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Thread: 120/208 Single Phase

  1. #1
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    120/208 Single Phase

    Can someone please explain how 120/208 single phase is derived from 208Y/120 three phase. I don't quite understand, mathematically, how it works and I have a client requesting this type of set-up; using a Meter-Pack Meter Center. Thanks for your help.

  2. #2
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    Re: 120/208 Single Phase

    I’m not sure I understand your question, since the “slash notation” (i.e., 120/208) necessarily refers to a three-phase system. Also, I am aware of no explicit rules that require a different meaning if the low number appears first (120/208) as opposed to second (208/120).

    On a three-phase panelboard, connecting a circuit to one single-pole breaker and to the grounded bus gives you 120 volts. Connecting a circuit to one double- pole breaker without a wire attached to the grounded bus gives you 208 volts. Is this what you are referring to? I can’t be sure, because it didn’t require math.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    Re: 120/208 Single Phase

    Charlie, What I am trying to ask is this, I have a three-phase service going into a terminal box and using single-phase meter stack in conjunction with the terminal box. Since I am now going from three phase to single phase, I am questioning the terminology of using 120/208v and not 120/240v on the single-phase side if the incoming voltage is three phase 120/208v? I understand how I am getting the single phase it's all about the nomenclature. I believe that once we do not use one of the legs from the terminal box and create single phase that it should be 120/240v, since there is 120v on each of the two legs and mathematically it is impossible (without some kind of load or draw) to obtain 208v on that circuit. Am I wrong?

  4. #4
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    Re: 120/208 Single Phase

    T-pataz, the voltages on a three phase 208Y/120 system are 120 between any phase and a grounded conductor, and 208 between any two phases. There is no 240 invloved.

    The mathmatical relationship is the square root of three (1.73 x 120 = 208). I will note you should be using all three legs of the service and not just two. To do so will will unbalance the system and could create a fire danger if enough load is developed.

  5. #5
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    Re: 120/208 Single Phase

    What is happening is you are providing two legs of a three phase source to a meter that has a neutral potential lug (normally at the 9:00 position) so the customer (normally residential in an apartment building) will have a normal single phase service. The problem is that the two legs are separated by 120 degrees instead of 180 degrees and for water heating, electric ranges, ovens, and electric dryers, he gets 208 volts instead of 240 volts. The meter center gets all three phases and each meter gets different pairs of phases. If a meter center has 12 meters, 4 would have A and B, 4 B and C, and 4 C and A. I hope I haven't muddied the water any.
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  6. #6
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    Re: 120/208 Single Phase

    Exactly. But, according to manufacturers like Square D, I can have a multi meter set up using two of the three legs and a the neutral for each of the meters (five jaw), and then have a legitimate single phase service after that meter.

    You are correct the load will not be balanced on the primary side but apparently Square D is not concerned with that, since they sell equipment that support this scenario.

    So after the meter I will have 120/240v, no?

    Since at that point each leg will be 120v and there will only be two plus the neutral.

    I know this may all seem a bit repetitive but I have to answer to others and I need a clear understanding so I can explain what is going on to them. Or direct them in another direction.

    Thanks.

  7. #7
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    Re: 120/208 Single Phase

    You will still have a 208-volt, single-phase service for each meter despite the fact you are only using two of the three phases. No matter how you use them the voltage between any two phases is still 208.

  8. #8
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    Re: 120/208 Single Phase

    Originally posted by t_pataz:So after the meter I will have 120/240v, no?
    As Dereck and “the other Charlie” have mentioned, what you will have is 120 volts between either phase wire and the grounded wire, and you will have 208 volts between the two phase wires. What is this called? I regret to report that it is frequently (and inappropriately) called “120/240.” We don’t have a phrase in our technical language for “120/208 single phase.” Perhaps we should. At least the customer won’t be misled into believing that 240V is available.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  9. #9
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    Re: 120/208 Single Phase

    This may help. This diagram is actually missing a little bit but will help show you the system.

    The second green line should be between (A) and (C), and there should be a green line between (A) and (B), both showing 208v.

    Now,
    I know this may all seem a bit repetitive but I have to answer to others and I need a clear understanding so I can explain what is going on to them. Or direct them in another direction
    This is obviously over your head and can not really be taught in a single thread.

    I'm not sure what your profile means as far as "Team Leader", and this is not meant to be demeaning, but you should get a professional to assist you.

    Roger

    [ April 09, 2003, 01:39 PM: Message edited by: roger deas ]

  10. #10
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    Re: 120/208 Single Phase

    208 sucks...Only 120 volt outlets should be operated from a 120/208 supply

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