The Electrical Code

215.11 Circuits Derived from Autotransformers. To understand this article a little bit better the word “Autotransformer” needs to be explained. An autotransformer is a transformer that has an input and an output. It uses only one winding or better said is has a power and a neutral. The input is the line and the output is the load. The autotransformer uses that voltage and either reduces or increases the voltage so it can be used for lights, motors, etc……

“Feeders shall not be derived from autotransformers unless the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the auto‐transformer.

Exception No. 1: An autotransformer shall be permitted without the connection to a grounded conductor where transforming from a nominal 208 volts to a nominal 240-volt supply or similarly from 240 volts to 208 volts.

Exception No. 2: In industrial occupancies, where conditions of maintenance and supervision ensure that only qualified persons service the installation, autotransformers shall be permitted to supply nominal 600-volt loads from nominal 480-volt systems, and 480-volt loads from nominal 600-volt systems, without the connection to a similar grounded conductor.”