2 or 3 phase?


Old 03-19-06, 02:43 PM
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Question 2 or 3 phase?

Hi everyone,

I was just wondering what does it mean when people refer to outlets as two or three phases.
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Old 03-19-06, 03:22 PM
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I have no idea.

Residential power in the US (with very few exceptions) is single phase.

Commercial power is a different story.
Old 03-19-06, 07:44 PM
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Is it possible that whoever was making that reference actually wasn't talking about phases? As noted, anything other than single phase power in residential installations is very unusual - although I have run into it. Someone completely unfamiliar could be referring to 2-wire and 3-wire which has nothing to do with phases.

Many older homes were wired with only 2 wires - hot and neutral. For many years now, an additional ground wire has been required. Most folks are familiar with the old "plug adapters" that allow you to use a 3-wire plug in a 2-wire receptacle (which is a bad idea unless the box is actually grounded and the grounding tab or wire from the adapter is connected to the plate screw).
Old 03-19-06, 08:24 PM
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3 phase power

Single phase power is used in homes and most commercial buildings (120 volt and 240 volt ).
Three phase power is used in industrial applications that have large machines or equipment that require more power to operate. Most common 3 phase applications are 480/460 volt circuits. I am an electrician and have not heard of 2 phase circuits unless someone was talking about a 240 volt circuit that has 2 hot wires versus a 120 volt circuit that has only one hot wire. 3 phase circuits have 3 hot wires.

Hope this helped!

Old 03-19-06, 09:55 PM
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Two-phase WAS a legitimate power system that is no longer used anywhere that I am aware of. It consisted of two legs of power that were not in phase, nor were 180 degrees apart. For today's practical considerations, it does not exist.

Single phase power is typically two hots with a neutral in the middle. Sometimes WITHIN this power system, some people refer to the two legs as two different phases, and that is admittedly confusing.

Three-phase power is three hots, typically found in industrial settings, as mentioned. Further, there are different three-phase systems, wye-connected and delta-connected. Each has different voltage patterns and use the neutral and ground references differently, but I digress.

For the two power systems in modern usage, look at it this way. The power at your house has TWO hot legs, but ONE way to get from a hot to a hot, therefore SINGLE-phase. A three-phase system has THREE ways to go from one hot to another, therefore THREE-phase.
Old 03-20-06, 09:24 AM
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Actually, I believe that the "Single" phase and "Three" phase refers to the timing of the power waveform, not the number of ways you can go from hot to hot.

In the single phase system, regardless of whether you go from hot to hot or neutral to either hot, the timing (phase) of the power is the same. Plotted on an oscilloscope, they would synchronize. In the three phase system, the three "hot to hot" phases are staggered by 120 degrees - which is what allows motors to generate rotating magnetic fields, rather than just alternating ones (hence the need for starting devices in single phase motors).

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