Connecting more than one wire on or under a single lug or connection point is called “double lugging,” and it is ONLY allowed in line voltage wiring under one condition, according to NEC 110.14:

If the terminal, lug, or connector is specifically rated for more than one wire

In the case of a conductor splice, like a wire nut or a split bolt, they are only designed for two wires unless they specifically state otherwise on the box, on the connector itself, or in the instructions/product data.

That means that wiring in a surge protector under the same lugs as the main or jamming as many wires as you can make fit under a split bolt or wire may be common, but it is not allowable according to NEC 110.14.

HVAC techs and installers will often double-lug contactors when making a repair, or they will connect to the closest and easiest point when installing 120v or 240v accessories, such as a UV light, humidifier, or air purifier.

In all of these cases, it is best to take a few minutes and find an approved and permanent method of making the connection instead of taking the easy way out.

It is also worth mentioning that some connections are rated for copper only and will be marked CU, while others designed for aluminum will be marked AL or ALR. Some will be marked as CU/AL, which means that either copper OR aluminum may be used but not necessarily that copper and aluminum may be MIXED.

There are very few connections that allow the mixing of copper and aluminum, and if they do, they must be specifically listed for that purpose.

—Bryan