314.16(A) Box Volume Calculations.
Code Change Summary: Code language was added to address box volume for a fixed barrier inside a box.
Sometimes the code requires a barrier inside a junction or device box. Section 404.8(B) states that a snap switch shall not be grouped or ganged in enclosures with other snap switches, receptacles, or similar devices, unless they are arranged so that the voltage between adjacent devices does not exceed 300 volts, or unless they are installed in enclosures equipped with identified, securely installed barriers between adjacent devices. Similar language for receptacles is found in 406.5(J).
Section 314.16 provides rules on calculating box fill and includes an allowance for conductors, devices, internal cable clamps and support fittings but has never required a specific allowance for a fixed barrier installed inside the box.
Now, in the 2017 NEC®, a divider or barrier inside a box must be counted since it takes up space inside the box. Some barriers are already marked with their volume to indicate how much space they take up inside the box while other barriers are not. Metal barriers that are not marked with their volume are now considered to have a cubic inch volume of ½ inch. Nonmetallic barriers that are not marked with their volume are considered to have a cubic inch volume of 1 inch.
Below is a preview of Article 314. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC® edition of NFPA 70.
2014 Code Language:
314.16(A) Box Volume Calculations. The volume of a wiring enclosure (box) shall be the total volume of the assembled sections and, where used, the space provided by plaster rings, domed covers, extension rings, and so forth, that are marked with their volume or are made from boxes the dimensions of which are listed in Table 314.16(A).
2017 Code Language:
314.16(A) Box Volume Calculations. The volume of a wiring enclosure (box) shall be the total volume of the assembled sections and, where used, the space provided by plaster rings, domed covers, extension rings, and so forth, that are marked with their volume or are made from boxes the dimensions of which are listed in Table 314.16(A). Where a box is provided with one or more securely installed barriers, the volume shall be apportioned to each of the resulting spaces. Each barrier, if not marked with its volume, shall be considered to take up 8.2 cm3 (½ in3) if metal, and 16.4 cm3 (1.0 in3) if nonmetallic.