A double tap is a term used to describe when two wires are attached to the same breaker or terminal (also called a lug). Unless the terminal or circuit breaker is designed for more than one conductor, it’s not a good idea. Overheating of the wires is possible. When I use the term double lugging, I usually reserve it for times when the main supply terminals to an electrical panel are double tapped. This is a fairly common and potentially serious defect.
Can you see the double tapped terminals in the panel above? When a panel is full, at capacity, how do you get more circuits? One solution was double tap the main lugs to supply power to an electric stove. That is not ok. The problem is that the stove is not protected by a breaker, so if something goes wrong– a short, over-heating of wires, etc–there is nothing to shut it off. The stove should probably have a 50 amp breaker.
Here above is another double tapped main lug, it was created to supply an auxiliary panel. The system needed to be upgraded for more capacity but the owner, or electrician, or likely a handyman found a way to add on with out changing the panel–but this is not acceptable. The double tapped conductors have no fusing to protect them if they were to become overloaded. Can you see the double tapped circuit breaker also present in the panel? Other problems?
Ok, here is another double tap, this time at a main disconnect. First off, this disconnect panel is not safe for residential use, you could get shocked trying to change the fuse, even with the disconnect off. Second the terminals are double tapped and they were not designed for that–If a terminal is designed for more than one conductor, it will say right on the terminal how many and what size conductors can be installed. And Third, if you look at the white wire in the center you can see that copper and aluminum conductors are double tapped. Aluminum and Copper are never combined under the same lug or attachment with out specialized equipment for connecting dissimilar materials.
The photo above has a neutral lug double tapped. This example not an overcapacity issue like the ones above. In these two photos, the neutral conductors are attached to the wrong lugs. The service entrance neutral goes through the main panel (above) and double taps in the subpanel (picture below). And a neutral for a electric stove is tapped in there as well. At least they were both copper.
So anytime you see a double lugged main terminal, there is likely a problem.