I am sure that most real estate agents have heard the term “double tap” so many times that I am likely to lose most of you due to not being able to bear having to hear about them even one more time.
Hang in there--because this will be a slightly different conversation about “double taps.”
First of all I need to clear up a “technical issue.” Technically an electrical installation that is generally referred to as a “double tap,” by many home inspectors and the general public, is really a “double lug.” In electrical installations a double tap is an entirely different thing and is typically a point where wires are spliced together---not typically related to two wires placed under the same screw terminal---as is a double lug (or multiple lug as the case many be for more than two wires).
Now the average real estate agent’s eyes will start to glaze over when the inspector goes into his or her spiel to their mutual client as to what is wrong with double lugging. The discussion will revolve around how when two wires are placed under one screw it is hard to be certain that equal pressure is being applied to both wires. If there is not equal (enough) pressure on one of the wires then there is the possibility of arcing at the connection. No one should need any education regarding the problems with arcing of electrical connections---unless of course you are doing welding and then arcing can be very useful.
The correction is remarkably simple and typically takes less time than it does to go through the whole process of making the agent’s eyes glaze over. It is a repair that can usually be done when the electrician is at the home doing something else. Correction is as simple as taking the two wires off the breaker and wire-nutting them together with an additional wire that is then run to the breaker. As a side note, there are some breakers that will allow two wires to be properly connected to them, but for our discussion here, we are going to ignore them.
Of course the bigger problem with double lugs is that they are an indication that the service panel may no longer be adequate to accommodate the electrical needs of the home---that or Uncle Homeowner was too lazy to go and buy another circuit breaker. So what turns into a 2 minute fix turns into a complete service change to the home.
There is one type of double lugging on circuit breakers that deserves more immediate attention than the typical installation of two same gauge wires under the same screw. It is not to hard to get one’s brain around the notion that two wires of different gauges (thicknesses) will very likely not have equal pressure on each wire. An even worse variation of this theme is when the wires are different types---as when one is solid wire and one is stranded type wire.
In the picture below we see three circuit breakers---the top two have double lugging.
The top breaker has two wires of the same gauge under one screw. The middle breaker has two wires under the same screw but one is solid wire and the other is stranded.
Where as you would have to be very unlucky for the top breaker connections to be a problem, with the lower breaker you would be pressing your luck and an arcing condition would be much more likely.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board