About AC supply

Discussion in 'General Electronics Chat' started by LDC3, Jun 25, 2013.

  1. LDC3

    Thread Starter Active Member

    Apr 27, 2013
    Taken from another thread....
    This message is mainly for Stuntman to education me. How do businesses get 3 phase power from the same distribution as residential houses if there is not 220V between each of the phases?
  2. Papabravo


    Feb 24, 2006
  3. LDC3

    Thread Starter Active Member

    Apr 27, 2013
    The article states:
    Most household loads are single-phase. In North American single-family dwellings, three-phase power generally does not enter the home; multiple-unit apartment blocks may have three-phase power but three-phase power is not used for household appliances. Utilities that supply three-phase power for lower-load-density area homes typically distribute only one phase to individual loads.

    This would provide about 110V. In order to get 220V, one would use 2 of the phases. This is what I have been saying, but Stuntman says I'm wrong.
    (OK, I'm wrong about the 550V)
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    You should note that three phase equipment has more connection wires/terminals and the internal wiring is quite different from single phase.

    So the four wires are brought to three phase equipment.
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
    Businesses DON'T get their power from the same place as houses. Somewhere between the high voltage (several thousand volts) line and the business' 480V receptacle, there is a transformer. This is not the same transformer that feeds 240V into your home breaker panel.
  6. LDC3

    Thread Starter Active Member

    Apr 27, 2013
    Actually, they do since there is only one high voltage transmission line.
    The high voltage transmission lines are 3 phase. You can easily see this since there are multiplies of 3 lines on the towers. (There may also be a single line at the very top of the tower, but this is for lightning control: it doesn't carry any power)
    Unfortunately, the view of the substations is usually blocked so it is difficult to see how the transformers distribute the power. It probably is true that businesses get a stepped down voltage from the transmission line and that the residential houses have another transformer, but I still believe that 2 of the 3 phases are used to provide 220V. After all, why can't phase 1 and 2 be supplied to the houses east of the substation, phase 2 and 3 go south and phase 1 and 3 go west (or whatever direction you want). If the same number of houses are on each pair, then the 3 phases are close to being balanced. I guess only a person working for the utility company can answer me.
  7. Stuntman

    Senior Member

    Mar 28, 2011

    The reason behind me calling you out on this is misinformation about power distribution seems to run rampant.

    Let's talk about a few things:

    A.) Yes, power is usually generated and distributed in 3phases (120° apart). There are inherent advantages to this both in transmission, generation, and use (especially large industrial settings).

    B.) Power is not distributed on your power lines at 120V, 240V, 440V, 600V, or even generally less than 1000V. I'd guess many of the power lines you see outside of the city are closer to 100kV, while those inside may be around 2kV. Substations and that those grey transformers on the pole behind your house make these high voltage lines usable for you, the consumer.

    Here is an excellent diagram from Wiki to outline what I'm talking about:

    C.) What a resident uses (the topic of discussion) is almost always a transformation of only 1 phase (perhaps sometimes 2) of these three phase distribution system.

    So into more detail, I will, from hereforth, assume a standard residential setup.

    Where you house hooks to the electricity service (or close nearby), you'll find a transformer. (notice the poll/transformer on the "Secondary Customer" in the diagram). This takes the in-town distribution voltage (perhaps 10,000V phase-phase) to what we in America, use in our homes (120V and 240V).

    Now, this isn't just any transformer, this is a center tapped transformer. Which means your house is not being fed by two wires, instead, it is fed by three. One, is a neutral (bonded to earth at the pole) and two 120V volt leads that are 180° out of phase (one is black, the other generally red).

    Now, one might be tempted to say "this is two phase power". Well, yes, they are two distinct phases, but they have nothing to do with three phase power, or the phases from power company, they are simply the result of a center tapped 240V transformer. (ETA: This statement could be argued if interpreted deep enough, I am simply stating that the two 120V phases going to the home are not related in the same way 3-phase power phases are related)

    So, once inside your home, if you hook an appliance (through your meter, main panel, and branch circuits) between the black wire, and the white wire, you have 120V (RMS) power. If you hook an appliance between your red wire, and white wire, again, you have 120V power. However, since these two phases are 180° apart, hooking an appliance between a black, and a red wire, will give you 240V power.

    This gives a basic breakdown of what happens to get power to your home.

    Now, let's make a few more notes. You mention the 120° phases. Yes, there are places that have all three phases from the power company at their disposal. At the pole that has the three main power lines (phases coming from the substation), there is three transformers (or a transformer pack with three transformers inside). Each transformer feeds 1 wire that goes to the building (phase wires) along with a ground wire. The phase can come in different voltage levels depending on transformer, the common voltages being 120, 277, and 347. These voltage levels refer to the voltage of each leg with respect to ground (earth). However, taking the voltage between any of the two legs, will give you 208, 480, and 600V respectively. Remember, they are not 180° out of phase, so phase to phase voltage does not double phase to ground voltage as it did in the residential example. Now, you may note, that in the 120V three phase example, there is no reason you cannot use a 120V appliance between a single phase and the ground wire. You may also notice that the phase to phase voltage is 208, roughly what one COULD use to power some 240V equipment (much is dual rated).

    There is much theory and practice that happens with three phase power in the US (and other countries). Although it is undoubtedly confusing how all the different systems work, there were a variety of non-truths and misleading phrasing in your statement, hence my post.
    Last edited: Jun 25, 2013
    LDC3 and tshuck like this.
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
  9. crutschow


    Mar 14, 2008
    If one phase of a three-phase Y circuit gives 110V to neutral then the voltage between any two phases gives √3 or ≈1.73 times the single phase to neutral voltage since the two phases are 120° out-of-phase and thus you must add them in vector fashion.

    If you have a 3-phase Δ (delta) connection, then the voltage between any two phases is the same.

    The 110V-220V connection that goes to a residence is a single-phase center-tapped winding from the transformer with the center-tap being the grounded neutral. Thus you get 110V from either hot wire to neutral and 220V across the two hot wires.
  10. Stuntman

    Senior Member

    Mar 28, 2011
    This statement also applies to Wye (Y) topologies (the other three phase topology).

    The differences come in when you measure phase to neutral or ground. In Y configurations, each phase to ground voltage is the same. In Δ configurations, this is not the case.

    However, for 3-phase equipment (that does not reference ground or neutral) there is not an operational difference between the two.