# Question about USA electrical system

1. Jul 2, 2006

### Micko

Hello,
I live in Europe in house we use 3 phase (230 V every phase, 400 V voltage between phases) and null. I heard that in USA there are two phases (120V phase, 240V between two phases) and I wonder if that is true.
Do they use 3 phase power transmission to homes or what?
Thanks

2. Jul 2, 2006

### Danger

I don't know about the US, but you have to pay extra to get 3-phase in Canada. Standard household input is 240VAC 2-phase. That's only used for heavy appliances such as driers. The normal outlets are 115VAC.

3. Jul 2, 2006

### Averagesupernova

Micko, I'm not sure I understand what you are talking about concerning how Europe is wired. I would assume that the voltages you mention are in a wye configuration and not delta? The null being the center of the wye?
-
Here in the U.S. it is typical to have SINGLE phase running into your house. It is NOT 2 phase. This is a common misconception. There is a single pair of wires feeding the primary side of the transformer. The secondary side has a center tap which is referred to as neutral. It is bonded to the earth with a ground rod. Each end of the secondary winding is obviously 120 volts to neutral and 240 volts between themselves. 3-phase power costs more to have run to a building. Commercial buildings such as offices usually have 3-phase wye connected transformers and industrial buildings which use heavier machinery usually have delta connected transformers.

4. Jul 2, 2006

### Micko

Yes, exactly that was what I meant. If I measure voltage in my wall connector (sorry, don't know exact english word) voltmeter shows 230 V and in three phase wall connectors voltage between any two phases is 400 V.

5. Jul 2, 2006

### Averagesupernova

That ratio correspondes to the 208/120 volt wye setup in the U.S.

6. Jul 2, 2006

7. Jul 2, 2006

### GENIERE

Give it a little thought Astronuc. I think you'll find, as averagesupernove stated, the typical residential voltage in the USA is single phase 120/240vac.

8. Jul 3, 2006

### NateTG

The feed to the house is typically two-phase 240. However, the outlets are typically wired with one-phase+neutral rather than both phases.

9. Jul 3, 2006

### Averagesupernova

Nope. It is SINGLE phase. There is such a thing as 2-phase power but it is very uncommon. Someone on this forum has mentioned it in previous threads. True 2-phase power has the phases out of phase by an odd angle. Just by using a transformer to invert 180 degrees does not count as 2 phase.

10. Jul 3, 2006

### FredGarvin

You guys are a bit confused by the term "phase." The basic input to a house is two legs (two hot wires) of a single phase. You have a double tap on the transformer with the ground in the center. There should be no phase difference between the two AC forms in your electric panel.

11. Jul 3, 2006

### Micko

It would be really nice if some post picture to see clearly and therefore avoid this confusion...

12. Jul 3, 2006

### Averagesupernova

Technically there is a phase difference of 180 degress between the 2 legs of a residential service. Otherwise, there would not be a voltage between them. It is still single phase though. If there is a single transformer involved and just a pair of wires feeding the primary side then it is single phase. More than 2 wires feeding the transformer(s) is polyphase of some sort.

13. Jul 3, 2006

### FredGarvin

I guess that makes sense. I was thinking in different terms, i.e. 120 degree phase shift. The 180 degreee would give you the double voltage. Good catch.

14. Jul 3, 2006

### Staff: Mentor

Ah, that seems right and along the lines of Fred's comments. I just checked the transformer outside and it has one tie to the distribution line, but it has two hot lines out. So I would it would seem there would be two coils (windings) coming off - and with opposite polarity which puts them 180° out of phase.

I seem to remember the electrician talking about two phases, but then that is a different reference to the 3-phase distribution system.

Adding to the confusion I found this:
This statement from the electric company infers two phases. We have a three wire system. But it appears they mean two hot lines and a neutral, with the hot lines coming from two coils (windings) on the same core as one of the 3 T&D phases.

Sorry for the confusion.

Last edited: Jul 3, 2006