Dear Mr. Electrician: What are the actual hole saw sizes for the various trade sizes of electrical conduit?  I plan to install a lot of electrical conduit.  I know that I will need to drill with hole saws or use a knockout punch in junction boxes and the circuit breaker panel.

Answer: I have posted the hole saw and knockout sizes for electrical conduit below.  For electrician’s hole saw kits see here.  When drilling with a hole saw it is important to use a slower spinning drill with a lot of torque rather than a high speed drill.  It is best to drill at 400 revolutions per minute (RPM) or slower when using a hole saw.  Drilling at a slower speed will help the hole saw last longer, and there is less of a danger of the hole saw getting jammed and causing damage or injury.  NOTE: Text links go to applicable products on Amazon.com

There is a list of old Greenlee hand knockout punch replacement part numbers at the bottom of this post.

I usually use my 1/2″ Milwaukee Electric Right Angle Drill when using a hole saw as the torque is very good and the RPM’S are just right.  The extra length of the right angle drill makes it easier to maintain control of the drill if the hole saw does get jammed.  I suggest wearing work gloves when using a right angle drill.

When drilling into metal, it is a good idea to oil the hole saw a few times to prevent premature wearing of the hole saw teeth.  I read in a trade magazine of one electrician who puts an oil soaked cut-to-size piece of Felt inside of his hole saws whenever he drills into metal.  As the hole saw heats up from drilling, the oil will trickle down onto the metal being drilled.

Put something on the floor to protect it from oil leaks such as flattened boxes or an old piece of plywood.  Although you can probably use most general purpose or motor oils for lubricating when drilling, there are oils made especially for drilling, tapping, and threading.  Amazon sells cutting oil which is used for these purposes.

  Electricians Hole Saw Kits on Amazon

My two types of non-interchangeable hole saw arbors with standard pilot bits
My two types of non-interchangeable hole saw arbors with standard pilot bits

Two types of hole saw arbors are depicted in the above photo and they are not interchangeable.  The three arbors on the top left can only be used with a hole saw that has the rectangular round shape.  The others screw in to one of two standard size threads.

Electrician’s Hole Saw Sizes

For 1/2″ conduit, drill using a 7/8″ hole saw or 1/2″ conduit knockout punch (.885″ or 22.5mm).

For 3/4″ conduit, drill using a 1 1/8″ hole saw or 3/4″ conduit knockout punch (1.115″ or 28.3mm).

For 1″ conduit, drill using a 1 3/8″ hole saw or 1″ conduit knockout punch (1.362″ or 34.6mm).

For 1 1/4″ conduit, drill using a 1 3/4″ hole saw or 1 1/4″ conduit knockout punch (1.701″ or 43.2mm).

For 1 1/2″ conduit, drill using a 2″ hole saw or 1 1/2″ conduit knockout punch (1.951″ or 49.6mm).

For 2″ conduit, drill using a 2 1/2″ hole saw or 2″ conduit knockout punch (2.416″ or 61.5mm).

For 2 1/2″ conduit, drill using a 3″ hole saw or 2 1/2″ conduit knockout punch (2.914″ or 74mm).

For 3″ conduit, drill using a 3 5/8″ hole saw or 3″ conduit knockout punch (3.539″ or 89.9mm).

For 3 1/2″ conduit, drill using a 4 1/8″ hole saw or 3 1/2″ conduit knockout punch (4.044″ or 102.7mm).

For 4″ conduit, drill using a 4 1/2″ hole saw or 4″ conduit knockout punch (4.544″ or 115.4mm).

For 5″ conduit, use a 5 1/2″ hole saw.

For 6″ conduit, use a 6 5/8″ hole saw.

Ebay Sells New and Used Knockout Punch Sets

Electrical Conduit Trade SizeHole Saw Size
For Conduit or Fitting
Drill or Hole Saw Size For
NPT Tapered Threaded Tap.
Taper is 3/4"per foot.
Tap Thread Size
1/8" Pipe or Fixture Nipple13/32"Q (21/64" or .332")27 Threads Per inch
1/4" Pipe or Fixture Nipple35/64"7/16"18 Threads Per Inch
3/8" Pipe or Fixture Nipple43/64"9/16"18 Threads Per Inch
1/2" or 16mm Conduit7/8" or 22mm45/64" 14 Threads Per Inch
3/4" or 21mm Conduit1 1/8" or 28mm59/64" or 7/8"14 Threads Per Inch
1" or 27mm Conduit1 3/8" or 34mm1 5/32" 11.5 Threads Per Inch
1 1/4" or 35mm Conduit1 3/4"1 1/2"11.5 Threads Per Inch
1 1/2" or 41mm Conduit2"1 47/64"11.5 Threads Per Inch
2" or 53mm Conduit2 1/2"2 7/32"11.5 Threads per Inch
2 1/2" or 63mm Conduit3"2 5/8"8 Threads per Inch
3" or 78mm Conduit3 5/8"3 1/4"8 Threads per Inch
3 1/2" or 91mm Conduit4 1/8"3 3/4"8 Threads per Inch
4" or 103mm Conduit4 1/2"4 1/4"8 Threads per Inch
5" or 129mm Conduit5 1/2"5 9/32"8 Threads per Inch
6" or 155mm Conduit6 5/8"6 11/32"8 Threads per Inch

As an alternative to hole saws, knockout punches can be used.  You would drill a pilot hole sized for the draw stud of the punch using a step drill or a small hole saw and then use a hand knockout punch or a hydraulic knockout punch to make the desired size hole.

Knockout Punch Operating Instructions

I found an old set of knockout punch instructions with a parts list among my father’s tools.  I scanned it and broke it up into sections for easier viewing.  The paper is not in the best condition as I surmise that it is from the early 1960’s.

Below are the operating instructions for Greenlee electricians hand knockout punches.

A section of an old operating manual for Greenlee hand knockout punches
A section of an old operating manual for Greenlee hand knockout punches

Greenlee Hand Knockout Punch Replacement Part Numbers

The following two images depict the replacement part numbers for Greenlee hand knockout punches.  I do not know how useful they will be today for obtaining replacement parts for Greenlee knockout punches.

In addition the hole size of each knockout punch is provided.

Piece of an old (Maybe 1960's) Greenlee Hand Knockout Punch operating instructions. Shown are dimensions of the knockout punch parts, and their Greenlee part numbers
Piece of an old (Maybe 1960’s) Greenlee Hand Knockout Punch operating instructions. Shown are dimensions of the knockout punch parts, and their Greenlee part numbers

Listed above are dimensions of the knockout punch parts, and their Greenlee part numbers.

Section of a very old (Maybe 1960's) Greenlee Hand Knockout Punch operating instructions. Depicted here are dimensions of the knockout punch parts, and their Greenlee part numbers.
Section of a very old (Maybe 1960’s) Greenlee Hand Knockout Punch operating instructions. Depicted here are dimensions of the knockout punch parts, and their Greenlee part numbers.

Depicted above are dimensions of the knockout punch parts, and their Greenlee part numbers.

The front and rear pages of the Greenlee Knockout Punch Operating Instructions and Parts List is below.

Front and back page of old operating instructions and parts list for Greenlee knockout punches and cutter
Front and back page of old operating instructions and parts list for Greenlee knockout punches and cutter

Click For Electrical Conduit Dimensions:

1/2″ (16)        3/4″ (21)        1″(27)

1 1/4″ (35)        1 1/2″ (41)        2″ (53)

2 1/2″ (63)         3″ (78)        3 1/2″ (91)

4″ (103)          5″ (129)         6″ (155)

See my post describing various electrical conduit types.