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single phase from 3-ph

Discussion in 'Electrical Engineering' started by Chief McGee, Jul 28, 2004.

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  1. Chief McGee

    Chief McGee Guest

    This was mentioned a few days ago but would someone please clarify it for
    I am installing 3-ph to the shop. It is 200A delta wound, 2
    transformers on pole, 3 hot wires 1 neutral from pole, ground rod driven
    into the dirt and hooked to the neutral buss bar. Drawing with the meter
    box shows the neutral wire coming from the middle of one side of the
    triangle that forms the delta. Is this what is called "center tapped"?
    I think I want to run 5 wires to each machine. 3 hots, 1 neutral, and
    one ground from the machine base all the way back to the neutral buss bar in
    the sub panel. Is this right?
    I also want to have a 120 outlet and 120 work light on each machine. AT
    THE MACHINE, I am going to pick one of the hot legs(NOT the power/stinger
    leg) and the neutral. Will connect the ground leg in the light to the base
    of the machine. Is this right or do I have to go all the way to the sub
    panel with the wires?
    Next, I have a 5-hp 220V single phase air compressor. For that I
    think I need only 4 wires. 2 hots(NOT the power leg) 1 neutral, and one
    ground from chassis all the way back to the neutral buss bar in the sub
    panel. Is this correct?
    I want this to be safe and legal. Thanks for your time, Chief.
  2. Jon Elson

    Jon Elson Guest

    That side of the triangle, with the center tap, is identical to standard
    single-phase service. The two hots are 2 of your 3 phase line
    terminals, as well.
    The additional transformer provices the 3rd phase line terminal.
    Yes, that sounds right, so you can have 120 at each machine location.
    The safety
    ground should actually have a separate ground bus bar in the panel, but some
    installations don't separate the neutral and the ground.
    Mostly. It seems OK to me to use the same ground conductor for the machine
    AND the light/socket. But, you can't use ANY line terminal for the 120!
    You must use one of the two that are on the center-tapped side. Going from
    Neutral to the 3rd line terminal will give you roughly 208 V, as this is not
    a center-grounded (balanced) Wye system.
    Maybe. If it is entirely 220, including the contactor/motor starter,
    you only
    need 3 wires. Any two hots (but, maybe best to run it off the extra
    to reduce load on the center-tapped one that is going to get a lot of
    single-phase loads.
    Just be sure to get it across one of the transformers. IE, if the tapped
    is supplying phases A and B, and the extra transformer is wired from B to C,
    don't connect the compressor from A to C. This L-L circuit will have more
    variation from 240 than the other 2 possible connections. If you need 120 V
    there for the contactor, then use A-B, and run the neutral as well.
    I believe the recent NEC has changed requirements, and grounding through the
    conduit alone is no longer permitted.

  3. Randy H.

    Randy H. Guest

    If you run your wire thru conduit, that is your ground leg. Your breaker box
    and all is grounded to the ground rod. You should only need to four wires in
    the conduit.

    Please note I am not an electrician. But I done a lot of my own wiring. (no
    fires yet!)

  4. Sounds like 120/208 to me...(though I'm 15 years rusty, I work on Traffic
    Signals these days)
    Nope. Run a 3 phase circuit for your machine, 3 hots and a ground. Fed
    off the proper size 3 phase breaker.

    Run a seperate circuit in a seperate conduit off say a 15 amp or 20 amp
    single pole circuit breaker. Yes, some machines have a built in
    transformer for a couple reasons, one to reduce the higher voltage to
    single phase voltage for control contactors, single phase coolant pumps
    and even lighting but you will find that there is a fuse usually on
    either the primary or secondary side, sometimes both to protect the lower
    current (amp) circuits. Put a light and related wiring on a 30amp 3 phase
    breaker, and something goes wrong, the wiring will likely fry before a
    breaker trips. That's why I suggest you run a dedicated circuit just for
    your machine lighting.

    See the above, Also, I believe Jon Elson to be correct. the NEC requires
    a seperate ground conductor. You can't rely on the conduit itself for a
    ground. Why? Because they have found that electricians sometimes forget
    to tighten the screws or lock nuts to keep the ground continuous, conduit
    pulls apart, guess what, no safety ground.
    Yup for reasons states above, need a seperate ground now, you should be
    running flexible metallic conduit to the compressor, you need the
    seperate ground in that for the compressor, you could run heavy SO cord
    too. As long as it was 4 conductor.

    You have doubts, you should run the conduit, and call an electrician to
    finish the job. Running the conduit is a good bit of work, let him pull
    and terminate the wire. My disclaimer, my advice may or may not be
    accurate, consult a licensed electrician for help....gawd I hate that....

  5. Don sez: "Not necessarily. He could have the Scott 4 wire."

    Yep! Seems like this could be true -- Scott 2 single-phase circuits derived
    from 3-phase. How about this Peter H. What say you?

    Bob Swinney |
  6. Randy H.

    Randy H. Guest

    Thanks guys for the info. I can see why they made the changes in the code as
    a good idea. I will take my comment as not having any fires as being lucky
    on my part.

    My building was built in 1954. I'm sure I will have a lot to change

    Thanks again,

    Randy Hansen
    SC Glass Tech
    Scam Diego, Comi-fornia
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