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  • 240 volt AC question

    This is for somebody with more electrical experience than I've had. I hail originally from the UK, where 240 volts AC single phase is standard for dwellings at least.

    I've done some 110vac electrical installation here in the US, and as far as I understand it, the 'normal' supply to dwellings is three wires - two positive legs at 120vac each, and a neutral eventually going to earth.

    For 240vac, the theory I've heard, ( which makes sense) is that if you need 240 vac, you can get this across the two positive legs - so you wire both legs to the 240vac socket , and leave out the neutral, unless on say an oven, you have a 110 vac fan or something similar.

    All this makes perfect sense, until I noticed recently that all the 240vac breakers I've seen take both their 110vac connections from the same leg. I've never seen any breaker system that has one connection on one leg and a different connection on the second leg, so how is one able to get 240 vac from just one positive leg?

    Thanks in advance

    Richard in Los Angeles

  • #2
    RPM,

    If you look more closely, you will see that the buss bars have alternating lugs. A double breaker takes 120 from each buss bar. Go have a look.

    Cheers,

    George

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    • #3
      You have the theory correct. The 240 two pole breakers do not take power from the same leg, it just looks that way. If you look carefully at the construction of the breaker panel, You'll see that the blades that the breakers connect alternate legs. Two adjacent blades are connected one to each pole.

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      • #4
        Your observation about the breakers is incorrect. A double pole breaker as used for 240VAC connects to two tabs in the panel that are the opposite 120VAC legs. You may have been looking at a two pole half width breaker. They do connect to only one leg and cannot be used for 240V. The panel tabs are arranged so that each leg connects to alternate tabs.
        Don Young

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        • #5
          Richard, in the panel every other lug is the other leg A B A B A B etc. In a three phase panel it is : A B C A B C etc.
          As a side note the legs are not ''positive'' they are AC so they are positive and negative (60 cycle) usually just called a hot leg.

          Steve

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          • #6
            I guess I'm a slow type-r

            Steve

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            • #7
              Thanks guys,
              I took a look, and of course it was just as you described -I should have been more observant :-(

              Richard in Los Angeles

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              • #8
                240 Vac

                RPM

                Be sure to run a ground even if you are only going to use the 240 VAC lines. This way you won't find yourself acting as the ground if a short develops in your appliance or whatever. Note that on some older systems the neutral was also used as a combination ground and neutral. You will probably run into some of these systems from time to time. Be careful.

                I have been bitten by a few of your UK systems long ago - about 1954/56.

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                • #9
                  Also, what you're calling "positive" is usually referred to as "hot".
                  Any products mentioned in my posts have been endorsed by their manufacturer.

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                  • #10
                    A 3 phase generator has 3 output wires, L1, L2, L3. The center wire is called neutral. The only time you use the neutral wire is when you one of the other wires to get singla phase at a lower voltage.

                    In industry if you have 480 volts between L1/L2 or L2/L3 or L3/L1 then you will have 240 volts between neutral and any of the L terminals.

                    In home use if you have 240 volts between L1/L2 or L2/L3 or L3/L1 then you will have 120 volts between neutral and any of the L terminals.

                    The L terminals are all 120 degrees out of phase with each other.

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                    • #11
                      Originally posted by RPM
                      This is for somebody with more electrical experience than I've had. I hail originally from the UK, where 240 volts AC single phase is standard for dwellings at least.
                      The difference is, or at least was that the UK uses one 240v phase to a grounded star point of a 3 phase transformer, every so many residences were on the same phase, used to be every three.
                      So your neighbour may be on a separate phase.
                      Max.

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                      • #12
                        Power

                        Gary
                        In most industrial power areas where 480 volts are used the power panel does not use a neutral unless it is designed for a Y connection and the you have 277 volts between a hot leg and the neutral (center of the Y connection). The 277 power is used mainly for lighting circuits. Where 120 volt single phase service is required a transformer is used to obtain the 120/240 volt service.

                        JRW

                        Comment


                        • #13
                          Originally posted by Don Young
                          Your observation about the breakers is incorrect. A double pole breaker as used for 240VAC connects to two tabs in the panel that are the opposite 120VAC legs. You may have been looking at a two pole half width breaker. They do connect to only one leg and cannot be used for 240V. The panel tabs are arranged so that each leg connects to alternate tabs.
                          There are also some half width breakers for 240. They take up two standard width slots and connect to two adjacent fingers on the bussbars. They have four separate, half width breakers and the inner two provide one 240 circuit while the outer two provide another. The inner two are strapped together and the outer two are strapped together.
                          Paul A.
                          SE Texas

                          Make it fit.
                          You can't win and there IS a penalty for trying!

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                          • #14
                            Originally posted by gary350

                            In home use if you have 240 volts between L1/L2 or L2/L3 or L3/L1 then you will have 120 volts between neutral and any of the L terminals.
                            Except it's 208 phase to phase for that type service. (when it's 120
                            phase to neutral).
                            ...lew...

                            Comment


                            • #15
                              Gary,

                              Are you sure of that? A 3 ph line is 120 volts per leg 120 degrees out of phase, all three legs are 120 volts. I don't think there IS a Neutral.

                              220 AC has 3 wires, 2 of which are 110, and the third neutral, plus a possible green for ground. Neutral is not required to get 220, but highly recommended, and possibly required by NEMA .

                              "In home use if you have 240 volts between L1/L2 or L2/L3 or L3/L1 then you will have 120 volts between neutral and any of the L terminals.

                              The L terminals are all 120 degrees out of phase with each other."

                              I have to question this, too. You have too many "L" terminals for 240 V home power. You have L1, L2, N, and Ground. L1 and L2 are 180 out of phase. That is why you have 240 V. If they were IN phase, you would have 120.. You are describing 3 phase.
                              Sorry.

                              George

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