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  • single phase and three phase?

    I am not sure if it makes sense or not....

    There is a system using 208V/3 phase. if we use single phase instead 3 phase, do we need two single phase or three single phase?


    I think...

    first, two single phase with ground total voltage will be 240V (line1 and line 2)

    second, when I check 208V/3phase any line voltage with neutral is 120V

    which one is right? and is it working when 208V/3phase system is connected to single phase?

    Thank you.

  • #2
    Re: single phase and three phase?

    Phase to phase (reading with a volt meter) will be 208V. Any phase to neutral will be 120V.

    Comment


    • #3
      Re: single phase and three phase?

      i don't know what you are working on, but you will have less power on a 208v 2 wire circuit than on a 240v 2 wire circuit. about 90% if i remember correctly. it's been years. volt will know. breid..............

      Comment


      • #4
        Re: single phase and three phase?

        Originally posted by breid1903 View Post
        i don't know what you are working on, but you will have less power on a 208v 2 wire circuit than on a 240v 2 wire circuit. about 90% if i remember correctly. it's been years. volt will know. breid..............
        When it comes to power, regardless of the voltage, power is power. Larger wires are needed for lower voltages in order to deliver the higher amount of current to supply the same amount of wattage required.
        Last edited by tailgunner; 01-22-2010, 10:14 PM.

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        • #5
          Re: single phase and three phase?

          khlee815

          Please tell about what you're planning to power. One thing you need to think about is not creating a major phase load imbalance issue.

          Comment


          • #6
            Re: single phase and three phase?

            ok. if you have a 240v 750w electric baseboard heater, you will have 563w at 208v...... 240 - 208 = 32,,, 32/240 = 13%. HP = (V x I x EFF)/746. V= volts, I = amperage and EFF = efficiency. less volts = less horsepower(hp). if larger wires worked, why not just use 120v with even larger wires? hell run it on 12v.............. woussko is right. you need to balance your load across all 3 phases. or as close as possible. make a triangle on a piece of paper. then list your loads on each flat until they are as close to equal as possible. letter each point as a, b, c. label you conductors a, b, c. it makes no difference which is which. ab is one flat, bc is one flat and ca is one flat. keep it simple. works for me. it will work for you. ................it would help if we had a better idea of what you are doing. breid..................

            Comment


            • #7
              Re: single phase and three phase?

              Originally posted by breid1903 View Post
              ok. if you have a 240v 750w electric baseboard heater, you will have 563w at 208v...... 240 - 208 = 32,,, 32/240 = 13%. HP = (V x I x EFF)/746. V= volts, I = amperage and EFF = efficiency. less volts = less horsepower(hp). if larger wires worked, why not just use 120v with even larger wires? hell run it on 12v.............. woussko is right. you need to balance your load across all 3 phases. or as close as possible. make a triangle on a piece of paper. then list your loads on each flat until they are as close to equal as possible. letter each point as a, b, c. label you conductors a, b, c. it makes no difference which is which. ab is one flat, bc is one flat and ca is one flat. keep it simple. works for me. it will work for you. ................it would help if we had a better idea of what you are doing. breid..................
              Because that particular appliance is basically a giant resistor, the simplest of circuits. So yes, when rigged for 208, a heater will use less wattage, but give off less heat for that same appliance. Yes, for a heater rated at 750 watts @ 240 volts will only produce 563 watts of heat @ 208 volts. So what do you do when you need 750 watts of baseboard heat? Simple, get a bigger heater for 208v supply, which means, the circuit requires a larger set of conductors to handle the increase in current for the same amount of wattage.

              Comment


              • #8
                Re: single phase and three phase?

                This may help.



                James
                Attached Files

                Comment


                • #9
                  Re: single phase and three phase?

                  Originally posted by khlee815 View Post
                  I am not sure if it makes sense or not....

                  There is a system using 208V/3 phase. if we use single phase instead 3 phase, do we need two single phase or three single phase?


                  I think...

                  first, two single phase with ground total voltage will be 240V (line1 and line 2)

                  second, when I check 208V/3phase any line voltage with neutral is 120V

                  which one is right? and is it working when 208V/3phase system is connected to single phase?

                  Thank you.
                  What are you trying to power?

                  James

                  Comment


                  • #10
                    Re: single phase and three phase?

                    Thank you for your answers.

                    The reason why I posted this question is someone ask me abut that.

                    When I heard, I just thought

                    208V/3 phase equal to two single phase.

                    but after I realized that 208V 3 phase each load with neural voltage is 120V.

                    So I think we need 3 single phase with neutral if the 208V 3 phase have to change to single phase.

                    Maybe it is stupid question but I also want to know about that.

                    Thank you again.

                    Comment


                    • #11
                      Re: single phase and three phase?

                      Originally posted by khlee815 View Post
                      Thank you for your answers.

                      The reason why I posted this question is someone ask me abut that.

                      When I heard, I just thought

                      208V/3 phase equal to two single phase.

                      but after I realized that 208V 3 phase each load with neural voltage is 120V.

                      So I think we need 3 single phase with neutral if the 208V 3 phase have to change to single phase.

                      Maybe it is stupid question but I also want to know about that.

                      Thank you again.
                      If I had the choice on which system to use for an electrical service. It would be 3 phase all the way, as long as it's not the old 3 wire delta system.

                      James

                      Comment


                      • #12
                        Re: single phase and three phase?

                        Why don't you like the delta system?
                        The high leg?

                        In a location where three phase is a small part of the system and the rest is predominate single phase, it appears to me to be a good way to go.
                        Last edited by BHD; 01-24-2010, 01:09 PM.
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                        • #13
                          Re: single phase and three phase?

                          Originally posted by BHD View Post
                          Why don't you like the delta system?
                          The high leg?

                          In a location where three phase is a small part of the system and the rest is predominate single phase, it appears to me to be a good way to go.
                          It's an obsolete system, not as versatile as the Wye.

                          High Leg, 200volts to ground when the other two would read 120V to ground.

                          James
                          Last edited by James P; 01-24-2010, 02:00 PM.

                          Comment


                          • #14
                            Re: single phase and three phase?

                            To the OP,

                            Are you possibly confusing 3 phase power with the 240 volt single phase power going to the typical home? I know I used to, especially since some of he voltages discussed in 3 phase power is similar to that 240 volts.

                            For those that are confused like I used to, let me try to explain.

                            A house is NOT supplyed by a 120 volt power source. It's actually powered by a 240 volt single phase power source. This power comes from a transformer that sits just outside your house, like one of these: http://science.howstuffworks.com/power9.htm. The voltage between the two wires coming out the ends of the transformer feeding your house is 240 volts. But not only are you supplied the two wires from the ends of the transformer, you're also supplied with a wire that is tied to the center of the transformer. When you measure the voltage from the center tap to either outside leg of the transformer, you measure 120 volts. But if you measure the voltage from the two ends of the transformer, you get 240 volts.

                            So where three phase power gives you three hot wires that are each 120 degrees out-of-phase, in your home, you get two hot wires that are 180 degrees out-of-phase.

                            What all this does for your home is gives you the option of wiring a device to 120v by connecting it from the ground to one of the two hots, OR you can wire a device to 240v by conecting it between the two hots. When you connect a device to the 240 volts, you cut the current requirement in half. So rather than your stove being 120 volts requiring 80 amps, its powered by 240 volts needing only 40 amps.

                            Comment


                            • #15
                              Re: single phase and three phase?

                              Originally posted by HooKooDooKu View Post
                              To the OP,

                              What all this does for your home is gives you the option of wiring a device to 120v by connecting it from the ground to one of the two hots, OR you can wire a device to 240v by conecting it between the two hots. When you connect a device to the 240 volts, you cut the current requirement in half. So rather than your stove being 120 volts requiring 80 amps, its powered by 240 volts needing only 40 amps.
                              I'm sure that HooKooDooKu understands this, but it probably should be clarified that any specific device MUST be intended for connection to the 240 circuit. Some motors and other devices offer optional wiring for either connection, but in general if you connect a 120V device to 240V, you don't halve the current... you'll burn the thing out, and possibly get a visit from the fire department. The panel breakers won't keep the 120V device from catastrophy.

                              So, never connect a 120V device across 240V unless the device is "dual voltage". And even then, make sure you know what you're doing.

                              And don't connect a 240V device across 120V, either. In the case of an ac induction motor (most common type), under load, it WILL draw a lot of excess current and is very likely to burn out.

                              However, since the original poster mentioned 208V, it sounds like actual three phase power to me. You can't measure 208V line to line on single phase residential power.

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