Wednesday, March 1, 2023
Dear CQD, are 10/32 screws required by the code for grounding & bonding purposes, or can any 32 TPI machine screw be used? Also, does the screw have to be green? (Not a device screw, an installed machine screw for grounding and bonding, for instance to a device box)? Thanks, Jackson Roos
Hi Jackson, great question and one that brings a lot of discussion. When the Code is silent on a topic like thread type of machine screws for grounding, it often helps to look at the “Must Do” or the “Shalls”:
- Section 110.2 requires conductors and equipment to be approved.
- Section 110.3 provides methods that an AHJ as defined in Article 100, can use for approval of an installation or equipment.
- Section 250.8 provides requirements for the connection of Grounding and Bonding Equipment. More specifically, Section 250.8(5)&(6) provide guidance on using machine screws for grounding and bonding:
- (5)-Machine screws must engage at least two (2) threads or are secured with a nut.
- (6)-Thread-forming machine screws that engage at least two (2) threads.
- Section 250.148(C) mandates that the connection method for equipment grounding cannot be used for other purposes. For instance, using a box clamp machine screw it attaches the equipment ground and the box clamp would not be permitted.
Looking at the “Must Do” or “Shalls”, there is no requirement for the ground screw to be listed, or green, or a 10-32 machine screw. However, the installation of the ground screw is required to be approved by an AHJ. To approve the installation of a machine screw for grounding, the AHJ must be able to verify that there are indeed two (2) threads engaged in the enclosure. To engage at least two (2) threads when using a 32-pitch thread machine screw, the enclosure box would need to be at least 1/16” thick. For this reason, most standard metal boxes are pre-tapped for a 10-32 machine screw, and the manufactures of ground screws make them 10-32 machine screws. Additionally, they are made green in color for easy identification of their intended use to aid in the demonstration of compliance with Section 250.148(C).