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.
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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 included with them that look like 8/32 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, but I have used my Klein Tools 6-in-1 tapping tool, if needed, to make them true 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 ground 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.
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 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 to buy them in terms of low price and availability. Most electrical supply companies do sell Klein Tools, as many of their products are used by electricians
If you need to drill for conduits, click for my hole sizes for electrical conduits post, including taps.
ELECTRICIAN’S SCREW DRILL AND TAP SIZES
Electricians Tap Drill Bit Sizes for Common Machine Screws Used in USA Electrical Wiring
| Screw Size and Pitch | Drill Bit Size for Tap at 75% | Decimal Equivalent | Next Fractional Size SMALLER Drill Bit | Next Fractional Size LARGER Drill Bit | Nearest Metric = |
|---|---|---|---|---|---|
| 4/40 | #43 | .0890 | 5/64" (.0781) | 3/32" (.0938) | 2.25mm (.0886) |
| 4/48 | #42 | .0935 | 5/64" (.0781) | 3/32" (.0938) | 2.35mm (.0925) |
| 5/40 | #39 | .0995 | 3/32" (.0938) | 7/64" (.1094) | 2.5mm (.0984) |
| 5/44 | #37 | .1040 | 3/32" (.0938) | 7/64" (.1094) | 2.60mm (.1024) |
| 6/32 | #36 | .1065 | 3/32" (.0938) | 7/64" (.1094) | 2.7mm (.1063) |
| 6/40 | #33 | .1130 | 7/64" (.1094) | 1/8" (.1250) | 2.9mm (.1142) |
| 8/32 | #29 | .1360 | 1/8" (.1250) | 9/64" (.1406) | 3.4mm (.1339) |
| 8/36 | #29 | .1360 | 1/8" (.1250) | 9/64" (.1406) | 3.4mm (.1339) |
| 10/24 | #25 | .1495 | 9/64" (.1406) | 5/32" (.1562) | 3.9mm (.1535) |
| 10/32 | #21 | .1590 | 5/32" (.1562) | 11/64" (.1719) | 4mm (.1575) |
| 12/24 | #17 | .1730 | 11/64" (.1719) | 3/16" (.1875) | 4.4mm (.1732) |
| 12/28 | #15 | .1800 | 11/64" (.1719) | 3/16" (.1875) | 4.5mm (.1772) |
| 1/4"- 20 | #7 | .2010 | 3/16" (.1875) | 13/64" (.2031) | 5.1mm (.2008) |
| 1/4"- 28 | #3 | .2130 | 13/64" (.2031) | 7/32" (.2188) | 5.4mm (.2126) |
Click Here to See Amazon Has #1 – #60 Drill Bits Used For Tapping Holes
Something else that I keep in my tool bag is a thread-restoring file. It does an excellent job of restoring damaged threads on screws, bolts, and threaded rods. It is also good for repairing the threads on screws, bolts, and rods that have been cut.
There is a total of 8 screw thread pitches on the tool above. Two or three models of thread restorer tools are available to accommodate all of the screw pitches out there.
Tap Drill Bit Sizes for Bolt and Cap Screws Used in Construction
| Screw Size and Pitch | Drill Bit Size for Tap at 75% | Decimal Equivalent | Next Fractional Size Smaller Drill Bit | Next Fractional Size LARGER Drill Bit | Nearest Metric = |
|---|---|---|---|---|---|
| 1/4"-20 | #7 | .2010 | 3/16" (.1875) | 13/64" (.2031) | 5.1mm (.2008) |
| 1/4"-28 | #3 | .2130 | 13/64" (.2031) | 7/32" (.2188) | 5.4mm (.2126) |
| 5/16"-18 | F | .2570 | 1/4" (.2500) | 17/64" (.2656) | 6.5mm (.2559) |
| 5/16"-24 | I | .2720 | 17/64" (.2656) | 9/32" (.2812) | 6.9mm (.2717) |
| 3/8"-16 | 5/16" | .3125 | 19/64" (.2969) | 21/64" (.3281) | 7.9mm (.3110) |
| 7/16"-14 | U | .3680 | 23/64" (.3594) | 3/8" (.3750) | 9.30mm (.3661) |
| 7/16"-20 | W | .3860 | 3/8" (.3750) | 25/64" (.3906) | 9.80mm (.3858 |
| 1/2"-13 | 27/64" | .4219 | 13/32" (.4062) | 7/16" (.4375) | 10.50mm (.4134) |
| 1/2"-20 | 29/64" | .4531 | 7/16" (.4375) | 15/32" (.4688) | 11.50mm (.4528) |
| 9/16"-12 | 31/64 | .4844 | 15/32" (.4688) | 1/2" (.5000) | 12mm (.4724) |
| 9/16"-18 | 33/64 | .5156 | 1/2" (.5000) | 17/32" (.5312) | 13mm (.5118) |
| 5/8"-11 | 17/32" | .5312 | 33/64" (.5156) | 35/64" (.5469) | 13.50mm (.5315) |
| 5/8"-18 | 37/64" | .5781 | 9/16" (.5625) | 19/32" (.5938) | 14.50mm (.5709) |
| 11/16"-11 | 19/32" | .5938 | 37/64" (.5781) | 39/64" (.6094) | 15mm (.5906) |
| 11/16"-16 | 5/8" | .6250 | 39/64" (.6094) | 41/64" (.6406) | 15.50mm (.6102) |
| 3/4"-10 | 21/32" | .6562 | 43/64" (.6406) | 43/64" (.6719) | 16.50mm (.6400) |
| 3/4"-16 | 11/16" | .6875 | 43/64" (.6719) | 45/64" (.7031) | 17.50mm (.6890) |
| 7/8"-9 | 49/64" | .7500 | 3/4" (.7500) | 25/32" (.7812) | 19.50mm (.7677) |
| 7/8"-14 | 13/16" | .8125 | 51/64" (.7969) | 53/64" (.8281) | 20.50mm (.8071) |
| 1"-8 | 7/8" | .8750 | 55/64" (.8594) | 57/64" (.8906) | 22mm (.8661) |
| 1"-12 | 59/64" | .9219 | 29/32" (.9062) | 15/16" (.9375) | 23.50mm (.9252) |
| 1"-14 | 15/16" | .9375 | 59/64" (.9219) | 61/64" (.9531) | 23.50mm (.9252) |
A thread pitch gauge is handy if you need to frequently identify different threads on screws and bolts.
Click here for a simple chart on bolt grades and their strengths.
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