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Thread: No primary protection required?

  1. #21
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    Quote Originally Posted by ActionDave View Post
    It's no problem to reverse feed a transformer. 450.11(B) makes this clear. Besides, it's only been recently that the code had anything to say about it at all and I don't think the manufacturers did anything different to their transformers other than add a note to the instructions that says, "It's ok if you want to backfeed this".
    Manufacturers vary on this topic. GE says their transformers are capable of stepping up as long as the OCPD is set high enough to handle the inrush current and meet the NEC. Hammond on the other hand states that it would require a special order transformer - same for Eaton Cutler-Hammer.

    http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White%20Papers%7CTransformer2%7Cg eneric




  2. #22
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    Quote Originally Posted by Isaiah View Post
    Manufacturers vary on this topic. GE says their transformers are capable of stepping up as long as the OCPD is set high enough to handle the inrush current and meet the NEC. Hammond on the other hand states that it would require a special order transformer - same for Eaton Cutler-Hammer.

    http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White%20Papers%7CTransformer2%7Cg eneric



    That is why I did not want to get into the design. It can get tricky real quick.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Quote Originally Posted by jumper View Post
    That is why I did not want to get into the design. It can get tricky real quick.
    What are your thoughts regarding the grounding aspect for secondary side at 480V?

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    Quote Originally Posted by Isaiah View Post
    Manufacturers vary on this topic. GE says their transformers are capable of stepping up as long as the OCPD is set high enough to handle the inrush current and meet the NEC. Hammond on the other hand states that it would require a special order transformer - same for Eaton Cutler-Hammer.

    http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White%20Papers%7CTransformer2%7Cg eneric



    It's a hunk of iron with wingdings around it. If it gets to the level of design needed to meet NASA specs I'll worry a little bit more about it. We back fed them for years and years without any added instructions or problems.
    If you don't think too good, don't think too much.

  5. #25
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    Quote Originally Posted by Isaiah View Post
    What are your thoughts regarding the grounding aspect for secondary side at 480V?
    I think an option would be to use a transformer with a center-tapped 480v winding for the first transformer, and ground the center tap, creating two 240v-to-ground conductors. This would, among other things, reduce the electrical stress on the insulation.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Quote Originally Posted by LarryFine View Post
    I think an option would be to use a transformer with a center-tapped 480v winding for the first transformer, and ground the center tap, creating two 240v-to-ground conductors. This would, among other things, reduce the electrical stress on the insulation.
    Yes - and would also provide a reference point to ground. The client's specification calls for a grounded system vs ungrounded w/detectors.
    I'd have to pull a white, grounded conductor (not a neutral) with the two phase conductors, through the second transformer primary OCPD and ground the 2nd tranny center tap on the primary. Of course the secondary side would also be grounded, 120V (ph and neutral) to the Control Panel CB.

    Basically both transformers (120-480V and 480-120V) would be grounded on both the primary and secondary windings. Is this the way you see it?

  7. #27
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    Quote Originally Posted by Isaiah View Post
    Yes - and would also provide a reference point to ground. The client's specification calls for a grounded system vs ungrounded w/detectors.
    I'd have to pull a white, grounded conductor (not a neutral) with the two phase conductors, through the second transformer primary OCPD and ground the 2nd tranny center tap on the primary. Of course the secondary side would also be grounded, 120V (ph and neutral) to the Control Panel CB.

    Basically both transformers (120-480V and 480-120V) would be grounded on both the primary and secondary windings. Is this the way you see it?
    You are mixing up terms.

    Grounded = physically connected to the earth

    Neutral = the conductor we usually ground

    Equipment grounding conductor = a conductor used to clear a fault

    You would pull two hots and an equipment ground ,ie, green, from the first transformer to the second, you derive a "neutral" at the secondary of the second transformer and ground it and bond it to the transformer case or first disconnect and also connect the EGC from the first transformer.
    If you don't think too good, don't think too much.

  8. #28
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    Quote Originally Posted by jumper View Post
    For this install I believe that 240.4(F) allows the primary protection for the first tranny to protect the primary and secondary of the tranny and secondary conductors also.
    Quote Originally Posted by Isaiah View Post
    I'm more concerned about the primary windings of the second tranny.
    240.4 is about the conductors, not the transformer protection requirements in 450.3. 450.3 doesnt have an allowance to protect a transformer through another transformer - but does it need to? It seems we could just use common sense, electrical theory, and the allowance in 240.4(G) and 408.36(B) exception as supporting evidence

    OP said the wire was already run, but the best way to do this is use the loophole in 250.30(A)(1) exception to save a conductor.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  9. #29
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    I like your idea of using 250.30(A)(1) but I still have to agree with Jumper - OCPD needs to be there for primary windings of second transformer in accordance with 450.3(B). This same overcurrent device would also serve to protect the secondary conductors of the first transformer secondary in accordance with 240.21(C)(4).


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  10. #30
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    Electro felón. Yes the conductors have already been run between transformers, they pulled a 3/C with ground. Going by your post the white (Grounded) conductor could be eliminated and simply use the EGC to bond the center tap of each 480V winding. In essence two phase conductors with an EGC to provide fault path - and coil/tape the ‘extra’ conductor - correct?


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