stacking ground lugs

[tkb]

Could it have something to do with the amount of surface area of the lug that is in contact with the mounting surface? The bottom lug would obviously have the entire surface in contact but the top lug would only have the surface area of the lug below it to mate up to.

So you mean something like this?

From page 465 of the 2011 White Book (Wire Connectors - ZMKQ):

INSTALLATION
Wire connectors are intended for use in installations covered by ANSI/
NFPA 70, ??National Electrical Code?? (NEC), and should be installed using
the prescribed manufacturer?s installation instructions.
Stacking of connectors (multiple connectors assembled using a single
bolt, nut and washers) may be permitted where mechanical interference is
reduced or eliminated with the use of offset tangs, stacking adapters, and
the like. The surface contact area of the mounting tang should make complete
contact with the mounting surface or the previously stacked connector.

So it is permissible IF you use lugs designed to be stacked or some sort of adapter. Notice that the Thomas and Betts catalog lists both stacking and non-stacking lugs.

Only the tangs would have to be stacked to be installed per UL listing.
The photo shows a UL complient installation that is done every day and has been done since Thomas Edison was a kid.

 

[infinity]

Only the tangs would have to be stacked to be installed per UL listing.
The photo shows a UL complient installation that is done every day and has been done since Thomas Edison was a kid.

Yup, very often when the lug is installed on the bus stab only about half of the lug is in contact with the bus. Many lugs have a ridge on the back that only allows half of the lug to be in contact with the surface. The ridge also keeps it from turning while tightening the lug.