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Conduit knockout diameters (NEMA Bulletin 71): 1/2 to 6 in trade sizes, minimum, nominal and maximum, with bushing and locknut diameters + hole gauge plates

Name: Conduit knockout diameters (NEMA Bulletin 71): 1/2 to 6 in trade sizes, minimum, nominal and maximum, with bushing and locknut diameters + hole gauge plates dimension sheet
Creator: Xem Studio
License: https://creativecommons.org/licenses/by/4.0/

What size hole is a 1/2 inch or 3/4 inch conduit knockout, and how big are 1-1/4, 1-1/2 and 2 inch knockouts? · Standard part · basis: NEMA Engineering Bulletin No. 71 (reaffirmed 2011), Tables 71-1A and 71-1B, read twice; three Xem-drawn hole gauge plates at the nominal diameters

Also called: 1/2 knockout size, 3/4 knockout hole diameter, conduit knockout size chart, electrical knockout sizes, 1 inch knockout diameter, 1-1/4 knockout size, knockout hole size for 1/2 inch conduit, 7/8 knockout 1/2 conduit, 1-1/8 knockout 3/4 conduit, knockout diameter chart mm, nema bulletin 71, knockout punch sizes, 2 inch conduit knockout size, knockout hole gauge stl, conduit hole size for panel, ele

Exact dimensions. Every file is drawn at true size with no tolerance added. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your process's clearance before making the part. Mating features

length: 298.5 mm
width: 78.7 mm
height: 3.0 mm

Notes

Knockout diameters for single or concentric knockouts in enclosures (outlet boxes, panels, cabinets), by conduit trade size, from NEMA Engineering Bulletin No. 71 (issued June 1965, revised 9 Sept 2004, reaffirmed 2011), Table 71-1A. The text of the table was read twice and agreed on every row. The figures exclude any projection of breakout ears or tabs (note B).
Table 71-1B in the same bulletin gives the diameter of the flange of a metallic bushing and of the locknut covering diameter for a nonmetallic bushing (Dim. B), which sets how far the fitting overlaps the hole: bushing and locknut overlap minimums are listed per size there (for 1/2 in: 0.033 in for the metallic bushing and 0.001 in for the nonmetallic locknut covering; 3/4 in nonmetallic: 0.000). A change in the knockout diameter, bushing flange, locknut or thread dimension can make an incompatible assembly (note A); dimensions for 1/2 through 1-1/4 are from UL 514 Table 20.2 (note E).
Trade size is a name, not a dimension: 1/2 in conduit is 0.840 in OD (rigid) or 0.706 in OD (EMT), and the knockout is a larger hole (0.875 in nominal) so the fitting's thread or body passes. Conduit and EMT outside diameters are on the pages emt-conduit and rigid-imc-emt-conduit-sizes-to-4-in; the metric designator column is the identification number for the same sizes (not a millimetre size).
Files: three hole gauge plates (1/2 to 2 in; 2-1/2 to 4 in; 5 and 6 in) with a through hole at each NEMA nominal knockout diameter (inches × 25.4: 22.225 mm for 1/2 in and so on, not the rounded mm column). Use them to check a punch, a step drill or a template. Plate thickness 3 mm, 8 mm between holes and 3 mm corner radii are Xem's choices. Hole diameters are exact; open them by your process's allowance if the gauge must pass a fitting. First plate 298.5 × 78.7 mm.
Not covered: the outline of breakout ears and tabs, concentric-knockout ring diameters, and the thread major diameters (Table 71-1C) of rigid conduit and IMC (the bulletin gives them; not copied).

Conduit knockout diameters (NEMA Bulletin 71): 1/2 to 6 in trade… size chart

trade size (in)	metric designator	knockout min (in)	knockout nominal (in)	knockout max (in)	knockout min / nominal / max (mm, as printed)	metallic bushing flange Ø B (in)	nonmetallic locknut covering Ø B (in)
1/2	16	0.859	0.875	0.906	21.82 / 22.23 / 23.01	1.00	0.937
3/4	21	1.094	1.109	1.141	27.79 / 28.17 / 28.98	1.23	1.189
1	27	1.359	1.375	1.406	34.52 / 34.93 / 35.71	1.54	1.485
1-1/4	35	1.719	1.734	1.766	43.66 / 44.04 / 44.86	1.92	1.859
1-1/2	41	1.958	1.984	2.016	49.73 / 50.39 / 51.21	2.18	2.109
2	53	2.433	2.469	2.500	61.80 / 62.71 / 63.50	2.68	2.635
2-1/2	63	2.938	2.969	3.000	74.63 / 75.41 / 76.20	3.20	3.156
3	78	3.563	3.594	3.625	90.50 / 91.29 / 92.08	3.83	3.812
3-1/2	91	4.063	4.125	4.156	103.20 / 104.78 / 105.56	4.40	4.343
4	103	4.563	4.641	4.672	115.90 / 117.88 / 118.67	4.94	4.890
5	129	5.625	5.719	5.750	142.88 / 145.26 / 146.05	6.05	6.062
6	155	6.700	6.813	6.844	170.18 / 173.05 / 173.84	7.20	7.110

Mating features

Drawn at the exact size. Open press, slide, run and socket features by your process's clearance (tolerances by process); leave designed gaps and clamp fits as drawn.

feature	exact	mates with	fit	files
1/2 in trade-size knockout	Ø22.225 nominal (21.82 to 23.01 printed); metallic bushing flange Ø25.4 (1.00 in)	1/2 in conduit fitting: its thread or body passes (RMC OD 21.34, EMT OD 17.93), locknut or bushing larger than the hole	designed	knockout-gauge-1-2*
3/4 in trade-size knockout	Ø28.17 nominal (27.79 to 28.98 printed); metallic bushing flange Ø31.24 (1.23 in)	3/4 in conduit fitting (RMC OD 26.67, EMT OD 23.42)	designed	knockout-gauge-1-2*
1 in trade-size knockout	Ø34.925 nominal (34.52 to 35.71 printed); metallic bushing flange Ø39.12 (1.54 in)	1 in conduit fitting (RMC OD 33.40, EMT OD 29.54)	designed	knockout-gauge-1-2*
2 in trade-size knockout	Ø62.71 nominal (61.80 to 63.50 printed); metallic bushing flange Ø68.07 (2.68 in)	2 in conduit fitting (RMC OD 60.33, EMT OD 55.80)	designed	knockout-gauge-1-2*

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

NEMA	Engineering Department Bulletin No. 71, Knockout Diameters and Fitting Dimensions to Assure Mechanical and Electrical Continuity (Tables 71-1A, 71-1B; reaffirmed 2011)	web	NEMA publication; dimensions are facts
McMaster-Carr	Knockout bushings (per-part CAD; the standard body publishes no CAD) (free STEP/IGES per part, for your own design use; no redistribution)	STEP, IGES, PDF	McMaster-Carr site terms: personal design use, no redistribution verify

Free files

★ Free files 3 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json

knockout-gauge-1-2-to-2in	knockout hole gauge plate, 298.5 × 78.7 × 3 mm, 6 through holes at the NEMA Bulletin 71 nominal knockout diameters for the 1/2 to 2 in trade sizes (22.225, 28.169, 34.925, 44.044, 50.394, 62.713 mm), smallest first, 8 mm between hole edges; plate thickness, spacing and corner radius are Xem choices	STEP · STL
knockout-gauge-2-1-2-to-4in	knockout hole gauge plate, 429.4 × 133.9 × 3 mm, 4 through holes at the NEMA Bulletin 71 nominal knockout diameters for the 2-1/2 to 4 in trade sizes (75.413, 91.288, 104.775, 117.881 mm), smallest first, 8 mm between hole edges; plate thickness, spacing and corner radius are Xem choices	STEP · STL
knockout-gauge-5-to-6in	knockout hole gauge plate, 342.3 × 189.1 × 3 mm, 2 through holes at the NEMA Bulletin 71 nominal knockout diameters for the 5 and 6 in trade sizes (145.263, 173.050 mm), smallest first, 8 mm between hole edges; plate thickness, spacing and corner radius are Xem choices	STEP · STL

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Plain text: sheet.txt · JSON: /api/geometry?object=conduit-knockout-diameters-nema-bulletin-71