Explain Out of Phase Amperage?

Discussion in 'Electrical Forum discussion & Blog' started by bens, Mar 22, 2010.

  1. bens

    bens New Member

    Messages:
    13
    Location:
    Midwest
    I'm just trying to fit my head around something.
    I have seen 240 volt circuits run to two 20amp breakers connected with a bar, sitting next to each other in the panel.
    I understand that you get your 240 volts out of the two sides of the power drop from the electric company which are out of phase 180 degrees to each other.

    What I'm trying to understand is how this affects the amperage through each side of the circuit at any given time. My gut would tell me that since each of these breakers are 20amps, that means that either side will only have a max of 20 amps flowing at any given time or the breaker would trip. If this is true then why do you need 10 gauge wire for a 40 amp circult instead of just being able to use 12 gauge for each leg? That just doesn't seem right though...looking for why.

    Thanks!
  2. drick

    drick In the Trades

    Messages:
    392
    Power is measured in watts, not amps. You do have only 20A of current for a 2 pole 20A breaker or for a single pole 20A breaker, but your wattage doubles when you go from 120V single pole to 240V double pole. So you are getting twice the power over the same gauge wire.

    120V X 20A = 2400Watts

    240V X 20A=4800Watts


    -rick
    Last edited: Mar 22, 2010
  3. Lightwave

    Lightwave New Member

    Messages:
    98
    Location:
    Vancouver, BC
    A 240 volt circuit fed with two 20A breakers is a 20A circuit. It can be wired using 12awg cable.

    A 40A 240V circuit is fed by two 40A breakers and requires 10awg cable.
  4. bens

    bens New Member

    Messages:
    13
    Location:
    Midwest
    So is drick agreeing with the notion that I can indeed use 12 gauge wire if I have two 20-amp breakers wired to a 240-volt device?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,995
    Location:
    New England
    The 20A circuit breaker, regardless of the fact two are tied together, will prevent more than 20A from going through any one wire in that branch, so 12g wire should be sufficient unless the runs were exceptionally long. Essentially, you've got 20A swinging back and forth from one side to the other, they don't add in current. You get more power because of the increase in voltage (Power=volts * amps). the CB protects from over current, not over power (which are related, but different things).

    In a conventional house, you really have one phase power (220-240vac). Neutral is the center-tap of the transformer producing the 120vac, so it is mid-point referenced to each end. So, stuff referenced from neutral to one side is 120v, and 240vac across between the two sides of the transformer secondary. Since the sine-wave is crossing zero in the middle (neutral connection) each half is opposite in phase.
    Last edited: Mar 23, 2010
  6. drick

    drick In the Trades

    Messages:
    392
    Yes, 12 gauge wire is sufficient for 20A 240V.

    -rick
  7. andrew79

    andrew79 Licensed Electrician

    Messages:
    18
    Location:
    Ontario Canada
    small FYI....40A circuit should have number 8's not 10's
  8. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,623
    Location:
    Cave Creek, Arizona
    Two 20 amp breakers on opposite legs of the 240v circuit do NOT make a 40 amp circuit, they TWO 20 amp circuits, and therefore your initial premise is flawed. Also if you have #10 wires, then you use 30 amp breaker. If these questions apply to a job you are doing, then maybe you should stop doing it, because you are going to burn something down or cause some other problem.
  9. andrew79

    andrew79 Licensed Electrician

    Messages:
    18
    Location:
    Ontario Canada
    i was refering to this.

    Also just because there's number tens there doesn't mean that you should use a 30A breaker. You would need to know what's on the other end of the line. It may be an appliance that's required to have a 20A breaker as protection and they just ran the wrong size wire, again this happens more often in comercial and industrial than residential...or they could have upsized the wires for voltage drop although i've rarely ever seen a house big enough for this to become a factor.
    look at it this way....if you have an A/C unit that's required to have a 40A breaker but for some reason number 6's were installed and for some reason a 50A breaker was put in then effectively that a/c unite could overheat and start to draw more than it's 40A load without tripping the circuit. Hopefully at this point the thermal overloads of the motor would kick in and shut it down but anythings possible. best case scenario is that you burn out the motor....worst case is that it starts on fire. Never oversize your breakers unless your sure of what it does even if the wires large enough to handle it.
    I agree with HJ on this one....i think your better off having a professional handle it.
    Last edited: Mar 25, 2010
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