I recently bought some new lighting fixtures and noticed the warning about needing supply wires rated for 90 degrees celcius. The old wire is knob and tube. I'm not sure if it can handle high temps, and it would be a major ordeal to replace the wiring. Can I try to pigtail new wire on the old knob and tube or is this warning just the lighting manufacturer covering itself against lawsuits.
A major problem working with obsolete knob-and-tube 14g wiring is that the old insulation on the wires becomes very brittle and will usually flake off when trying to work with them, exposing the bare wires and creating a fire hazard.
An example: My mother-in-law had an 1883 "National Register of Historic Homes" 6,300 sq. ft. colonial with K&T wiring that burned due to the faulty wiring of a new bathroom light fixture by a brother-in-law. That's precisely where the fire started, according to fire investigators.
And unfortunately, my elderly mother-in-law had let her homeowners insurance lapse the year before, resulting in a $600,000 uninsured loss.
A family dog died in the fire, but she and my brother-in-law escaped.
K&T is best left to pros who are experienced with working with it, IMHO, and should be replaced at the first opportunity.