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220 volt double pole breakers (US)

Discussion in 'General Electronics Chat' started by ke5frf, Nov 18, 2009.

  1. ke5frf

    ke5frf New Member

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    An interesting question just came up that I can't answer.

    A field service rep at my facility today was servicing our equipment and took a call from another customer. The other customer's equipment is on a 220 volt service with a double pole breaker labeled 20 Amp on each pole.

    Evidently there has been an ongoing problem with this breaker. The equipment in question is rated for 40 amp service.

    I'm no electrician i.e. building power distribution and wiring, but I do understand that in the US 220 volt circuits are fed out of phase with two parallel 110 volt legs. My understanding was that 20 amps drawn from either leg would trip the breaker in unison since they are connected.

    But the question that came up, is this considered a 40 amp service or 20 amp service? Is the rating of the breaker the COMBINED rating or the max rating for either leg?

    So, if I were looking at a breaker with two poles, each labeled "20", how many amps on either leg would trip the breaker, 10 or 20?

    This is important to understand based on the rating of the equipment. It is foreign manufactured for European standard 220 volts, which is single phase as I understand it. I am not European, but I would assume that European breakers are single pole switches.

    Thanks if you know the answer.

    Edit: The reason why I mentioned the difference between European and US ratings is because if the equipment was labeled for Europeans using single phase power, then 20 amps would be literally 20 amps on a single pole switch. My thought process is that with a 2 phase system, each leg is protecting its own leg of the circuit. 20 amps drawn through either leg would trip it, so in effect the "trip current" is 20 amps....yet, lets say for discussion the load was drawing near maximum rating through each leg...would the total load of the service be around 40 amps?

    It confuses me because we aren't talking about parallel legs from the same voltage source, thus all currents add together. We are talking about two voltages fed out of phase. My logic says that the total current would STILL be the sum of each phase leg, but I don't fool with 220 AC very much so I'm confused.
     
    Last edited: Nov 18, 2009
  2. ke5frf

    ke5frf New Member

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    I suppose technically it is 240 volts we're dealing with, but for whatever reason my mind has always been geared to the old 110/220 standard.
     
  3. Mark_R

    Mark_R Member

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    A 2 pole 20 ampere breaker is called a 20 amp breaker. It is just 2 single pole 20 ampere breakers tied together at the factory with an internal "common trip" mechanism. More than 20 amperes on either leg will trip both.

    US single phase power is generally center tapped 240v (120 on each leg). Three phase is generally 120/208 or 277/480. With single phase power the phase to phase voltage is 2x the line to neutral voltage. With three phase the phase to neutral voltage is the phase to phase voltage divided by the square root of three. you can work out the proof in with trigonometry if your into that sort of thing.

    If the equipment in question is rated for a 40 ampere service, they have the wrong breaker. Take the full load amps (at whatever voltage it runs on), multiply by 1.25 and round up to the next standard breaker.
     
    Last edited: Nov 18, 2009
  4. dave

    Dave New Member

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  5. smanches

    smanches New Member

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    I think this is the source of your confusion, as this information is incorrect. Houses are fed from a single 120-0-120 transformer. There is nothing "out of phase" about it.
     
  6. Mark_R

    Mark_R Member

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    To expand on this further...

    I think you have it twisted up. Your confusing amperage with power. Lets say we have 2 motors, one 120v and one 240v, each have a full load amperage of 5 amperes. Lets assume unity power factor (which wouldn't be the case with a motor). The 120 volt motor is dropping 120v lead to lead at 5 amps, so using Ohm's law we get 120x5=600 watts. The 240v motor is using 240x5=1200 watts - twice the power on the meter.

    The voltages are not "out of phase", your still dealing with one sine wave, it just has twice the magnitude. The mirror image sine wave your picturing does not exist because the motor does not use the neutral center tap point of the system.
     
  7. Sceadwian

    Sceadwian Banned

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    smanches not if the center tap is grounded you'll have L1 ground and L2 L1 and and L2 will both be 120 volts to ground, but theyl'l be 180 degrees out of phase giving you 240 volts when used together. As was discussed in another thread it's not technically two phase it's split-phase, but there is a phase difference between the two branches. I've heard stories of horrible miss wired exension cords shorting L1 and L2 together and vaporizing the extension cords.
     
    Last edited: Nov 18, 2009
  8. Mark_R

    Mark_R Member

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    :confused: Huh?
     
  9. ke5frf

    ke5frf New Member

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    If they aren't out of phase, how do they add together?

    You can't at two voltages in parallel and get anything more than the original voltage with twice the ampacity...unless they are combined out of phase with respect to neutral.

    Please expand if I learned something wrong.
     
  10. smanches

    smanches New Member

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    I'm now getting confused by your confusion. :p

    It's a simple center tapped transformer. It works just like any other center tapped transformer.

    Am I missing something?
     
  11. Mark_R

    Mark_R Member

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    It's a matter of reference. The voltages are not in parallel, its the same sine wave / winding, you are either going across the entire winding or half or it. Picture using the horizontal position knob on an O-scope. You are changing your point of reference.

    OK, that's not a very good example. I must have a good diagram around here, I'll try to find it later.

    EDIT; I meant vertical position, up and down.
     
    Last edited: Nov 18, 2009
  12. ke5frf

    ke5frf New Member

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    Seriously, I want to re-learn if I learned something incorrectly.

    My mentor years ago drew a sinewave like the following for me when he taught me about 240. Honestly, I never saw it on a scope, just took his word for it and never saw anything to the contrary. Again, I don't fool with anything inside of walls, never have.

    Pardon the crude quick sketch.
     

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  13. ke5frf

    ke5frf New Member

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    Right, well I understand center tapping a transformer.

    Ah, so neutral is center tapped. OK I think I'm picturing this but it is still sort of mind blowing because I always had it wrong I'm thinking.
     
  14. ke5frf

    ke5frf New Member

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    So, hmmm. I am still confused. Oh wait a minute.

    I was doing a mental excercise with my voltmeter, measuring from hot to hot on a 240 circuit and seing 240 on the meter. In my mind, I'm looking at two in-phase hot wires going up and down together. At any point between the 2, the voltage would be 120 volts.

    But then I thought, my meter doesn't work that way, it is a true RMS meter, thats the key, right?
     
  15. ke5frf

    ke5frf New Member

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    Because to get 120 volts on an AC RMS meter, at time x, the peak is 60 volts above neutral, and at time x+1, the valley is 60 volts below neutral. So, my drawing throws all of that off, because the meter can't see it as a sine wave.

    This is a booger to get my mind around but I think I see.
     
  16. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    The center tapped 240 volt transformer is the most correct definition.
    Being that center tap is also used as the common reference and earth ground point in most systems that makes it so that only a maximum AC voltage potential of 120 Volts AC is possible in reference to the common line and earth ground planes from a single line.
    To get the full 240 volts you must have two opposing lines feeding the load.
    a double pole circuit breaker is 20 amps per line. That equals the same power of a single 40 amp 120 volt line.
     
  17. ke5frf

    ke5frf New Member

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    If someone could draw what it looks like on a scope, it would clear it up for me. I could't find it on google.
     
  18. ke5frf

    ke5frf New Member

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    Its very wierd, having thought incorrectly for all these years makes it hard to even picture this, but All About Circuits has a nice tutorial that makes sense of it. I never even realized that the transformer secondary outside my home was 240 and center tapped. I see it clearly now what is happening. Doh!!!
     
  19. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    240 volt AC looks just like the sine wave of 120 VAC. It just has twice the amplitude.
     
  20. ke5frf

    ke5frf New Member

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    The neutral wire is what throws it all off, makes it counter intuitive. It doesn't seem to serve any purpose on the 240 circuit that I can see. It is what caused me to believe the voltages had to be out of phase all these years.
     
  21. Sceadwian

    Sceadwian Banned

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    Mark, people do some pretty stupid stuff with extension cords/power strips. Like using male to male adapters or extension cords to tie things together in weird ways.
    Just as an example someone wires an extension chord to two seperate outlets that are controled by switches that that if either switch is on it'll power the strip/device. This is perfectly fine if they're both the same branch circuits because they're in phase, so you'll only get a little bit of current from the voltage drop difference in the two circuits, but it'll work. If you connect two different branches together it would also work fine, as long as only one was ever on at the same time, turn on both at the same time and the two 180 degrees phases will add up to 220V's and the extensio cord will be a dead short, it'll draw a fair bit of current before the breaker trips.
     

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