Why do we say residential service is single phase?


Old 04-19-16, 04:17 PM
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Why do we say residential service is single phase?

Why do we say that residential service is single phase in the US?

Doesn't the service provided by a center tapped transformer (which we all have) actually produce two opposite phases? Sure, most small appliances only use one, but every home or apartment I've ever seen has both phases available in the panel, and larger appliances often use both (240V).

Every time I hear someone say they can't buy a 3 phase shop tool because they only have single phase service I cringe. You have two phases!
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Old 04-19-16, 04:31 PM
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You have two phases!
Sorry, but that is wrong. Residential single phase power comes from one phase of a 3-phase high voltage circuit typically found at the top of a distribution pole. A 3-phase commercial service comes from all 3 phases of a 3-phase high voltage circuit.

In a typical residential loadcenter you have two legs and not two phases.
Old 04-19-16, 04:48 PM
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But each 'leg' is 180 out of phase with the other, no? The single phase drawn from the 3 phase generator at the power plant is converted back into two opposite phases by the transformer before it ever gets to the residential meter.

Last edited by Paul Steichen; 04-19-16 at 04:51 PM. Reason: Clarification
Old 04-19-16, 05:00 PM
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It is not so much about what is technically correct but a naming convention so everyone knows what is going on. I like the term split-phase for standard residential service.

Another way to look at it as single phase is to move your reference point from ground to one of the legs. Then the waveform is identical to the center tap/neutral as to the other leg, just twice the amplitude.

Two phase is used to describe a system that is 90 out of phase.

If you have two legs of a three phase system (120) what would you call that?
Old 04-19-16, 05:01 PM
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I agree it's confusing, because the two legs of residential single phase service are 180 degrees out of phase. So it seems reasonable to refer to them as two separate phases. But as CJ points out, because both legs are derived from a single phase of the 3 phase power grid, by convention, it is referred to as single phase.

It's kind of like little p phase and big P Phase. Little p phase just means the mathematical relationship between two waveforms. Big P Phase refers to one of the three phases of the power grid. We don't actually use little p and big P, but it would be less confusing.
Old 04-19-16, 07:28 PM
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I've actually worked around two-phase systems, not too many of us left. Two-phase uses FOUR conductors and in some systems FIVE conductors. ALL two-phase systems are ancient these days and as far as I know there are no two-phase generators still in use, the two-phase power used is derived from standard three-phase systems by the use of special transformer connections.

I know of one installation that has 2400 volt two-phase derived from a 4160 volt Y-connected supply with a single-phase 2:1 step-down transformer. Neutral and phase A is one 2400 phase and phase B with phase C (4160) feeding the high voltage side of the transformer the low voltage side becomes the second 2400 volt phase. The actual vector angles aren't exactly 90 degrees but it is close enough to work. More common arrangements use a Scott connection utilizing two transformers with special taps and the Taylor connection that utilizes three transformers.

I don't know about today but even into the 1970s all the drawbridges in Seattle had 2400 volt two-phase motors as that was the distribution at the time they were built in the early part of the century. They all had the required transformer banks added after Seattle went to a three-phase system.
Old 04-19-16, 07:45 PM
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Another way to look at it is to count the windings. Single phase has a single winding in the transformer that is center taped for the neutral.
3 phase has three windings which either has one winding center taped and grounded for the 120 volt power (120/240v High leg delta) or a grounded midpoint of all three windings as in a 120/208 Wye.
Old 04-19-16, 08:19 PM
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Consider one of the hots, say the one with a red wire, as your reference point.

Then, using an oscilloscope, measuring red to white, red to black (and also white to black) you get the same (same phase) sine wave except for size (amplitude).

Thus single phase.

Trigger the oscilloscope using a second pair of test leads that are not moved around as you measure different things, i.e. don't use the subject waveform as its own trigger.
Old 04-20-16, 08:10 AM
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The point of counting the windings is a good one. Residential is single-phase; it's one winding; one waveform -- the fact that the winding is center-tapped just changes the point of reference. For example, a 240V motor cannot start without a dedicated built-in start winding and centrifugal switch simply because there are not two reference points to create a force from a stand still. It's a hack to compensate for the lack of multi-phase. Multi-phase motors start automatically because they actually have real separate phases from which a force can be derived.

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