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I actually have 3 phase in my home. Notes you said it was not possible. Not sure why, but it is here. Why would anyone ever want it?

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Where do you live (country) and is this an apartment building? –  Fiasco Labs Jul 30 '13 at 5:39
    
What makes you think you have 3 phase? What kind of connection (outlet) is it? –  Elliott B Jul 30 '13 at 5:56
    
Sometimes you'll have equipment designed for three phases. Those are very powerful heaters, very powerful AC systems and industrial equipment (such as an industrial sewing machine) induction motors. –  sharptooth Jul 30 '13 at 9:26

4 Answers 4

Here's an example:

My home was provided with a three-phase supply when it was built in 1990 because the builder intended to install an electrical storage-heating system.

I bought the house before the heating system was installed and opted for an oil-fired system instead.

So, in my area, it's an option you can request from the power company if you have a specific need for it.

So far as I know, three-phase supply is commonly used for equipment that requires substantially more power than most domestic household appliances. Some industrial electric motors require a three phase supply. In my area a single phase is 230 V so domestic households almost always use only a single phase. I expect the phases are separated at the local substation where high voltage local distribution is reduced to 230 V.

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Plus 3-phase motors are more efficient and self starting without extra circuitry. –  Fiasco Labs Jul 30 '13 at 18:44
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The transformer(s) on the pole or pad-mount will be the only place that determines what phase(s) are used. Not the sub-station. –  Kris Aug 8 at 20:11
    
In extreme cases, someone with a shop with heavy-duty power tools might have 3 phase installed. –  TomG Aug 9 at 0:43

Three phase power distribution is used in cases, such as industrial setups, where large motors or other loads are attached to the AC power mains. The advantage of 3-phase over single or two phase is that more net power can be delivered to the load over a 3-phase system for a given conductor size.

If you happen to actually have 3-phase power delivered to your site then it is possible that a previous owner or site occupant had loads of the type that demanded the more efficient power transmission.

Three phase power installations use a different type of main load center distribution panel than a single or dual phase system. Thus it is not a good idea to even think of mixing the two.

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Three phase systems are also useful for lighting when you need to mitigate against the stroboscopic "Wagon-Wheel" effect, where high-speed rotating objects appear to have a different rotation direction/speed (or even appear stationary) –  MrJamin Jul 31 '13 at 11:50
    
It's very possible to have single or two phase subpanels off a 3phase main panel (in either a high leg delta or a wye configuration) -- in fact, it's not at all rare, even –  ThreePhaseEel Aug 8 at 23:25

3-phase residential power is actually quite popular for highrise condominiums, but not so popular for traditional single family homes. A person living in a highrise condo will more than likely have a 3-phase panel supplying their home.

In most units there is just one 3-phase main panel that supplies 208V for the double pole appliances and 120V for the single pole lights and small electronics/appliances. Occasionally you will find a sub-panel that is fed off the main 3-phase panel for the residential portion of the home, while the main panel supplies vital resources to the home such as elevators, garage doors, fresh air exhaust fans, AC units, and whatnots.

I have only worked in one house that was initially 3-phase. The homeowner eventually went back to a 600A single phase because a whole house 3 phase generator was not an option.

The utility co. uses what is known as a WYE 208/120Y for residential 3-phase. The neutral will carry the non-linear loads of all 3 phases while delivering adequate power to the many units at a cost efficient and effective way.

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Some houses have a 3 phase supply which feeds adjoining houses with just a conduit with old VIR carrying the power from one of the fuses.

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