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Thread: Using 240 volt single phase to run a 208 volt single phase appliance.

  1. #1
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    Using 240 volt single phase to run a 208 volt single phase appliance.

    I have a 208 single phase toaster appliance that I need to hook up, I was told by the sales person who sold it to me that I need to buy a transformer because the power in my restuarant is a 3 phase 240 system. My question is can't I just run a 240 volt circuit to the receptacle the appliance will only draw 208 volts, less volt more watts?? right. Just looking for some input before I have to buy a $300 transformer.

    Store power is 3 phase 240.
    appliance needs 208v single phase, 19.5 amps.

    Thanks

  2. #2
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    I would first make sure you have 240v 3Ø power. It is not that common. It is usually 208v.

    If you indeed have 240v, and there is no 240v rating on the appliance, I'd say you need the transformers(s).

  3. #3
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    Most heating appliances are tagged with both 208 and 240 volt and the watts the appliance will use for either.

    HOWEVER, if the appliance is only designed and labeled for use on 208 and you supply it 240 it will very likely overheat and either break down, perform badly or even cause a fire.

    If it was only designed for 208 and you push it to 240 that will be a significant increase in heat output of the unit.

  4. #4
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    There is a basic misconception that I think you need to have clarified. An appliance does not simply “draw,” from the available volts, only the amount of volts that it needs. Rather, the available volts are going to impose themselves on whatever is connected to them, regardless of whether the load is capable of handling it. “Voltage” can be thought of as an amount of “push.” The higher the voltage, the stronger the push. What is being pushed is electrons, and the resulting current is essentially a measure of the flow of the electrons.

    A common analogy used to explain the relationship between voltage and current is water flowing through a hose. Suppose you took a garden hose, one that is designed to handle water being “pushed” by the household water pressure. Suppose that instead of connecting it to an outdoor faucet, you bring in a high pressure pump, and connect the hose to that pump. What happens is that the higher pressure will be too much for the hose to handle, and the hose will probably rupture.

    If you use a 240 volt source to “push electrons” through a component that is designed to handle no more than 208 volts, what will happen is similar to the hose rupturing. As Bob said, the appliance will overheat. If you are lucky, it will simply stop working. If you are unlucky, it will set the place on fire.

    Therefore, it is important to know for certain whether the appliance has been rated, by its manufacturer, for both voltage levels. Also, it is important to be certain of the voltage level that your building actually uses. As sparky mentioned, 240/3 phase is rare.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  5. #5
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    Quote Originally Posted by pearsv1
    appliance needs 208v single phase, 19.5 amps.

    Thanks
    That 19.5 Amps. is probably why it's only rated for 208 and not 208/240.
    This appliance probably has a 20 Amp. plug that would work in most locations ( 20 Amp. receptacle ). If they rated it for 240 then it would need to be hard wired or come with a 30 Amp. plug.

    Edit: It's probably cheaper to by a different toaster.
    Last edited by growler; 11-05-07 at 04:35 PM.
    "A sight for sore eyes to the blind would be awful magestic"---Wax Fang

  6. #6
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    Actually, if it draws 19.5 amps at 208 volts, it would draw 22 1/2 at 240 volts.

  7. #7
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    If I may add another explanation, we have to separate the constants from the variables. The supply voltage and the appliance impedance are the constants, and the current and the resultant power are the variables.

    Using 208v and 19.5a, we calculate that the impedance is (R = E/I = 208/19.5) 10.67 ohms, and the resultant power is (P = E*I = 208*19.5) 4056 watts.

    With 240v applied to the same impedance, the current would be (I = E/R = 240/10.67) 22.49 amps, and the power would be (240*22.49) 5397.6 watts.

    The moral of the story: when you raise the voltage across a given impedance, the current also increases, and since P=E*I, the resultant power increases.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  8. #8
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    I just thought of something. He has 240 V three phase which means he has a high leg ( stinger ). He needs 208 on a 20 Amp. circuit. Any rule against running this appliance off the high leg.

    If it were lighting you would do it. A toaster is just a heating coil. There is probably not a lot of load on the high leg.


    Unless anyone can think of a reason not to I see a 20 Amp. Single pole breaker rated for 250 V to supply a toaster at 208 volts 20 amps.

    ( the breaker can't cost that much ).

    Edit: Skip that stupid idea, it appears that such a breaker doesn't exist. That always slows me down.
    Last edited by growler; 11-05-07 at 05:35 PM.
    "A sight for sore eyes to the blind would be awful magestic"---Wax Fang

  9. #9
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    Quote Originally Posted by pearsv1
    I have a 208 single phase toaster appliance that I need to hook up, I was told by the sales person who sold it to me that I need to buy a transformer because the power in my restuarant is a 3 phase 240 system. My question is can't I just run a 240 volt circuit to the receptacle the appliance will only draw 208 volts, less volt more watts?? right. Just looking for some input before I have to buy a $300 transformer.

    Store power is 3 phase 240.
    appliance needs 208v single phase, 19.5 amps.

    Thanks
    Your profile says elect. contractor, your post implies that you own a restaurant. Are you both?:confused:

  10. #10
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    using the wild leg immediately came to mind when 208 is needed
    ambidextrious wirenutter
    All quoted sections are from NEC 2008 unless otherwise stated

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