What is the most common electrical problem found during a home inspection? Many in the real estate business would probably answer double taps. I have come to this conclusion after being asked more times than I can possible count by the agent while I am inspecting the electric panel, "any double taps?"
While double taps, two or more wires on a circuit breaker, are a common defect and do not only apply to just the breakers, I have also come to the conclusion they are not the only common defect.
Second or sub electric panels are quite common in many houses. People add on or remodel and in doing so require additional electric circuits which the current main panel can not accommodate. Instead of replacing the entire main panel, not a terrible idea by the way, a second electric panel is added. The problem begins with who actually installs this new panel.
In my experience I would say about half the time the new service has been installed by the contractor or homeowner doing the construction, not a licensed electrician. The motivation is I'm certain cost savings. Professionals with knowledge and a license are after all expensive. And necessary.
An unskilled person will almost certainly make errors that render the new system unsafe. Further the work is undoubtedly not permitted, never inspected and ultimately completed with everyone believing everything is hunky dory.
While the double tap seems to reign as the most common electrical defect, it is not by far as dangerous as an entire electric panel wired incorrectly by a non professional.