Although the cable sheathing and insulation on wiring eventually deteriorates, it takes a long time. The wiring that is currently manufactured is estimated to have an 80 to 100 year lifespan. Original wiring in pre-1940 homes that is knob-and-tube type will definitely have brittle and flaking insulation by now, but most post-war wiring should still be in acceptable shape inside walls.

    Older wiring in the attic is a different story, and is more likely to have problems for two reasons:

1. The extreme heat in an attic in the summer accelerates the the aging and progressive brittleness of the insulation of wiring manufactured before the 1980s, when the thermal resistance of plastic insulation was increased.
   
   
2. Also, wiring in an attic can be damaged by people crawling over it to do repairs or home improvements, and storage boxes being pushed back-and-forth across it. 

Rodents or squirrels in the attic gnawing on the wires can also be a problem, although we see this issue rarely. But we look for damaged wires, like in the photo below, whenever evidence of animal activity in the attic is found, such as urine stains, fecal pellets, and nesting material.


    Because wiring inside a wall is not accessible without tearing out wallboard, it is entombed and protected in place for life. Only an unlucky nail into a stud without the required protective plate over the electric cable crossing point has the potential to damage the wiring. Any electrical shorts or arcing inside a wall are more probable around the connections at the outlet and switch boxes that are accessible to meddling by weekend warrior home-improvers.

    Also, see our blog posts What causes copper wires to turn green or black in an electric panel? and Why is undersize electric wiring in a house dangerous?

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Here’s links to a collection of our other blog posts about ELECTRICAL WIRING:

• Which house appliances need a dedicated electrical circuit?

• What is the maximum spacing requirement for securing NM-cable (nonmetallic-sheathed cable)?

• Is it alright to just put wire nuts on the end of unused or abandoned NM-cable or wiring? 

• What are typical aluminum service entrance wire/cable sizes for the electrical service to a house?

• Why is it unsafe to bond neutral and ground wiring at subpanels?

• Should I get a lightning rod system to protect my house?

• Why is a strain relief clamp necessary for the cord connection to some electric appliances?  

• Does a wire nut connection need to be wrapped with electrical tape?

• What is the minimum clearance of overhead electric service drop wires above a house roof?

• What are the requirements for NM-cables entering an electric panel box? 

• What is the color code for NM cable (Romex®) sheathing?

• Why is undersize electric wiring in a house dangerous? 

• Why are old electrical components not always "grandfathered" as acceptable by home inspectors?

• How can I find out the size of the electric service to a house?

• What is an open electrical splice?

• What is "knob and tube" wiring?  

• What is the code requirement for receptacle outlets in a closet?

   Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.