Too many variables!!!! If installed properly in environment intended and well maintained, 25-30 years would be a good estimate. This for generally in a raceway or ductbanks installation.
For exposed or overhead installation I do not really know.
We are doing a project which includes replacing equipment and LV cables installed in 1949 in good operating conditions. I would consider it an exception.
In 1989-1994 I had the opportunity to do extensive work on a 15kv power system installed in 1944. The client ended up replacing a lot of cable that did not look good during testing, but had not yet failed. A large part of his reliability problems were related to failing splices in his system of PILC (lead-sheathed, oil-impregnated paper insulated) underground feeders.
Admittedly, properly installed PILC makes a very robust installation, impregnable to the hydrocarbons available in this particular environment, but it does give an idea of the lifespan.
In the same environment there are some original switchgear components and transformers still plugging away today, SIXTY years later.
In 1994, I tested a station service transformer, single-phase, 45kv-120/240v that was dated 1902. Still in service. Lightly loaded, but still in service.
Hoever, a betting man would say thrity years max, twenty years for a safe bet, assuming proper installation and a reasonable environment.
I just came from a school that was build about 40 years ago.
Although I didn't go and move it around, the wire looked fine. Several others of the same vintage looked about the same.
I have seen at industrial site where the insulation was 25 years old and if was cracked, although it was said that a combination of a very hot location and defective insulation was the cause.
My house was built in 1967 and the original wire looks ok, it's not brittle. That's 39 years and holding.
I've also seen houses built in the 70's that were real POS.
My conclusion? You gotta look at each situation individually. BUT, good wire in a good location installed properly will last a "long" time.
A couple of weeks ago I was on loan to another part of our company and down at an old power plant built in the 1950's. The switchgear, transformers, and most of the cable was original build as far as I could see. Other than some superficial rust on the exposed parts of the PILC/SWA cable armour as they entered the compound boxes, they looked in fair condition. Maintenance records didn't pick up abnormal levels of PD activity. In common with many things from that era, that plant was built up to a standard instead of down to a price. It certainly showed in the equipment fifty odd years after it was originally commissioned.
