Page 1 of 3 123 LastLast
Results 1 to 10 of 24

Thread: Converting 120/208 to 120/240

  1. #1
    Join Date
    Nov 2010
    Location
    Dayton,Ohio
    Posts
    8

    Exclamation Converting 120/208 to 120/240

    Hello all,
    I have a 120/208 panel in a mechanical room. The panel services only 120 circuits. I now have a need in this area for a 240 circuit. Being that I already have a 25 KVA tranformer in stock , I was wanting to change the feed to the panel from 120/208 to 120/240 from this tranformer. I can't see any problem with this other than having to be clearly marked, but having never done this type of conversion with a panel before, I was wanting to know if anyone had experience doing this, or any safety concerns that might arise with me doing this. Thank you for your time, JSCIARLO

  2. #2
    Join Date
    Jul 2005
    Location
    Tennessee
    Posts
    12,776
    Can be done, but, depending on your load, it might be easier and less expensive to purchase a buck & boost transformer for that one load
    (buck 208 to 240)
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  3. #3
    Join Date
    Nov 2010
    Location
    Dayton,Ohio
    Posts
    8
    yes i agree, but some time before I started someone intalled a tranformer right above the MCC that now feeds the line side of the panel. So since it is there i figured i would try to use it. The panel now is a 100 Amp

  4. #4
    Join Date
    Jul 2005
    Location
    Tennessee
    Posts
    12,776
    Most 25 kva 240/120 transformers are single phase. Is the one you plan on using single phase ?
    You state your panel is 208/120. That often means a 3 phase panel. Is the panel 3 phase ?
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  5. #5
    Join Date
    Sep 2010
    Location
    Indianapolis
    Posts
    13

    208

    In my experience 208 is three phase wye connected secondary. Everything else relating to 220-240 would be single phase or three phase or open delta type configurations. I don't recall ever seeing a system that was 208 and not 3 phase.

    Then again, I have a lot more experience with controls than power distribution.

  6. #6
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    21,838
    J, welcome to the forum!

    The simple answer, presuming the transformer and loads are single-phase, is a 208-to-240v transformer, and the secondary center tap needs to be bonded as an SDS.

    If everything is three-phase, you won't have a neutral, so you'll end up with 240 only, unless one secondary has a center tap, and you'll end up with a high-leg secondary.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  7. #7
    Join Date
    Nov 2010
    Location
    Dayton,Ohio
    Posts
    8
    Yes the panel is 3 phase, and yes the transformer is single phase. The plan was to mark the B phase dead in the panel, and put covers on all the b phase breaker possitions. I know it doesn't sound likem the most ideal way. Just trying to use what i have!

  8. #8
    Join Date
    Jul 2005
    Location
    Tennessee
    Posts
    12,776
    You need to look at the branch circuits and see if any are multiwire (shared neutral). If they are, and were installed based on a 3 phase
    supply, you neutrals will be subject to overload if you change to a single phase arrangement.
    Otherwise, a bit unortahdox but permissible.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  9. #9
    Join Date
    Jun 2003
    Posts
    5,631
    So you just happen to have a 208 volt to 240/120 volt transformer there? Or is the transformer voltage something else???

    Steve

  10. #10
    Join Date
    Nov 2010
    Location
    Dayton,Ohio
    Posts
    8
    The tranformer that is there is a 460 primary to 120/240 secondary

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •