Rodent Damage
How Common Is It For Rodents To Chew Wires?
Rodents chew many things around a home. It keeps their teeth from overgrowing. It has even been noted that they may chew on things more after they have been poisoned.
But if you are asking this question because you have an outage or short circuit of unknown cause, realize that even if you have had rodents in your house, these electrical problems are still much more often due to other causes.
If you have an outage or tripping circuit breaker, I urge you to eliminate all other possibilities first, as explained in the Diagnostic Tree. When it is then clear that a breaker or poor connection at an outlet box, switch box, light box, the main panel, or a junction box is not to blame, then you can consider how to determine where a wire has been damaged in some other way -- including by rodents chewing on it.
Rodent/Wire Myths or Unsubstantiated Claims
Over the years I have solved many hundreds of outages and short circuits, with only five or six being due to rodents. So it is a myth that outages, lights flickering, and short circuits are commonly from rodent damage. Rodent-caused wire damage that does NOT result in these problems may be fairly common. Here are more myths or undocumented notions:
- "Many fires start from rodent-chewed wiring." I have not heard that anyone has data on this.
- "Rodents go after wiring particularly." A common reason a rodent will damage a wire to the point that you lose power is that the animal is trying to make its way from one place to another in a wall or ceiling, and it finds nice holes already drilled in studs or joists, but the holes will only be big enough to squeeze through if the rodent chews away some of this soft-coated round stuff someone put in these holes! So he chews.
- "Chewed wires will easily set sparks going." There are two ways sparks or heat might develop, and neither one would be a usual result of rodents gnawing on wires. One is poor contact along the circuit's path. In a wirenut or at the terminal of a receptacle, this can make some heat; but if a mouse bothers the continuity of a circuit, it will be by chewing the copper of one wire through. In that case the wire will either have no contact at all (so no heat will be made) or will be broken in place (also hard to sustain enough current to make heat). The other way damaged wiring could be a fire danger is by a short circuit being set up. When the outer cable-sheath and some inner wire insulation is gnawed away, this in itself does not let the wires touch each other (=short). They are still held apart by nearby parts of the cable that were not chewed on. No short circuit I was called to solve ever proved to be directly from chewed wires. It is true, though, that if such exposed wires are exposed to water (or rodent urine!), a somewhat conductive path can form between the wires, creating a short circuit then. If this is enough that the breaker trips, then the breaker has prevented any sparks from continuing. If the path is not conductive enough to trip the breaker, then you would have a fire hazard.
Story of Wires Chewed By a Rat in a Mobile Home
Here is what I recently had to do to end up finding rodent damage to a wire in a mobile home. This was the case of an outage on most of a general purpose circuit that extended from the panel in the laundry room to (in order of proximity to the laundry room):
- Kitchen lights
- Dining room light
- Bathroom lights and outlets
- An outdoor outlet
- One wall of a bedroom (which the owner was not aware was dead because she did not often use these two outlets behind furniture).
It took me a good deal of time to determine that the actual electrical order the wiring was done in was:
- An outlet beneath the laundry room, discovered after a while
- The wall of the bedroom
- Bathroom lights and outlets
- Outdoor outlet
- Dining room light
- Kitchen lights
In other words, the first dead item after a working outlet underneath the laundry room was way at the other end of the home, from which point power was fed back to all these rooms, back toward the panel. Mobile home manufacturers have their reasons, I guess. Once I knew what the last working item and first dead one were, I checked the connections at each (since the bad connection can be in the box of a working item just as easily as it can be at the first dead thing).
More time and trips were necessary for this job because this outage would come and go at will. Things would be dead for a few hours or weeks and then be back on for hours or days.
Finally convinced that a wire between the laundry room and the bedroom wall must be broken or (if someone had made an inaccessible splice in the wall) loose, I plugged a buzzer into one of the dead outlets and went all over the place, banging on the walls, high and low, and on the floor and ceiling, anywhere generally between those two points. On one wall in the bathroom about ten feet from the first dead (bedroom) outlet, my banging set the buzzer off. To confirm that I had found the right spot, I banged some more until banging a particular place on the wall made the buzzer stop. (If the outage had been permanent rather than intermittent, I would have had to try some wire tracers I have, to approximate where the wire was apart.)
I cut the wall open at that place and saw that the cable had been gnawed into, not only where it went through one stud (done by a rodent to fit himself through the hole) but also at the next stud I could see. Even though I could have solved the immediate problem by bypassing the spot where a wire had been broken (using new cable and junction boxes on each side of this damage), it was becoming clear that this skinning of this particular cable was probably not limited to two studs. In the future, conditions might be ripe for starting a fire somewhere else along the cable.
Since I knew the box this piece of cable came from and the one it went to, once I figured a way to run new cable from the panel to a box of this same circuit in the nearby kitchen (feeding power in the opposite direction from the way it had fed), I disconnected the damaged cable at both ends. With the new cable in place, the homeowner could sleep better at night, although the condition of the cables of other circuits is unknown.