There's a lot of industrial 277V in North America that's a few volts too hot to support level 2 EV charging. It just needs to be knocked down to roughly 240V, and that's not that hard to do with a small bucking transformer, thusly, outputting 241V (close enough).

For instance at 48A, the bucking transformer is 36V x 48A = 1.7 kVA. Not a big transformer. So this is pretty achievable.


simulate this circuit – Schematic created using CircuitLab

Note that the current economics are a little weird here - even though 48A is flowing through the outer ring circuit, giving 48A @ 241V (11.5 kW), and the transformer is drawing 6.2 amps, the 277V draw isn't 48A + 6.2A = 54.2A... it's actually 48A - 6.2A = 41.8A.

However, 277V-36V transformers are far less common than 240V-36V transformers. Is it possible to feed the bucking transformer from its own output? Like this


simulate this circuit

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    \$\begingroup\$ This is called an autotransformer. Should work. \$\endgroup\$
    – RussellH
    Commented Sep 21, 2023 at 22:03
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    \$\begingroup\$ Forgive my ignorance, but would it make more sense to just use a 12kVA 277V to 240V transformer? \$\endgroup\$ Commented Sep 21, 2023 at 22:03
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    \$\begingroup\$ @TomCarpenter because 12 kVA transformers cost and weigh a lot more than 2 kVA ones. An EVSE with a 2 kVA transformer inside could be lifted without rigging and shipped UPS. \$\endgroup\$ Commented Sep 21, 2023 at 22:49
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    \$\begingroup\$ @TomCarpenter - an autotransformer can be much smaller (cheaper) than an isolating transformer because the magnetic field only needs to transfer a small portion of the total power delivered to the load. \$\endgroup\$ Commented Sep 21, 2023 at 23:49
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    \$\begingroup\$ @manassehkatz I suspect it's more because Tesla knows that "Tesla Taps" exist and that other cars don't support 277V. Which I think is more of an oversight than a masterplan to save $2 a car. \$\endgroup\$ Commented Sep 22, 2023 at 18:24

1 Answer 1


You are essentially creating an autotransformer. You can make such an autotransformer by wiring together the primary and secondary windings of a step-down transformer, and it will work fine providing you observe all the voltage and current specs for your transformer. Note that an autotransformer is not isolating. Make sure you don't need an isolating transformer in your application.

One other point worth mentioning. In your schematic, you place polarity dots on your transformer. In practice, it is often not obvious which way to connect a primary lead to a secondary lead. The easiest solution, AFAIK, is to test one way, and if that gives the wrong transformer ratio, swap either the primary lead, or the secondary lead. But it is important in general to do this testing at less than the normal operating voltage. Not only is it safer for you, but it is also safer for the transformer.

  • \$\begingroup\$ Side note: his original schematic doesn't provide isolation either. \$\endgroup\$ Commented Sep 22, 2023 at 17:43
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    \$\begingroup\$ Yeah for the EVSE application it's not required @Jerry. \$\endgroup\$ Commented Sep 22, 2023 at 18:10

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