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household wiring

Discussion in 'Electronic Basics' started by Jamie Morken, Sep 6, 2005.

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  1. Jamie Morken

    Jamie Morken Guest


    I am a bit confused about 240V/120V household wiring.
    If there are 3 phases on the big powerlines, ie. X Y and Z,
    and they are 120 degrees out of phase with eachother, then
    how do you generate 240VAC and 120VAC from those three signals?

    I assume that for 120VAC you use only one of the three phases,
    ie X, and then you need a common (the white wire) as the current
    return, but where in the circuit is the common hooked up to?

    Do you always use the same phase for the 120VAC or do you try to
    balance the load between the different phases, ie X and Y phases
    each powering some of the 120VAC loads.

    For 240VAC, I would have thought that the two live wires (red and black)
    would be 180 degrees out of phase, but they must only be 120degrees out
    of phase since the phases are all 120 degrees out of phase?

  2. Tom Biasi

    Tom Biasi Guest

    Hi Jamie,
    Start here:

  3. Art

    Art Guest

    Unless you have a commercial license it is highly doubtful that the service
    connected to your residence is actually tree phase. Normal Residential
    Service is single phase. Measuring 208 -230 VAC across the outside
    conductors and measuring 105-125 VAC from either of the outside conductors
    to the earth (reference). 220 VAC appliances are connected to the mains, via
    a fuseing device, directly to the outside conductors with the earth lead
    normally going to the case of the device. 110 VAC devices are connected via
    a fusing device to one side of the service, therefore using 110 VAC and not
    the full supplied 220VAC.
    As suggested, the link referred to has a lot of relevant information.
  4. They connect a transformer between any two of those phases to produce
    a single phase source. The secondary of that single phase transformer
    has a center tapped secondary. That center tap is grounded and
    becomes the neutral going into your house. Each end of the secondary
    has 120 volts with respect to that neural. The two ends have 240
    volts with respect to each other (when one end swings positive, the
    other end is swinging negative, see-saw fashion)

    So the single phase voltage at your house may have a different phase
    than that at your neighbor's house, since his may be derived from a
    different pair of the 3 phases on the pole. Or your whole
    neighborhood may be fed by only a single pair of high voltage phases.
    Each pole transformer usually supplies one or more nearby homes with a
    single phase. The power company tries to rotate which two phases are
    loaded so that the 3 phase load is roughly balanced at the substation
    That is how it works.
    No, the 120 and 240 volt supplies represent taps on a single winding.
  5. Not as simple or correct as it might be. What do you make of this
    quote from that page?
    "In a single phase unit the power falls to zero three times during
    each cycle,"

    The coil diagrams showing 4 magnet poles but the first phase windings
    wrapped around only the two norths is also a bit strange. Just
    showing a north south pair (2 pole field) would have made more sense.
  6. In my area, the pole transformers are connected between one high
    voltage phase and ground, not between phases.

    We have 3 phase HV running north, along the street at the end of my
    block. At each lane, one phase is tapped, and fed along the lane. If
    my lane gets phase A, the next lane north gets phase B, and the next,
    phase C, then phase A again, til we get to a commercial area, then all
    three phases run along the lane.
  7. Tom Biasi

    Tom Biasi Guest

    I haven't visited that site in some time.
    I was thinking that the OP would read some about 3 phase and then realize
    that he had 240 ct.
  8. Jasen Betts

    Jasen Betts Guest

    they take one of the single phases (at 1100V or whatever)
    and put it through a step-down transformer with a centre-tapped
    secondary. (more precisely they use a three phase step-down transformer
    with three cnetre tapped secondaries - one for each phase)
    common is the centre tap in the secondary, also connected to ground.
    they use X and -X phases (opposite ends of the secondary),

    that's in the americas anyway

    in much of the rest of the world where the domestic supply is (about) 230V
    they don't bother with X and -X phases - everything runs of a single phase,
    or if it uses more than one they are 120 degrees out of phase. and the
    voltage between phases is (about) 381 volts
  9. Terry

    Terry Guest

    Hope this isn't a troll to waste time?
    Jamie you are making it more complicated than it needs to be.
    Yes; we may have 3 phase on high power transmission lines at 166,000 volts
    or even higher across country.
    Also those three phases may be brought into a residential area or into a
    large building at voltages such as 15,000 or 46,000.
    Talking about North America now, not necessarily elsewhere ;
    But to your house consider just one of the phases, stepped down by means of
    a single phase local 'distribution transformer' either pole mounted, or not
    too far away on a cement pad or hidden in a manhole.
    Other houses and streets will be connected to other phases; in order to
    'balance the load' back through the transmission system to the
    network/generating station. Forget about them for a moment.
    That single phase, into your house is alternating at 60 hertz per second.
    There are three wires into a typical home. Forget about ground for a moment.
    The middle wire (typically white) is considered neutral and to all intents
    is zero volts. Note 1.
    One of the other wires (typically) red, you can consider that to be, as it
    were at positive 115 volts. Note 2.
    The third wire is (typically) black, you can consider that to be, as it were
    at negative volts 115 volts. Note 2.
    So between the centre wire (neutral-zero volts) and either the red or the
    black 'hot wires' there will be 115 volts.
    Between the red and black there will be 115 + 115 = 230 volts. This 230
    volts is used for heavier appliances such as clothes dryers, cooking stoves
    and water heaters. The (two supplies) of 115 volts each are both used for
    lights and wall outlets etc.
    Note 1. That neutral is grounded (only once) as it enters the house main
    panel. It is often/usually also grounded by the power company.
    Note 2. The red and black are frequently and incorrectly called 'Phases',
    even by electricians who should know better!
    A clearer designation is to call them 'Legs'; for example Leg A, Leg B. If
    you think about it Leg A and Leg B are the two outer ends of the single
    phase 230 volt supply to your house.
    Note 3. The whole darn thing is of course alternating, 60 times a second.
    That's a bit too fast for most people to see the flicker, although some
    people claim they can do so; so one sixtieth of a second after what was just
    said about 115 volt positive and 115 volts negative it will have
    alternated/reversed and will continue to do so ad infinitum.
    Note 4. The next street or another group of houses over may be fed from
    another phase of the power system! So if you ran a wire over and measured
    their alternating voltages on their single phase residential supply you
    would find that their service is exactly the same; BUT 120 degrees ahead or
    behind yours. And another group of homes could be supplied from the third
    phase and be 240 degrees ahead/behind.
    Any help or more confusing? Sorry about the long winded answer.
    My regards. Terry
  10. Kitchen Man

    Kitchen Man Guest

    Take your dog for a walk, finding a road with pole-routed 3-phase
    power. You'll soon pass a power line pole that has a transformer, or
    two or three. Note that each transformer takes a tap off only one of
    the phases, and the secondary output will be onto two insulated wires,
    and one bare. This is the 240Vrms that supplies residential power -
    240V between the insulated secondary outputs, and 120V between each of
    these lines and the bare-wire center-tap, which will be grounded.
    Following my first paragraph, you can see that indeed, typical
    residential power uses only one of the three phases. Your confusion
    lies in the fact that you don't realize that the typical residential
    feed is 240Vrms, not 120Vrms. The 120V results from splitting the
    phase with the grounded center-tap at the transformer. In your
    residence, you end up with two separate 120V supplies, one 180 degrees
    out of phase with the other, along with a single 240Vrms supply.
    The power generation, transmission, and supply companies work together
    to keep the loads balanced. You need not worry about balancing phases
    in your residence, as you have only one phase. Industrial power feeds
    typically use all three phases, and the industries' electricians sort
    it out from there.
    The 240Vrms is in phase with itself - the red and black wires in your
    residence correspond to the two different sides of the power supplied
    by the pole transformer. (It is possible that your 240Vrms might be
    120 degrees out of phase with a near or distant neighbor's residence,
    which is fed with a transformer on a different phase.) Part of your
    120V is 180 degrees out of phase with the other part, as the 120V
    service is actually the 240V service split to equal sides of ground.
    Make sense?
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