How much 90 wire do you need before splicing on to 60 degree wire? This is not well defined and my answer is entirly inferred from other sources. Almost all product standards use 1.2 meters of wire as the default length to dissipate heat introduced at 1 end of the wire. Breaker tests use 4 feet. pot lights only use about 18 inches of 110 degree wire to get to the external JB where 90c wire is required.
When inspecting older homes I regualry reject new lights on old wiring. except for luminaires with a canopy and pendant most luminaires now have a 90c minimum temperature rating. so where new high temperature fixtures are installed on old 60c wiring it is pretty common to see no insulation on the wires if they are even touched. by splicing in 4 feet of wire between the old box and new luminaire we generally avoid this issue but the old JB is all to often in the exact location they want the new JB and light to be mounted. Not too hard if you can lift the box from the attic but the extension can be very hard to accomplish. It is usually best to find a luminaire that can be connected to 60c wire.
