Dear Mr. Electrician:  What are the standard screw sizes for electrical switch and outlet boxes?

Answer:  6/32, 8/32, 10/32 are the most commonly used screws in electrical boxes installed in the USA.  6/32 is used for mounting the switch or outlet to the box and also for the wall plate.  8/32 is used to hold the covers on certain electrical boxes.  10/32 screws tend to be used for grounding metal outlet, junction, and switch boxes.  NOTE: Text links below go to applicable products on Amazon and EBay.

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.

Below is a table of screw sizes, some of which are used during the course of installing electrical wiring.  I have also included the drill bit size for each screw tap and the decimal equivalent.

Many of the 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 come in handy countless times.  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 is quick to clean them out.

If you need to drill for conduit click for my hole sizes for electrical conduits post, including taps.

ELECTRICIAN’S SCREW TAPS AND DRILL BIT 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)

Another tool that is always in my bag is my thread restoring file.  It works very well at restoring threads on screws, bolts, and threaded rod.  There is a total of 8 screw thread pitches on the tool above.  There are two or three models of thread restorer tools 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 identify different threads on screws and bolts frequently.

The drill bit size equivalent chart below was copied from my old html site.   It does not work well with WordPress.  I need to come up with a better idea for displaying the drill bit sizes.  This page is best viewed horizontally.

DRILL BIT SIZE EQUIVALENT

1/64 17/64 33/64 49/64 .1mm 8mm 17mm 3/64 19/64 35/64 51/64 1mm 9mm 18mm 5/64 21/64 37/64 53/64 2mm 10mm 19mm 7/64 23/64 39/64 55/64 3mm 11mm 20mm 9/64 25/64 41/64 57/64 4mm 12mm 21mm 11/64 27/64 43/64 59/64 5mm 13mm 22mm 13/64 29/64 45/64 61/64 6mm 14mm 23mm 15/64 31/64 47/64 63/64 7mm 15mm 24mm 16mm 25mm

Minimum and maximum drill sizes are indicated for many taps.  A smaller hole will have tighter threads, but will be more work to tap in hard materials.  A smaller hole (Where 75% of the thread surfaces will make contact) is a good idea in soft metals such as copper or brass.  Not responsible for errors and omissions regarding the content of this page.

 

 

 

 

 

 

 

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