If a device box for a single-pole switch is installed 1/2 inch behind the finished surface of sheetrock, is this a violation of Section 314.20?

The correct answer is that the installation is a violation of Section 314.20 of the National Electrical Code (NEC). This section specifies that electrical boxes must be installed in a manner that allows for proper access and use. Specifically, it mandates that the front edge of the box should be flush with the finished surface of the wall or, in some cases, slightly recessed, but never more than 1/4 inch back from the finished surface.

By installing the device box 1/2 inch behind the finished surface of the sheetrock, the box will not be accessible for connection to the switch. This situation can pose safety hazards, as it would prevent convenient access for maintenance or troubleshooting. Such a depth might also obstruct the switch operation if not aligned correctly, further emphasizing the necessity for compliance with the code.

It's crucial for electrical installations to adhere strictly to these requirements to ensure not only the effectiveness of the devices but also the safety of the installation as a whole.