One-Hole vs Two-Hole Compression Lugs and NEMA Hole Spacing
Sep 29, 2026
One hole or two is decided by what the lug bolts to, not by the conductor. Here is the rule, the hole spacing that trips people up, and how to read a bus pad before you order five hundred of the wrong one.
One-hole lugs
A single bolt through the tongue. Used on equipment terminals, disconnect and breaker lugs (where the equipment lug accepts a lug rather than bare conductor), grounding electrode terminations, and most panelboard and transformer landings. It is the default for 8 AWG through 500 kcmil on general construction, and the only choice where the pad has a single tapped hole or stud.
Two-hole lugs
Two bolts through a longer tongue, which stops the lug rotating under fault current and doubles the clamping area. Required on switchgear and switchboard bus pads, busway tap boxes, ground bars in data centers and telecom rooms, generator and transformer pads, and by most utility and industrial specs on 4/0 and above. Two-hole is standard on medium voltage terminations.
Hole spacing is the spec, not an afterthought
Two-hole lugs are made to NEMA drilling patterns. The common ones:
| Bolt size | Center to center | Where you see it |
|---|---|---|
| #10, 1/4" | 5/8" or 3/4" | Small ground bars, telecom bars, 8 to 4 AWG |
| 5/16" | 7/8" or 1" | Ground bars, small pads, 6 AWG to 1/0 |
| 3/8" | 1" | Switchgear pads, 1/0 through 250 kcmil |
| 1/2" | 1-3/4" | The switchgear standard, 250 through 750 kcmil |
| 5/8" | 1-3/4" | Large gear and utility pads, 500 kcmil and up |
Before ordering, read the pad or the equipment drawing for bolt size and spacing. Conversions Tech two-hole pages list spacing on the product; if the drawing shows something else, ask before you buy.
Ground bars
Ground bars are drilled on a fixed pattern (commonly 5/8" or 1" centers with 1/4" or 5/16" holes, or 1-3/4" centers with 1/2" holes on data center MGBs). Order the two-hole lug to match the bar, not the conductor. Two-hole lugs on ground bars are required by most data center and telecom specs (TIA-607) so the bond cannot rotate loose.
Common mistakes
- Ordering two-hole lugs by conductor and finding the spacing does not match the gear pad.
- Using a one-hole lug on a bus pad drilled for two, leaving a hole open. Inspectors reject it.
- Oversizing the hole (1/2" hole on a 3/8" bolt) to make a lug fit. It cuts contact area and fails torque.
- Mixing tongue widths in a stack of lugs on one pad. Match the family.
What to stock
One-hole standard barrel from 8 AWG through 4/0 in 1/4", 5/16" and 3/8" studs covers panels and disconnects. Two-hole long barrel in 1/0 through 500 kcmil with 3/8" on 1" spacing and 1/2" on 1-3/4" spacing covers switchgear and bus work. Every two-hole page on the site shows the spacing, the cross reference and the die index. Filter the compression lug collections by Number of Holes and Barrel Length. Same-day shipping on in-stock items.
Related: Which lug for which gauge · Long barrel vs standard barrel · Data center lugs and grounding
