Code Change Summary: A new subsection was added to clear up confusion about backfeeding a transformer.

Transformers are often wired in reverse with the primary conductors terminated to the secondary terminals and the secondary conductors terminated to the primary terminals. One of the most common applications is a dry-type transformer installed in a facility where the utility supplied voltage is 120/208 volts. If a specific piece of equipment is installed that requires 480 volts, a transformer can be installed.

Step-up transformers are not as readily available as step-down transformers. Often, the electrician will just install a step-down transformer and wire it in reverse in order to turn it into a step-up transformer.

In many cases, a backfed transformer will work fine without any issues but the 2014 NEC^® has new language making it clear that if reverse feeding a transformer, it must be done in accordance with the manufacturers installation instructions.

Following the installation instructions when backfeeding a transformer may result in selecting the maximum permitted overcurrent device on the primary side to account for the larger inrush current. This issue can occur because the low impedance 208Y/120-volt winding that was originally designed to be the secondary winding, is now serving as the primary which can cause the value of the magnetizing inrush current to be much greater than expected.

As well, when reverse feeding a typical step-down transformer, the taps will now be on the secondary side instead of the primary side, so their operation is reversed.

Below is a preview of the NEC^®.  See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).