The standard size and specifications for electrical boxes, outlets, switches and cover plates.
TYPE OF SCREWS
The standard type of screw for electrical outlets and switches to secure the switch or outlet to the electrical box is a flat head machine screw, they can either be Phillips or slotted screw. “Flat head” refers to the shape of the screw head, not the slot used to drive the screw. Cover plates typically use an oval head machine screw. The oval head closely resembles a flat head screw but has a slight roundness to the head.
The standard screw gauge for home electrical outlets, boxes, switches and cover plates is #6. The gauge size is the diameter of the screw.
THREADS PER INCH
The standard thread count on an electrical screw is 32, this means there are 32 threads per inch of shaft.
The lengths of the screws depend on the application.
Cover Plates: Electrical cover plates for switches and outlets are typically packaged with 5/16″ long screws. Solid or blank cover plates are usually packaged with 1″ long screws.
Switches & Outlets: 1″ long screws are packaged with most electrical outlets and switches.
Extended Screws: If you area dealing with a situation where you have a recessed outlet due to an added wall thickness at an electrical box due to either double thick drywall or tile, a screw that is 1-1/2″ long will usually be adequate for installing the switch or outlet. The screw length can be as long as you need it, but 2″ is probably the max length you would ever need in a typical residential application. The cover plate screw length can remain unchanged. Since most switches and outlets are packaged with 1″ screws you will need to purchase these screws separately. You can find these screws in the hardware section of a home improvement store or on Amazon: http://amzn.to/2s4QCkO
THE STANDARD ELECTRICAL SCREW SIZES
Cover Plates: #6-32 x 5/16″ Oval Head Machine Screw http://amzn.to/2sqggh9
Electrical Box Switches & Outlets: #6-32 x 1″ Flat Head Machine Screw http://amzn.to/2qSxUIB
Extended Outlet Screws: #6-32 x 1-1/2″ Flat Head Machine Screw http://amzn.to/2s4QCkO