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240 volts and 120 volts

  1. Apr 8, 2010 #1
    Ok i have a question about the difference between 240 volts and 120 volts. So from my knowledge power will come from a generator somewhere down the road. it will then make it to my house and a transformer will take it to 120/240. i get 240 120 from each line that are out of phase from each other. and i get 120 from one hot and the middle of the transformer or the neutral. does 120 use the earth as a reference point of 0 volts to have AC run threw the wires back and forth and 120 volts if it where hooked up to a circuit? and so 240 doesn't use the earth as a refernce point becuase it uses both phases... so does it just alternate current back and from each phase in your panel or does it go all the way back to the power plant i just don't understand how far the electrons will travel back and forth do they stop at the pannel or travel all the way back the the generator then back again. Im probley way off with most of this if anyone could explain it nice and simple for me that would be great thank you.

    And one other question if i have two phases making 240 and i hook up two 100 watt light bulbs in series will they light up and get 120 volts across each of them.. or will this not work becuase they arne't getting the earth as a nuetral
  2. jcsd
  3. Apr 8, 2010 #2


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    The electrons in the wires using AC do not travel very much at all. Essentially they are oscillating in place.
    Last edited: Apr 8, 2010
  4. Apr 8, 2010 #3


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    Slightly depends where you live.
    If you are in europe (and a few other places) the power is 240v (well 220-240V) this is the average (RMS) voltage above 0 (ground) of a sin wave.

    If you are in the US the wires are mostly 110V (RMS) above ground - except they also supply another wire that is 110V below ground - they are exactly out of phase so the voltage between them varies in exactly the same way as it would if you had a 0-220V supply in europe. You use this for big appliances.

    Don't think of electrons - it's not helpful, just think of connecting a battery.

    Depends what you are connecting the light bulbs between.
    If you connect two light bulbs between a 240V (europe) line and ground, then yes they will get (roughly) 110 volts accross each and would light up as normal (if they were US bulbs designed for 110V) or dimly (if they were eu bulbs designed for 220V).

    Phases, as in a 3phase supply is a little more complicated again.
  5. Apr 8, 2010 #4
    Ideally if the load is balanced there'd be no current going through the neutral wire at all, because the currents from each phase would add up to zero as they are out of phase by 120 degrees.

    And I don't understand how you get 240V if the phase voltage (line to neutral) is 120V, the line voltage (ie from one line to another) should be 120 times the square root of 3 as the voltages also are out of phase by 120 degrees so you can't just add them as scalars.
  6. Apr 8, 2010 #5


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    It's not 3phase, it's just two separate 110V lines in antiphase.

    It was a bit of expedient engineering (bodge) to allow high power appliances without changing the infrastructure too much. You just run an extra -110V line to the house and run a few appliances between them, with no local neutral connection (you have to be a little careful with the earthing)

    ps. US line voltage is really 120v +-6v, it's called 110V for historical reasons
  7. Apr 8, 2010 #6
    Okay sorry I had no idea about the extra line in antiphase, so I thought the OP was talking about three-phase when talking about line to line voltages. My bad. I don't think we have such a thing in Europe, I guess that's because it's 220V from line to neutral (though now we call it 230V) :P
  8. Apr 8, 2010 #7
    What if i live in canada and the voltage in my house is 120/240 its 120 from line to ground. But if i where to connect two lightbulbs 100 watts each in series to 240 would they light up with 120 volts across each? I asked this question to a Electrician friend he says it will not work becuase you don't have a neutral reference (ground).
  9. Apr 8, 2010 #8


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    Yes you can connect two * 110v bulbs in series across the 240V lines - really they are 110V above and below neutral so you could also connect the middle of the two bulbs to neutral and it would still work (except it would probably trip the GFD on you fuse panel)

    - the usual don't try this at home warnings apply.
  10. Apr 8, 2010 #9


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    It's an interesting American wiring thing, I imagine it came about when you started getting large appliances and needed to run them without having power cables the size of tree trunks.
    Even kettles are rare in the US because you can only take 15A @110V in stead of 13A @240V, and they don't have electric immersion heaters or electric power showers.

    But for washing machines, dryers you need to be able to supply 5-6kw without having 100A cable so the split supply is a good solution. It's also safer because you only have 110V from ground on either side, rather than 240V to ground.
  11. Apr 8, 2010 #10
    Yea i don't plan on ever doing this i would just use 110 from my panel im just trying to understand the concept. When you say you could also connect the middle of the two bulbs to neutral and it would still work.. im not understanding this.. and so the 110 bulbs in series across the 240 would work without doing this? what would be the point of connecting the middle of the two bulbs to neutral?
  12. Apr 8, 2010 #11


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    Consider 2 ordinary AA batteries

    -ve [battery] +ve ---> -ve [battery] + ve

    this is just the same as, your 240V supply

    -110V [electric] --- (0v) ----> [electric] +110V

    Connecting a pair of bulbs from -110V to +110v gives 240V, with 120V across each.
    the middle of the bulbs at 0v , even if you don't make any connection to neutral there.
    Remember the bulbs only care about the voltage across them, they don't knwo what the absolute voltage compared to earth is.
  13. Apr 9, 2010 #12
    thanks that makes total sense to me that the middle would be 0 volts. So would they light up and work without taping into the middle of the two lights. So am i correct and the electrican that i mentioned before that said it woudn't work wrong?
    Last edited: Apr 9, 2010
  14. Apr 9, 2010 #13
    In a single family dwelling you likely have 240V and split the phase for two 120V in anti-phase. This is topology mgb_phys talks about.

    But beware! If you live in an apartment you likely have 120V and 208V available to you. This is because you are fed two phases of a three phase system. For 120V you connect from either phase to neutral. If you connect from one phase to the other you get 208V because the phases are separated by 120 degrees.
    http://en.wikipedia.org/wiki/Three-phase_electric_power (Check out the single phase section)

    Appliances don't transfer well between house and apartment. Interesting bit of trivia to file away.

    Hope this helps clarify things for Bobbybear too - you were right for apartments and industry, but not detached homes. Generally speaking. With voltages there's potential for confusion.
  15. Apr 9, 2010 #14


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    In my neigborhood, the AC comes in via 2 lines at 6900 volts to a step down transformer that outputs 220 volts. The transformer output side is center tapped and 3 lines go out, "+110 volts", "zero volts" (neutral), "-110 volts" (I'm not sure how else to explain this). The 3 lines go through the meter into the homes and the households get 220 volts using the outer 2 lines, and the 2 110 volt output combinations are somewhat evenly distributed at the fuse box.

    A fourth ground wire is attached to pipe or rod implanted into the ground to give a true ground reference, and is used for the third connector on the wall sockets in a home, and also used by the ground fault interruptor wall sockets.
  16. Apr 9, 2010 #15


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    If I have 2 motors, the first motor is designed for 110V and the 2nd motor is designed for 220V (all other motor parameters are the same).
    If the 110V motor is working at 6Amper does is necessarily mean that the 220V motor is working at 3A?
  17. Apr 9, 2010 #16


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    Yes (give or take any small efficency differences)
  18. Apr 9, 2010 #17
    O: so then you can ruin your appliance if its meant for a chalet (120V) and you take it to your appartment and plug it into the 208V socket D=
    Isn't that confusing for consumers?

    Over here for residential use, whether in appartments or houses, all our sockets are 230V (line to neutral), we don't have 3-phase line to line voltage sockets in homes. So the 3-phase systems reaches our house or appartment building, and each of the sockets has a line and a neutral wire (apart from earth) (the three lines are distributed amongst the different rooms of the house/building to obtain a more or less balanced 3-phase load, but each socket only has one line and neutral). For industries which require machinery that use 3-phase current, like electric induction motors and stuff, you'd have 3 phase sockets, but that would be using all three phases plus neutral, so the socket would have 4 connections.

    Okay so I have to be aware of where I plug in my laptop when I visit the US lol :P
  19. Apr 9, 2010 #18
    No, I think the current absorbed by a motor would simply depend on the load it's moving (the mechanical torque).
    Yes, so if they're moving the same load, I suppose they should be consuming the same power, so the 220V one would absorb half the current..
  20. Apr 9, 2010 #19


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    No the 110 outlets (actually 120V) inside a house are different from the high power 240V sockets for washer/dryers.
    What you might have a problem with is that some commercial properties may have the high power outlets wired 208V (ie 110V 2phase) rather than domestic 240v (ie +-110v split)
    Since the only thing anyone uses these for is clothes washers they probably aren't all that fussy about exact voltages.
  21. Apr 9, 2010 #20
    mhm I see. It's interesting though, I hadn't realised that the line-to-line voltages of a 3-phase system (the 208V) is used for powering single-phase appliances. As I said, over here in Europe we only have phase-voltages (line to neutral of a 3-phase system) available in our houses/flats, but since the line wire of different sockets might come from different phases, I suppose one could derive cables from the line wires of two such sockets to obtain approx. 400V ( the phase voltage being 230V). Maybe I can connect my 230V lightbulb to that and see if it burns brighter:) I'll probably fuse it though ;p
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