happycamper002 wrote:
NEC disallows this auto transformers, but RV owners came up with a workaround by having them pluggable instead of hardwired—a loophole they think that outsmart the NEC Code Panel.
The section of the NEC that disallows autotransformers is specifically dealing with "Combined electrical systems" (i.e. the converter in an RV, and other devices that interact with both 120V and 12V circuitry). As such, it would appear to require simply that you simply can't construct a converter using an autotransformer but instead must use some circuit that provides galvanic isolation between the 120V and 12V sides. I don't think it applies outside of that context (though in this case the wording of the code is not as clear as one might hope).
Note that the term autotransformer applies to a certain method of building a transformer (namely, having the primary and secondary coils shared or at least interconnected, rather than entirely isolated), and not only to voltage step-up/step-down devices, such as the Hughes Autoformer, although such devices do typically employ autotransformers in their construction.
