Tag: Metric and Decimal Drill Bits

Dear Mr. Electrician:  What are the standard electrician's screw drill and tap sizes for electrical switch and outlet boxes? Answer:  6/32, 8/32, and 10/32 are the most commonly used electrician's screw drill and tap sizes in electrical boxes installed in the USA. NOTE: Some text links below go to applicable products on Amazon.  As an Amazon Associate, I earn from qualifying purchases. Table of Contents: Electrician Screw Sizes and Taps Construction Bolt Sizes and Taps 6/32 screws are used for mounting a wall switch or an electrical receptacle outlet to the electrical box in the wall and also for the finished wall plate.  #6 is the screw size, and 32 is the number of threads per inch. 8/32 holds the covers on certain electrical boxes and for light fixture support.  10/32 screws tend to be used for cable clamps and grounding metal outlet, junction, and switch boxes. I have noticed that some light fixtures made in other countries have screws and mounting hardware included with them that look like 8/32 drill and tap size screws but are a little tight when screwed into a standard ceiling electrical box in the USA. I discovered that they are not true 8/32 screws but are a metric equivalent made to accommodate the American lighting fixture market.  The hardware shipped with such light fixtures also uses these odd screws. I have used my Klein Tools 6-in-1 tapping tool to make the holes in the hardware true drill and tap size 8/32 without any problem. Some ceiling fan boxes use 10/24 screws for the fan mount.  10/24 screws were also used for the cable clamps in very old black enameled switch and outlet boxes. For grounding it is important to use fine threaded machine screws such as 10/32 to attach a grounding pigtail wire to a metal electrical box or a grounding lug to a metal enclosure.  The reason is that laboratory testing has determined that a minimum of two and a third screw threads must be in contact with the grounded metal for an effective ground connection. That is why sheet metal screws and wood screws cannot be used for grounding connections. Top Of Page Below is a table of drill and tap sizes, some of which are used during the course of installing electrical wiring.  I have also included the decimal equivalent. Many plastic switch and outlet box types do not have threaded holes.  The device screws thread into a precisely sized hole in the plastic box.  No tapping is necessary. I always have my 6-in-1 Klein tapping tool in my tool bag.  It has the most common drill and tap sizes for electricians and is handy.  If the threads are stripped on a metal outlet or junction box, you can sometimes re-tap the hole to the same size or the next bigger screw size.  If paint or joint compound gets in the screw holes, the 6-in-1 tapping tool quickly cleans them. Replacement taps are available for the Klein 6-in-1 tap handle.  However, I have found that an electrical supply company selling Klein Tools is the best place...