Sparky
There is always a design limit but it is not necessarily defined. It is the point where all the materials science runs out of data. For example a spec for sunlight resistance might require some type of insulation to withstand exposure to sunlight for 20 years. Well they don't stick the wire outside and wait 20 years. They stick the wire in a box and shine artificial light on the insulation for a period of time and at an intensity high enough to simulate 20 years of exposure. In the real world some will be installed in Arizona and some in Alaska. The UV protection is the same but the wire in Arizona is more likely to fail sooner than the wire in Alaska. The wire was engineered to last a period of time and any day after that is bonus.
So how long is a lifetime? I have heard that electrical equipment is engineered for something like 40 years. So we don't replace it at 41 years but the design was for 40 years and every day beyond is bonus. Our older houses were designed to accommodate a light bulb and a plug in every room. Most outlets beyond those installed for appliances were also expected to accommodate a plug in light since not that many other household appliances existed. We overloaded these circuits and overheated the wires. The insulation degrades and gets brittle from heat. Eventually the wire will lose it's protection and become a hazard.
There has also been the handyman adding and modifying the circuit as well as new hotter fixtures being connected.
The wiring system is on borrowed time. Even the newest K & T is at least 50 to 60 years old. It is all borrowed time. Sure there are houses where the old wiring is in pristine condition and serving the same function but there is no data to indicate the date of ultimate failure. Maybe it is 500 years or 200 but we all see that these old systems are on average, past the expiry date. Old wiring might last a very long time and well past the original design but like everything, it all eventually fails.
If only a product could predict its last day of operation and account for all the variables of use and abuse. We would wake up one morning and the circuit for the counter would just dissolve or stop working without all the safety hazards. The house computer could have called the electrician and arranged replacement dates and scheduled a work plan. It could predict what parts of the system are soon to fail and we would always have an up to date and failsafe system.
