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Thread: sizing transformer

  1. #11
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    Quote Originally Posted by Besoeker2 View Post
    Thank you for your most informative comment.
    It was off the OPs topic so ...start a new thread if you need more info

  2. #12
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    Quote Originally Posted by PVfarmer View Post
    You haven't said what voltage you are transforming from/to.
    240 secondary but no mention of primary voltage.

  3. #13
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    Quote Originally Posted by iwire View Post
    It was off the OPs topic so ...start a new thread if you need more info
    No need to. You answered my question.

  4. #14
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    Quote Originally Posted by ActionDave View Post
    You need to do a load calc for the trailers, al la Art. 220 or look for the tag that manufactured trailers have. If you do a load calc annex D is quite helpful. Just for comparison my whole house clacs out to 100A and pulls around 70A with every possible electric thing turned on.
    There are two trailers and so application of demand factor is required for calculating optimum size of transformer. So OP better visit other similar installations to find out optimum size transformer at work unless the needed demand factor is made available to him.

  5. #15
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    Quote Originally Posted by Sahib View Post
    There are two trailers and so application of demand factor is required for calculating optimum size of transformer. So OP better visit other similar installations to find out optimum size transformer at work unless the needed demand factor is made available to him.
    The OP needs to follow the NEC, not the advice of those untrained NEC.

  6. #16
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    Sorry I do not know how Iam violating NEC......

  7. #17
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    Quote Originally Posted by brock1221 View Post
    I have a situation where I am trying to size the transformer based upon the load.
    I was thinking 100 X 3 ==300A X 240V= 72kVA which equates to 80kVa transformer. Am I correct or wrong?
    iwire and the others are 100% correct here below. Don't just use 300A.

    Quote Originally Posted by iwire View Post
    But that is not what you seem to be doing. You seem to be sizing it by the breaker sizes. That will work but likely result in much larger transformer than really needed.
    Quote Originally Posted by kwired View Post
    I don't know what my whole house calculation is, I do have electric water heater, 3 ton heat pump, 15 kW back up heat, electric range/dryer, well, plus shop (probably seldom exceeds 3-4000 VA, and when it does is for brief periods of time) - All that only supplied by 25 kVA transformer by the utility.
    Quote Originally Posted by ActionDave View Post
    You need to do a load calc for the trailers, al la Art. 220 or look for the tag that manufactured trailers have. If you do a load calc annex D is quite helpful.
    Just for comparison my whole house clacs out to 100A and pulls around 70A with every possible electric thing turned on.
    Quote Originally Posted by iwire View Post
    240 secondary but no mention of primary voltage.
    You want to know the total load, meaning the electrical draw of both trailers when everything in them is turned on.
    So like ActionDave said- for "around 70A", going by this chart in the link, kwired's 25kVA transformer is plenty for Dave's 70A.

    And the price factor seems worse, if only going from 480V to 240V, the 100kVAs run nearly twice as much $$ as the 50kVAs and 3-4x the 25kVAs.
    I can't think of any reason to pay 2-4x as much as necessary for....anything!

    http://1y18vt40uea231dp7l2g1lsi.wpen...ansformers.pdf

  8. #18
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    Quote Originally Posted by Sahib View Post
    unless the needed demand factor is made available to him.
    I think everyone is simply telling the OP to go by the book and calculate the actual two trailer load. If you oversize the transformer when you don't have to, the xfmr isn't the only thing that becomes more expensive.

  9. #19
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    If these "trailers" are mobile homes, see 550.18 and 550.31 to determine what the load possibly can be calculated at.

  10. #20
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    Quote Originally Posted by kwired View Post
    If these "trailers" are mobile homes, see 550.18 and 550.31 to determine what the load possibly can be calculated at.
    Very constructive suggestion.

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