run into one. They were used, I believe, on railroads and a few large manufacturing facilities.

- Jim Katen, Oregon

As always, your superior understanding of this stuff is more than a little humbling.

I don't quite get it, but I promise to perform some research.

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Jim's right about 2 phase systems. 2 phase has the two phases 90 degrees apart and could mimic 4 phase, but three phase won out several decades ago and became the standard.

As for the OP...You can't have a voltage source without a return so that's why there's two conductors for single phase. Hot wire and return. Standard 120/240 V service to a residence is actually a 240 Volt single phase voltage source with the neutral connected to a centertap on the transformer for combined 240/120 volt single phase service.

The three phase configuration shares one return for all three sources, which comes out to four wires. When the loading on the three phases is balanced between the 3 phases, such as for a 3 phase motor load, the neutral current is practically zero and becomes redundant and is often omitted to save on conductor.

3 phase service to a residence isn't balanced at all and so it needs that neutral.

It's not a dumb question but let me ask you....is this a trick question to get me drawn into another long contentious thread on electrical?[;)]

Marc

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It's not a dumb question but let me ask you....is this a trick question to get me drawn into another long contentious thread on electrical?[;)]

Marc

Well, actually yes, but it looks like my attempt has failed. I hate it when that happens.

I'm kidding, clearly. Your explanation was quite helpful and it makes me understand what's going on phase-wise a little better. Thank you.

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Jon,

That was a great question. Thanks for exposing your soft underbelly for the masses.

Said underbelly was exposed for only the briefest of instants. Beware, for now I have flipped back over and the public's visage is limited to only my prick-ly side.

If you're reeaally looking for fodder, read my last FB status-update from a week or so ago . . . .

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Let's see...

Look up at the wires on the electric pole...

With 3 phase, there are 3 wires coming from the electric power plant.

You could say that these 3 wires are coming off the generator from three evenly distributed points on the generator. Or three points around a circle.

Then we take those three wires and use them to power an electric motor. THIS power would be like 3 horses walking around a circle and atttached to arms which go to a central rotating shaft. The "power" of the horses is evenly distributed from three points in a circle.

And because of this, 3 phase electric motors run more "efficiently". And that is the big benefit with 3 phase power, to run large electric motors for things like air conditioning, elevators, machinery, etc. Things you would find in a business, large building, or industry.

For homes, look up at the electric pole again. You will notice just one of those 3 wires being used for the house transformer and from there the wires going to the house. Just one phase is being used. Single phase.

Then the transformer does what it can with this single phase. That is to produce "opposites". While one is +, the other is -. Then it switches to where the first is - and the other is +. 60 times a second (60 cycles - or "alternating current").

BUT the two hot wires going to the home are doing their +/- business at the same exact time. Because this comes from that one wire above - just "one" of the phases.

As to horses in a circle, I suppose this would be like 1/2 horse going in a circle. And another 1/2 horse going the opposite direction in the circle. (Together they make one horse!) There is a point where they meet and there is a point where they are the farthest apart.

(Note: All 3 wires up above that are 3 phase alternate 60 times a second.)

3 phase picture...

http://upload.wikimedia.org/wikipedia/c ... rm.svg.png

Single phase picture...

http://www.electricalknowledge.com/imag ... veform.gif

More...

http://en.wikipedia.org/wiki/Three-phase_electric_power

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My basic electronics training kept me going through my years on submarines (I let the electrician's mates deal with the voltage/current/phasing my equipment used.)

In my civilian career I had to learn about three phase five wire wye supplies for the equipment we worked on, and, before I left that field, I was traveling around the country doing power quality surveys at hospitals where our systems were installed.

I never worried about sngle phase elect. much (though I did bring some of our test equipment home to scope out the house I was leasing back then (two wire, the only ground was at the service disconnect/panel) ---

Since I learned 3 phase, my somewhat autistic mind handles it better when I think of my household single phase as two phase, 180 degrees out ....

(edited to remove the typos I caught --- I don't like this laptop keyboard --- not forgiving at all)

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Let's see...

Look up at the wires on the electric pole...

With 3 phase, there are 3 wires coming from the electric power plant.

You could say that these 3 wires are coming off the generator from three evenly distributed points on the generator. Or three points around a circle.

Then we take those three wires and use them to power an electric motor. THIS power would be like 3 horses walking around a circle and atttached to arms which go to a central rotating shaft. The "power" of the horses is evenly distributed from three points in a circle.

And because of this, 3 phase electric motors run more "efficiently". And that is the big benefit with 3 phase power, to run large electric motors for things like air conditioning, elevators, machinery, etc. Things you would find in a business, large building, or industry.

For homes, look up at the electric pole again. You will notice just one of those 3 wires being used for the house transformer and from there the wires going to the house. Just one phase is being used. Single phase.

Then the transformer does what it can with this single phase. That is to produce "opposites". While one is +, the other is -. Then it switches to where the first is - and the other is +. 60 times a second (60 cycles - or "alternating current").

BUT the two hot wires going to the home are doing their +/- business at the same exact time. Because this comes from that one wire above - just "one" of the phases.

As to horses in a circle, I suppose this would be like 1/2 horse going in a circle. And another 1/2 horse going the opposite direction in the circle. (Together they make one horse!) There is a point where they meet and there is a point where they are the farthest apart.

(Note: All 3 wires up above that are 3 phase alternate 60 times a second.)

3 phase picture...

http://upload.wikimedia.org/wikipedia/c ... rm.svg.png

Single phase picture...

http://www.electricalknowledge.com/imag ... veform.gif

More...

http://en.wikipedia.org/wiki/Three-phase_electric_power

Wicked helpful, Billy Bob. Thanks.

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