A raised plaster cover (often called a plaster ring or mud ring) is permitted to increase the maximum number of conductors permitted in an outlet box when it is a. listed as a box extension b. by the same manufacturer as the box c. marked with its cubic-inch volume d. metallic and capable of being grounded
The correct answer and explanation is:
The correct answer is a. listed as a box extension.
A raised plaster cover, also known as a plaster ring or mud ring, is a device used to extend the depth of an electrical outlet box. It is often installed to bring the box flush with the finished surface of a wall when additional wall material like plaster, drywall, or tile is added. The key reason it can increase the maximum number of conductors allowed in the box is that it is considered a box extension.
Box volume and conductor fill calculations are critical for electrical safety and are regulated by the National Electrical Code (NEC). The NEC provides specific rules for calculating the number of conductors that can be safely contained in an outlet box based on its cubic-inch volume. When a box extension like a plaster ring is listed (meaning it has been tested and approved by a recognized testing laboratory such as UL), it is considered part of the box’s volume.
Because the plaster ring effectively increases the total cubic-inch volume of the box, it increases the allowable number of conductors that can be safely installed inside. If the plaster ring is not listed as a box extension, its volume may not be included in conductor fill calculations, and therefore it will not increase the maximum number of conductors allowed.
Options b, c, and d are incorrect because:
- Being by the same manufacturer (b) does not automatically allow the volume of the plaster ring to count unless it is listed as an extension.
- Simply being marked with its cubic-inch volume (c) is not enough unless it is listed as a box extension.
- Being metallic and capable of grounding (d) relates to grounding requirements but does not affect conductor fill limits or volume calculations.
Therefore, only when the plaster ring is listed as a box extension does it increase the maximum number of conductors permitted in the outlet box by effectively increasing the box volume used in fill calculations.