You're right, Table 314.16(A) can be confusing. Here's a breakdown of the bottom portion addressing FS and FD boxes, along with an explanation that hopefully clarifies the intent

Understanding the Problem: Why FS/FD Boxes are Different

Regular box fill calculations rely primarily on conductor counts and the number of devices. FS and FD boxes, however, are often used in industrial or outdoor environments and have different cover/gang configurations. The covers themselves, particularly when accommodating multiple conduit entries or devices, take up space inside the box.

Explanation of the Table Entries for FS and FD Boxes



FS Boxes - Designed for surface mounting and typically use a single threaded conduit entry. FD Boxes - Designed for surface mounting and typically provide multiple conduit entries. Single Gang/Cover

This refers to a box designed for a single wiring device or one device mounted to one cover. Think of a single receptacle or switch mounted on a single-gang cover.

Multiple Gang/Cover

This refers to a box or cover designed for more than one wiring device or the box with the cover will accept more than one wiring device. Think of multiple receptacle or switch mounted on a multiple-gang cover.

In summary, the "Single Gang/Cover" vs. "Multiple Gang/Cover" distinction DOES apply to FS and FD boxes.Why the confusion exists

The term "gang" is often associated with the number of device openings (like a two-gang switch plate). The NEC is using "gang" here to also indicate the presence of a wiring device in the box. The volume deductions in table 314.16A take into consideration the device mounted in or on the box. The same is true for devices mounted on the cover of the box.

Example Scenarios



Single Gang/Cover

You have an FS box with a single receptacle mounted on a single cover. You use the "Single Gang/Cover" volume deduction for the box's size.

Multiple Gang/Cover

You have an FD box with two switches mounted on a multi-gang cover. You use the "Multiple Gang/Cover" volume deduction for the box's size.

Key Takeaways

The "Single/Multiple Gang/Cover" distinction is about the presence of a single wiring device vs. more than one wiring device and the volume deduction that it takes up in the box. Don't just count conduit entries. Count the number of wiring devices that are mounted in or on the box/cover.

Important Note: Always refer to the latest edition of the NEC for the most accurate and up-to-date information. When in doubt, consult with a qualified electrical inspector or engineer. They can provide guidance specific to your installation and local codes. Flag for review