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Thread: 120/208 vs 120/240

  1. #1
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    120/208 vs 120/240

    I have to install a sub panel 120/240 single phase on to a 120/208 three phase system as per instructions of my boss. What I would like to know is there a problem with running 240 equipment on a 208 system and why do we have these two nominal voltage systems.

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  2. #2
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    Quote Originally Posted by kda3310 View Post
    I have to install a sub panel 120/240 single phase on to a 120/208 three phase system as per instructions of my boss. What I would like to know is there a problem with running 240 equipment on a 208 system
    It will be a problem if any of the equipment is not listed for use on 208 volt systems.

    and why do we have these two nominal voltage systems.
    120/240 is single phase and 120/208 is 3 phase. They co-exist because single phase is most practical for residential and most small/medium commercial while 3-phase is better for anything beyond that.

  3. #3
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    Quote Originally Posted by kda3310 View Post
    I have to install a sub panel 120/240 single phase on to a 120/208 three phase system as per instructions of my boss. What I would like to know is there a problem with running 240 equipment on a 208 system and why do we have these two nominal voltage systems.

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    Wait, do you have both voltages there now?
    If not and it's only a 208/120, you can't get 240V out of that.
    If you aim at nothing, you will hit it every time!

  4. #4
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    Quote Originally Posted by Little Bill View Post
    Wait, do you have both voltages there now?
    If not and it's only a 208/120, you can't get 240V out of that.
    But you can use a panel rated for 120/240 for a two out of three phase application. Just be careful with MWBCs.

  5. #5
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    Quote Originally Posted by GoldDigger View Post
    But you can use a panel rated for 120/240 for a two out of three phase application. Just be careful with MWBCs.
    Yes, but my reply was in thinking the OP was needing to have actual 240/120 voltages.
    I might have read him wrong as he could be asking about the panel rating only.
    If you aim at nothing, you will hit it every time!

  6. #6
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    Quote Originally Posted by Little Bill View Post
    Yes, but my reply was in thinking the OP was needing to have actual 240/120 voltages.
    I might have read him wrong as he could be asking about the panel rating only.
    I read it like that, too.

    You have to be careful that the 240v equipment is also rated for 208v. There may be taps to change internally especially if it has low voltage controls, like HVAC equipment.

    There's a condo complex that uses us for the maintenance and we get regular calls from condo owners that just replaced their range. They buy a standard 240v residential range and then are confused why it takes an hour for it to get up to temp. I usually explain the issue to them and tell them to call the appliance people back out. I've heard there are taps you can change in the ranges but I've never dug into myself.

  7. #7
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    Quote Originally Posted by MNSparky View Post
    I read it like that, too.

    You have to be careful that the 240v equipment is also rated for 208v. There may be taps to change internally especially if it has low voltage controls, like HVAC equipment.

    There's a condo complex that uses us for the maintenance and we get regular calls from condo owners that just replaced their range. They buy a standard 240v residential range and then are confused why it takes an hour for it to get up to temp. I usually explain the issue to them and tell them to call the appliance people back out. I've heard there are taps you can change in the ranges but I've never dug into myself.
    Some may have taps, but the ones I am familiar with require changing elements.
    For water heaters you can get either single voltage elements or dual voltage 208/240 with a tap.

  8. #8
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    Quote Originally Posted by Little Bill View Post
    Yes, but my reply was in thinking the OP was needing to have actual 240/120 voltages.
    I might have read him wrong as he could be asking about the panel rating only.
    This is our plant administration offices. It is all 3 phase 208. They want to hook up a mobile hearing test booth to it that has been running on a 1 phase 120/240 generator. I don't know what the both requires. I'm only doing what my boss is telling to do. I'm hoping there is not going to be any problems.

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  9. #9
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    Quote Originally Posted by kda3310 View Post
    This is our plant administration offices. It is all 3 phase 208. They want to hook up a mobile hearing test booth to it that has been running on a 1 phase 120/240 generator. I don't know what the both requires. I'm only doing what my boss is telling to do. I'm hoping there is not going to be any problems.

    Sent from my XT1565 using Tapatalk
    What's a mobile hearing test booth? Is that like a trailer with hearing test stuff and things like AC and interior lights? I would guess the test equipment and lights runs on 120v meaning that it wouldn't matter at all. The wall pack AC is probably 208-230

  10. #10
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    Quote Originally Posted by kda3310 View Post
    I have to install a sub panel 120/240 single phase on to a 120/208 three phase system as per instructions of my boss. What I would like to know is there a problem with running 240 equipment on a 208 system and why do we have these two nominal voltage systems.

    Sent from my XT1565 using Tapatalk
    Some 240V rated equipment will not be happy on 208V. A hearing test trailer should have 120V equipment for the testing, so whether it's fed from a 120/208 or 120/240V source *shouldnt* matter.

    As to your second question, residential service is often single phase (split phase or center tap neutral), with 120V to neutral but 240V line to line. Commercial transformers are often 3 phase and due to phase angle, 120V to neutral gives 208V line to line. There are open delta transformer banks that have 240V line to line, but the B phase is 208V to neutral. Why have 120, 208, 240, 277, 480V here? Different demands of equipment is one answer. A 900HP aeration blower would be wired to 4160V, you could wire it 480V or even 120V if you could make conductors that would carry 4,000+A. Cheaper to make better insulation and smaller wire.
    Electricians do it until it Hertz!

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