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Thread: Combining two 120V into a 240V connector

  1. #1

    Combining two 120V into a 240V connector

    I was wondering if someone more electrically inclined could figure out some type of device that would combine two 120V outlets and turn it into a 240V so that you could charge faster.

    What I'm imagining is if I went to a friends house or somewhere that didn't have a high power charger. I could use 2 extension cords and plug in two 120V plugs into a small device that would combine it to make a 240V. The extension cords would have to be plugged into different breakers since most house breakers have a 15 amp capacity.

    Is this possible?

  2. #2
    mod squad TEG's Avatar
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    This has been discussed before. I will dig up some old threads in a few moments.

    Short answer - it is only possible if you find two outlets that are are different halfs of "split phase" service.
    Most North American houses have half the outlets out of phase with the other half. With long enough cords, you can combine the two and get 240V, but you will still be limited by the circuit breakers behind them... For instance, perhaps 16A max current draw.
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    It is possible. The issue is to make it cheap, easy to use, reliable and compact.

    Probably the best way is to turn AC into DC, have some voltage regulation to ensure you have the same voltage from both connections, combine the DC, then turn the DC back into AC. You can make this sort of system very non-picky about the input. You could even combine a 240V and a 120V and generator outputs, if designed correctly.

    (I think it would probably be too expensive to make sense.)
    Planning on getting a Model S in 2014.

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    mod squad TEG's Avatar
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    Related old threads:
    220 charger from 110 voltage
    Fast charging from 120V


    Related product:
    220 240 Volts from 110 120 Volt Outlets - Catalog & Pricing

    - - - Updated - - -

    Quote Originally Posted by Yggdrasill View Post
    ...Probably the best way is to turn AC into DC, have some voltage regulation to ensure you have the same voltage from both connections, combine the DC, then turn the DC back into AC...
    Not the best way for USA! Just hunt for outlets that are out of phase with each other, then you can take the alternate "hots" from each and you get 240V. It is basically the same power lines that the house 240V outlets use, just they split them apart each to different halves of the house with hot+neutral for 120V. If you don't have access to a 240V outlet, you can make one once you find the two halves.

    Home Electrical Wiring- Diagram and Installation
    (Visualize some more 120V outlets on the left side of this diagram going Line 2 to Neutral...)
    Last edited by doug; 2013-12-26 at 04:19 PM. Reason: image removed
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    It is better from the ease-of-use angle. Just plug it in. (And maybe adjust how many amps to take from each connection.)
    Planning on getting a Model S in 2014.

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    mod squad TEG's Avatar
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    Quote Originally Posted by Yggdrasill View Post
    It is better from the ease-of-use angle. Just plug it in. (And maybe adjust how many amps to take from each connection.)
    But you would still be limited to ~1.5kW because of 15A or 20A breaker on each 120V outlet.
    Besides, modern EVs in USA all let you charge from 120V directly, so stepping up voltage with an AC->DC->AC to 240V really wouldn't get you anything (other than a big piece of equipment and wasted efficiency.)
    Doubling the voltage by getting both parts of the split phase gives you twice the power capability. (Like 3kW instead of 1.5kW)

    USA "split phase":


    Not to be confused with "2 phase":
    http://en.wikipedia.org/wiki/2_phase_power
    Three-wire, 120/240 volt single phase power used in the United States and Canada is sometimes incorrectly called "two-phase". The proper term is split phase or 3-wire single-phase. The two live outputs of a 3-wire single phase transformer secondary winding are properly called "legs".
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    No, you'd still take as much as each breaker can handle. 2 * 15A * 120V = 3600W. Efficiency losses would mean you could probably output 3 kW.

    But of course, you could combine other inputs without worrying about phases and the like. For instance: 2 * 15A * 240V = 7200 W.

    And it would have a dual purpose, as you could use it to clean up generator outputs and the like. Of course - if designed correctly.
    Planning on getting a Model S in 2014.

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    mod squad TEG's Avatar
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    Outlets nearby tend to be on the same breaker... So I don't know how you plan to get 2x 15A from your 120V->240V converter device.

    If you get 240V from two different sides of the house then you know you have at least 2x 15A because they are from different circuits.

    Anyways, your proposal of AC->DC->AC seems way "overkill" for something like this. You could just use a transformer if all you wanted was higher voltage...

    I think all this 'sidebar' talk of rectifications and transformation distracts from the fact that there is a simple solution available that just selectively connects existing wiring...
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    Quote Originally Posted by TEG View Post
    Outlets nearby tend to be on the same breaker... So I don't know how you plan to get 2x 15A from your 120V->240V converter device..
    Outlets nearby also tend to be the same split phase. So, by that reasoning you can't produce 240V.

    I'm proposing the exact same thing as you - plugging into two different outlets in different parts of the house. My solution is just less picky - it doesn't care if the two breakers are on the same split phase or different split phases. It doesn't care if the outlet is wired this way or another way. As long as there is a (for instance) 120-240V sine 5-20A supply - my solution will work with it.

    Quote Originally Posted by TEG View Post
    If you get 240V from two different sides of the house then you know you have at least 2x 15A because they are from different circuits.
    And you can plug in my solution into the exact same outlets, and get out 240V with nearly the same power.

    Quote Originally Posted by TEG View Post
    Anyways, your proposal of AC->DC->AC seems way "overkill" for something like this. You could just use a transformer if all you wanted was higher voltage...
    But it wouldn't be easy to combine the output from two outlets, so you gain nothing. (It is of course possible to do, if you make sure to use the same split phase. But this isn't something an idiot can do.)

    Quote Originally Posted by TEG View Post
    I think all this 'sidebar' talk of rectifications and transformation distracts from the fact that there is a simple solution available that just selectively connects existing wiring...
    The box of electronics you need is simpler, but it's harder for a layperson in use, as it requires being able to figure out how to wire it up. My solution can be made by an expert and used by an idiot. Your solution can really only be used by an expert.
    Planning on getting a Model S in 2014.

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    It doesn't take an "expert" to see when a light is on, that's the indicator on the "Quick220" device that's made commercially to get 240VAC, from seperate 120V outlets on opposite phases. If you plug into 2 outlets on the same phase, the indicator light doesn't light up... Not rocket science. There is a post above from TEG (#4) with a link to the various models of the "Quick220" availble.

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