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Thread: Transformer rating question.

  1. #1
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    Default Transformer rating question.

    I just got a transformer. It is made by a company that is no longer in business and I can't find additional data beyond the tag. It is a 120/240 single input and, 1 kva rating with 12/24 volt ac output. Is the kva rating for the low side or the high typically?
    Thanks

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  2. #2
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    kVA is typically for the output, the secondary, which would presumably be the 12/24V as you say. Lots of current at 12V.....
    1601

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    Hashim Khan

  3. #3
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    The KVA rating is for the transformer, and applies to both sides. Transformers don't actually have an input/output: they don't care which way the killiwiggels come in or go out.


    Cat

  4. #4
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    If you're going to get 1000 watts worth of current (83 amps) at 12 volts from your 120 volt input there should be some pretty serious copper in the secondary winding. If the secondary's leads/terminals/whatevers don't look up to carrying 83 amps continuously, be a bit suspicious.

  5. #5
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    Quote Originally Posted by Catshooter View Post
    The KVA rating is for the transformer, and applies to both sides. Transformers don't actually have an input/output: they don't care which way the killiwiggels come in or go out.
    Cat
    Theoretically true.

    Real world is not always the same.

    Some transformers, typically small ones at and under 1 kVA, may be wound to produce rated output, despite larger losses. This is done by altering the turns ratio to raise the output a bit to compensate for losses. When reversed, they produce considerably lower than expected output. I have seen these, used them, found them annoying.

    Then also, larger transformers may be wound with better line to ground insulation on the designated primary, because it will be connected to the less controlled incoming line, while the secondary, if wye, will be actually grounded, and is not subject to large transient voltages from lightning, etc. New NEC editions prohibit reversing unless the unit is listed for that use.
    1601

    Keep eye on ball.
    Hashim Khan

  6. #6
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    Sorry for the confusion but it is rated at .1kva. My phone keyboard "auto corrected" incorrectly. Pretty puny.

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  7. #7
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    8 Amps at 12 V or 4 Amps at 24 V is not all that puny, depending on what you want to use it for, anyway. I have designed many, many devices around a 12 V or 5 V transformer that was only rated for 1 Amp.

    You aren't going to weld with it, but it can serve for many, many uses.
    Paul A.

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  8. #8
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    Where do you get that current rating if the tag states .1kva? Just playing the devil's advocate iykwim.
    Thanks

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  9. #9
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    Quote Originally Posted by challenger View Post
    Where do you get that current rating if the tag states .1kva? Just playing the devil's advocate iykwim.
    Thanks

    Sent from my Pixel 2 using Tapatalk
    The "power" is known, and the voltage is known, so the current is then also known. So, if you divide the 100VA by the 12 V, you get 8.33 A. Half that at double voltage, obviously.
    1601

    Keep eye on ball.
    Hashim Khan

  10. #10
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    Just my personal viewpoint and coming from a guy who has worked in and around the electrical trade since 1962. Until my retirement in 2007 I held a master electrician license and was current on the NEC.

    I think the NEC has gone far beyond the original intent and is now gasping at straws to justify the committees existence. Frankly I would think anything wired to Code say 20 - 30 years ago with the GFI requirements at that time is about as safe as it gets.

    The issue is not the Code requirements but Joe Homeowner who has a friend or knows someone who is a electrician that told him that was the way to do it. Same thing is going to happen with that flex gas line that is now being sold to homeowners at the big box home improvement stores.

    Let the flaming begin.
    Retired - Journeyman Refrigeration Pipefitter - Master Electrician - MakerGear M2 3D printer- 20 Watt Ray Fine Galvo fiber laser, LightObject 40 watt co2 Laser Engraver

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