427.27 Voltage Limitations.
Unless protected by ground-fault circuit-interrupter protection for personnel, the secondary winding of the isolation transformer connected to the pipeline or vessel being heated shall not have an output voltage greater than 30 volts ac.
A new exception has been added for industrial locations. These units are manufactured as a system and in some cases heat fluid within pipes up to 6 miles long. This is accomplished by regulating currents upwards of 3000 ampere through these vessels. The conductive piping exchanges heat via the I2 R losses and this method is less than others. The transformers are 120/208V and the + - taps are set at 10%, thus the need to increase the output voltage to 132V. Exception: In industrial establishments, the isolation transformer connected to the pipeline or vessel being heated shall be permitted to have an output voltage not greater than 132 volts ac to ground where all of the following conditions apply:
