Autoformer

pianotuna wrote:
Hi Dutch,

NEC prohibits hard wired autoformers in RV's but when the unit is a plug in device, they are not prohibited. This argues for having a plug in unit. I choose to do plug in rather than hard wired.

Dutch_12078 wrote:
Be aware that some parks ban the use of "autoformers" (autotransformers), either out of ignorance, believing that they somehow "steal" power, or because either they or their insurance carrier are aware that the NEC prohibits their use in RV's. If nothing else, that could be a factor in deciding between a portable unit versus a built-in one that's out of sight.

The NEC is none to clear on the details, so I may well be interpreting it incorrectly, but I believe the prohibition is specifically on using an autotransformer in the converter, presumably to ensure that the 12V and the 120V systems are isolated from each other. The section on which the prohibition is listed is dealing with requirements for having separately derived electrical systems in an RV, i.e. the 12V system and the 120V system.

Note in this case "autotransformer" refers generically to a transformer in which the primary and secondary partly share the same winding, and not solely to the voltage correcting transformers used with RVs (which are indeed generally built as autotransformers).