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It's hard to see but on the overhead power lines, houses seem to only be grabbing one phase of the 3-wire transmission line. If each house is using a separate phase, what makes the 3-phase system an advantage if each house still has the 0-voltage point of the sine wave?

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  • \$\begingroup\$ Most houses don't need to power 100 HP motors. \$\endgroup\$
    – Matt Young
    Sep 5, 2013 at 2:59
  • \$\begingroup\$ Do the factories that have those motors have a separate transformer for each phase? \$\endgroup\$
    – skyler
    Sep 5, 2013 at 3:04
  • \$\begingroup\$ They will have 3 phase transformers, and probably several. This is not really my specialty. Short answer to your question is, yes, houses are typically single phase, and get 240V for the appliances that require it from flipping the polarity of that phase. \$\endgroup\$
    – Matt Young
    Sep 5, 2013 at 3:17
  • \$\begingroup\$ In Germany houses typically have all three phases connected (the stove is usually connected to all three), and the power lines to the houses run underground. \$\endgroup\$
    – starblue
    Sep 5, 2013 at 19:57

3 Answers 3

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Generally (in the US, anyway), several houses in a neighborhood will all be supplied by a common pole transformer, which will be attached to a single phase of the medium voltage (several to tens of kV) distribution network. Each house is connected to the center-tapped ("split phase") secondary, giving 120V-0-120V, or 240V across the ends, at anywhere from 100 to 200 amps max.

It's possible that all three phases of the medium voltage network are wired along the street, and the pole transformers will alternate among the phases, but I've seen quite a few areas in which all the pole transformers on an entire street or neighborhood are all on the same phase, and different streets are on different phases. In either case, the three-phase load that the power company sees balances out statistically, rather than rigorously.

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  • \$\begingroup\$ Actually, I've seen plenty of rural areas where only the outer two phases are ever used for single-phase loads, and the middle phase is used only for three-phase loads. \$\endgroup\$ Sep 5, 2013 at 3:46
  • \$\begingroup\$ The practice of 2-phase delta supplies to households isn't common in Europe as we have 240v Ph-N to start with. Quite common for houses in a street to alternate between the phases. In the UK at least, overhead supply to premises is also fairly uncommon except in very rural areas. Where there are a group of houses there might be a 3-phase overhead supply to a substation and then underground cabling from there. \$\endgroup\$
    – Marko
    Sep 5, 2013 at 9:10
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    \$\begingroup\$ @Marko: Europe has a 230V grid, not 240V (that was in the UK, but that's already 20 years ago). As far as power distribution is concerned, the US is more like a developing country :). In Belgium we don't have overhead powerlines to the houses either: after the last transformer station (10kV/230V) everything goes under ground. \$\endgroup\$
    – Johan.A
    Sep 5, 2013 at 10:02
  • \$\begingroup\$ @Johan.A One of the justifications given is that overhead lines are more earthquake resistant and more easily repaired than buried services. Overhead utilities seems the norm for suburban areas of Japan as well. They're also less commonly grounded as well shudder. \$\endgroup\$
    – Marko
    Sep 5, 2013 at 10:11
  • \$\begingroup\$ Earthquake resistant may be a good reason in California, but I've been told you can see overhead powerlines in downtown Washington DC too, and that's no more earthquake prone than western Europe. \$\endgroup\$
    – Johan.A
    Sep 5, 2013 at 11:30
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While 30 or 40 years ago it was common to get 3-phase power in many European countries, today as a rule you get one phase. If you get the R phase, your next door neighbor will get the S phase, and the person in the house next to that will get the T phase. Over hundreds of houses this more or less balances the three phases. 3-phase power is still available at an extra cost, but households hardly ever need it anymore. (I remember my mother had a Miele washing machine which ran on 3-phase power.)
Industrial customers often will have 3-phase power, either because they need lots of power, or they use machines which run on 3-phase power, like synchronous motors. Overall the three phases coming from a 10kV transformer station will be fairly balanced, with only a small residual current through the neutral.

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  • \$\begingroup\$ Stoves usually uses multiple phases in Sweden. Therefore almost all people with a four plate stove will have several phases availible. \$\endgroup\$
    – Gunnish
    Sep 5, 2013 at 17:18
  • \$\begingroup\$ @Gunnish: I live in Belgium and my four plate vitroceramic stove uses only 1 phase (as does all my apartment). \$\endgroup\$
    – flup
    Dec 19, 2013 at 13:14
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Different houses will connect to different phases so that they hopefully load them equally. Of course that's not possible at all times but even in the worst case scenario when all the consumers are on the same phase the current in the neutral will be equal to that in the used phase. So the system still allows you to use four wires instead of six and that's significant saving of wires.

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