120/208 confusion NOT FOR AN OVEN

Discussion in 'General Electronics Chat' started by Ngiltner, Sep 25, 2015.

  1. Ngiltner

    Thread Starter New Member

    Sep 25, 2015
    5
    0
    I work with a small custom panel shop. We mainly utilize 120 single phase, 480 three phase, or 600 three phase for primary power. We recently had a request for 208 single phase and 208 three phase. I am not electrical enough to be 100% on utilizing this. Particularly the single phase vs three phase. Some one gave me this simple diagram for 120 three phase going to (3) transformers (attached). There are three lines of 120 and a neutral. One line of 120 goes to each 120v tap on a transformer and the neutrals connect back to a main neutral out. That is pretty straight forward. However, I am confused by 208 single phase. The transformer has a 600v tap, 480v tap, 277v tap, 230v tap, and 120v tap. So..

    1. Is this Diagram correct for utilizing 208 three phase?
    1. Can I use this transformer for 208 single phase?
    2. How many wires are 208 single phase?
    3. How would these wires land on the transformer?

    Thanks for the help, I'm sure a very basic question.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,500
    2,364
    Normally to obtain 120v 1ph and star neutral a 208 secondary is used, often for balanced industrial lighting loads etc.
    upload_2015-9-25_12-29-50.jpeg
    It is 208 3ph and each conductor to neutral is one pair, L & N.
    208/√3.
    Not sure if this answers your question?
    Max.
     
    Last edited: Sep 25, 2015
  3. Ngiltner

    Thread Starter New Member

    Sep 25, 2015
    5
    0
    If the primary power is 208 single phase and I need to bring it to a transformer with a 120v tap is that possible or would the transformer require an actual 208 tap? 120v single phase would obviously be one line to the 120v tap and one line to the neutral + a safety ground...can 208 single phase be utilized on a transformer with 120v tap?

    thanks
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,500
    2,364
    Not completely sure of your question?
    What exactly do you have now, 208 3ph and no neutral or?
    If you have the configuration in your OP then you just need 120v primary 1ph transformer(s).
    You mention a transformer with a 120v tap, is this on the secondary of each phase of the transformer?
    Or if you have a 208v 1ph load then you would simply go across any two of the 208 secondary phases.
    Obviously if wanting to duplicate your DWG then you need a transformer with 208 three phase and neutral secondary or supply.
    Max.
     
  5. Ngiltner

    Thread Starter New Member

    Sep 25, 2015
    5
    0
    I have a transformer with a 120vac tap and neutral tap on the primary side. This would transform the primary side to 48vac and 24vac on the secondary side.
    I have some one wanting to use this transfer with 208 single phase. I obviously dont even know enough to know how to ask my question clearly, haha.

    Can 208 single phase be brought to a transformer with a 120vac tap and neutral tap or not at all?
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,766
    928
    Some basic configurations
     
  7. BR-549

    Well-Known Member

    Sep 22, 2013
    1,980
    388
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,500
    2,364
    What do you mean by 'Neutral' tap?
    Only IF the supply transformer is 208 as per your OP and it has a star neutral, which in that case you use one phase and neutral, otherwise I don't see how you are going to utilize a 120v transformer on a 208v supply!
    Max.
     
  9. Ngiltner

    Thread Starter New Member

    Sep 25, 2015
    5
    0
    So I think I have just been chasing my tail. If I have one transformer being powered by 120/208, I am literally just not utilizing 2 of the lines of 120, correct? I would land 1 line of 120 to the transformer and land the neutral.

    drawing attached to verify.
     
  10. JWHassler

    Member

    Sep 25, 2013
    201
    33
    Your actual question is still kinda vague, especially in view of the shock hazard.
    If you have a normal 208-volt service (two or three hot-wires, each reading 120VAC to a neutral) then your 20-48 transformer would be connected across a phase and the neutral.
    I say 'two or three hot wires' because 120/208 is sometimes used in residential service.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,500
    2,364
    Yes, you would only be using 1 of the phases to neutral, I would be surprised if you have a 208 transformer supply without the star neutral available.
    Max.
     
  12. Ngiltner

    Thread Starter New Member

    Sep 25, 2015
    5
    0
    Thanks for your patience and support.
     
  13. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,135
    200
    There is a typical high leg delta, delta and "Y" form of transmission to buildings. The presence of 208 and 277 is indicative of Y.
    See: https://en.wikipedia.org/wiki/High-leg_delta

    With ANY 3 phase system, you need to know if a Neutral is provided or not to the equipment. We had a bunch of bozo's spec power to the equipment and left out the neutrals to lot of 60 A 4-wire pieces of equipment.

    208/sqrt(3) is 120V; It's the line to neutral voltage in a 208 V "Y" system.

    480 is 277 * sqrt(3). I just flipped stuff. So, if the building had 480 V "Y", 277 single phase is available to neutral. That 277 is then used to power lighting loads.

    Same deal, 480 is available line to line.

    The real thing to watch out for is equipment needing 120 V and powered by 240 V high leg delta.

    Three phase 120 V as far as I know shouldn't really exist. FWIW: 2 phase and not split phase power was available in the US a long time ago.

    Households in the US are powered by "single phase". I prefer to call them "split-phase". The high voltage side is single phase, but a 240 V center tapped transformer is used for the secondary. The center tap is connected to ground/neutral at ONE point for the distribution.

    Hospitals and radio transmitter MAY have two different ground wires which are kept separate. Orange receptacles denote "isolated ground". I won;t go into that right now.

    Typically you won;t have both power systems in a building, BUT we did have a German system to support some German furnaces.

    The building had multiple internal feeds of 208 3 phase @ 200 Amps, and 277 V . 480 3-phase was pretty much in the electrical and mechanical rooms for 36 heat pumps.

    It was not uncommon to have a 5-wire (L1, L2, L3, N, G) 208 3 phase phase 60 A for one piece of equipment or have 4-wire (l1, L2, N, G) 208 single phase 60 A. One used 208 90A. And one used the full 208 at 200 A into it's equipment cabinet. Most 3 phase motor stuff GENERALLY does ot include the neutral. 120 V is stepped down for the controls from the higher voltage.

    The machine shop had a 480V 3 phase feed (smart move) with a breaker box for 208 3 phase above it. In my high school, there was a mushroom kill switch that killed everything in the shop.

    Power was ridiculously high for the building, It may have come in at 4,800 V 3 phase from a 12,600 3 phase feed.
     
    Last edited: Sep 28, 2015
Loading...