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Thread: Changing three phase voltage

  1. #1
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    Changing three phase voltage

    A project I have asked about is 120/240 3phase. I am told by POCO that I could switch to 120/208 3phase.
    A existing piece of equipment I called on I would need to switch heating elements on and a controller so I am thinking against changing voltages.
    What other things should I consider when thinking of changing voltages. And what other pluses and minuses are there.

  2. #2
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    A 120 / 240 3 phase service is a high leg Delta, a 120 / 208 3-phase service is a Wye.

    The latter is generally more versatile, the only real thing you need to look at is equipment that is designed for 230 or 240 volts only... Motors that are not 208- 230 / 460 will not be happy running on 208 volts.

    You need to look at what percentage of loads are single phase and what percent are three phase, cost of the existing service versus a new one, amperage, and so on.

    Heating elements that are designed for 240 volts will put out roughly 75% of their rating at 208 volts.
    Electricians do it until it Hertz!

  3. #3
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    The first step, I would think, is confirm with POCO as to what's available.. some power companies won't even supply 240/120 3 phase any longer,
    Next, list all your equipment and see what requires 240. If 208 is you voltage of choice, It is a fairly simple task to use a comparatively less expensive buck-n-boost transformer on items that are straight 240. If a neutral (120) is involved that option is a bit more complicated or not possible.
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  4. #4
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    Quote Originally Posted by arnettda View Post
    A project I have asked about is 120/240 3phase. I am told by POCO that I could switch to 120/208 3phase.
    A existing piece of equipment I called on I would need to switch heating elements on and a controller so I am thinking against changing voltages.
    What other things should I consider when thinking of changing voltages. And what other pluses and minuses are there.
    Pluses:
    1) Get to use the 'B' Phase for single phase loads when changing to 3 Phase.
    2) More simple to conduct a short circuit current study (opinion of course)
    3) You don't have to worry about having an atypical voltage that, while small, still represents a potential hazard for connection for maintenance/electricians, etc.


    Minuses:
    1)Resistive loads will increase current to meet same output/or changing of equipment not dual rated 208/240V.
    2)You don't get to say you have a Wild Leg anymore.
    3) Confirm the POCO has 3 Phases readily present and that they won't charge you to extend 3 phase to this location, if not present. A few delta services that I've come across the utility noted the same thing, but would require payment to extend an additional phase at the owner's expense.

  5. #5
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    Quote Originally Posted by augie47 View Post
    The first step, I would think, is confirm with POCO as to what's available.. some power companies won't even supply 240/120 3 phase any longer,
    Next, list all your equipment and see what requires 240. If 208 is you voltage of choice, It is a fairly simple task to use a comparatively less expensive buck-n-boost transformer on items that are straight 240. If a neutral (120) is involved that option is a bit more complicated or not possible.
    He said it is high leg system already and that POCO suggested possibly changing to 208/120.

    Quote Originally Posted by tw1156 View Post
    Pluses:
    1) Get to use the 'B' Phase for single phase loads when changing to 3 Phase.
    2) More simple to conduct a short circuit current study (opinion of course)
    3) You don't have to worry about having an atypical voltage that, while small, still represents a potential hazard for connection for maintenance/electricians, etc.


    Minuses:
    1)Resistive loads will increase current to meet same output/or changing of equipment not dual rated 208/240V.
    2)You don't get to say you have a Wild Leg anymore.
    3) Confirm the POCO has 3 Phases readily present and that they won't charge you to extend 3 phase to this location, if not present. A few delta services that I've come across the utility noted the same thing, but would require payment to extend an additional phase at the owner's expense.
    Resistance of such loads is usually fixed, so to get same kW output you physically need to replace heating elements. If you leave existing ones they won't be overloaded or overheated, they just put out about 75% of what they did on 240 volts. If marginally sized for the application to begin with then they may not be enough if run on 208.
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  6. #6
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    Quote Originally Posted by kwired View Post
    He said it is high leg system already and that POCO suggested possibly changing to 208/120.

    Resistance of such loads is usually fixed, so to get same kW output you physically need to replace heating elements. If you leave existing ones they won't be overloaded or overheated, they just put out about 75% of what they did on 240 volts. If marginally sized for the application to begin with then they may not be enough if run on 208.
    Yes, or if they are doing something like heating up water, it just takes a little longer if you leave the original heaters in place and give them the lower voltage.

    Quote Originally Posted by arnettda View Post
    A project I have asked about is 120/240 3phase. I am told by POCO that I could switch to 120/208 3phase.
    A existing piece of equipment I called on I would need to switch heating elements on and a controller so I am thinking against changing voltages.
    What other things should I consider when thinking of changing voltages. And what other pluses and minuses are there.
    As I said above, you may not need to change the heating elements if you can just live with it taking longer to get to temperature. I can't see having to change the "controller" though. that's probably coming from someone who wants to sell you more stuff...
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  7. #7
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    Quote Originally Posted by tw1156 View Post
    Minuses:
    1)Resistive loads will increase current to meet same output/or changing of equipment not dual rated 208/240V.
    2)You don't get to say you have a Wild Leg anymore.
    3) Confirm the POCO has 3 Phases readily present and that they won't charge you to extend 3 phase to this location, if not present. A few delta services that I've come across the utility noted the same thing, but would require payment to extend an additional phase at the owner's expense.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

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