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My ac system is 240V. I want to step it up to higher voltage to plug in some temporary equipment that needs at least 400-440 volts. If I use a 120v-240v step up transformer.. would it make my 240V become 480V?

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    I use step up transformers on long runs but keep them isolated from other lower voltage systems, inspectors have never red tagged me but a friend from the UK thought similar to what you plan to do had to split the systems and had to have 3 extra inspections that cost way more than splitting the systems in the first place, our rate back then was 50$ ea more now , so verify if using tap rules or split systems to isolate them and use the seperatly derived rules. – Ed Beal Oct 18 '18 at 0:42
  • I just need to use it for 5 minutes or less... so If I use a 120v-240v step up transformer.. would it make my 240V become 480V? Please confirm yes or no before I buy one. – Samzun Oct 18 '18 at 1:01
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    Voltage change is just a winding ratio, so the ratio from 120 to 240 is 2:1, the same as 240 to 480V. The problem is, voltage DESIGN is based on things like creepage distances and air separation, plus electrical dielectric properties of the insulation used. A cheap 240-120V transformer most likely will not be made with the proper clearances you need at 480V and it's also unlikely to have the right insulation rating. You may get lucky, but luck is a terrible strategy. That's what's behind Harper's response of "5 minutes to 5 days", but if the 480V side flashes over on you, maybe 5 milliseconds. – J. Raefield Oct 18 '18 at 1:20
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    Step up or down is (mostly) irrelevant, it's just a ratio change. – J. Raefield Oct 18 '18 at 1:45
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    Another comment you mention equipment at 400, 440. Most industrial equipment at 440 (in the US is referred to as 480) is 3 phase there are ways to convert single phase to 3 phase but this would be good to know to answer the question better. – Ed Beal Oct 18 '18 at 12:18
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Yes, for a short time, and then there will be a BANG and a fire will start.

Whether that happens in 5 minutes or 5 days is based on the quality of the build of the equipment. Siemens >> random junk off Alibaba.

If you plan to do that, then buy a 240/480 autotransformer in the first place, which can also be used for 120/240 at half the VA. This type is more likely to be available as an isolation transformer, for what that's worth.

  • What is the reason a 120v to 240v step up transformer can't handle a 240v input to the 120v primary? The winding in the 120v primary can really only take 120v and not higher voltage? Would the wire heat up or something even if the current is much less? – Samzun Oct 18 '18 at 1:16
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    @Samzun insulation quality. A 240V transformer is not insulated for 480V. – Harper Oct 18 '18 at 1:28
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    Or think about it another way. If 120/240 could be used as 240/480, why not keep going higher - chain a few of them together 120/240->240/480->480/960->960/1920. Do you really think the same device designed for 120/240 is safe at 960/1920? Probably not. The problem is we aren't the design engineers so we don't know how much of a safety margin is built in to a particular device. – manassehkatz Oct 18 '18 at 3:34

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