I want to install a new lighting fixture outside. It says that the supply wire must be minimum 75C. I'm thinking it means 75 degrees Celsius/Centigrade. It's an older house (1950s) and I want to know if the wires it currently has are able to withstand the heat it may generate.
Joun Himself is on the money. FYI, it is the heat that the bulb produces that can cook the insulation that is on the wires. It may have come with some sort of insulation to prevent this from happening. DON'T, I repeat, DO NOT DEFEAT IT'S PURPOSE!
Most electrical codes require wiring for lighting to be rated at 90 degrees C. Make sure the fixtures either have insulation where it mounts to the outlet and/or the light bulb stood off the base by the design of the fixture. Coach Lamps are designed this way and would be ideal for your purpose. This type of design keeps the heat from the building wiring. Purchase only fixtures that are rated for outdoor use. Good Luck
The 75C is the temperature rating of the insulation on the wire. TW and UF wires are rated at 60C (140 degree F). TH wire is rated at 75C (167 degree F). The wire in your older house is most likely a 60C wire. The fixture manufacturer had the fixture tested and listed (think UL) with a 75 degree wire.
You actually don't have enough information here to answer the question definitively, but I'm 99% sure you are okay. The new outside fixture is probably less than 150W on a120V circuit with less than 1000W of existing load on the circuit. If the existing circuit already has 1600W of load on a 20A breaker or 1200W of load on a 15A breaker, you need new wire to handle the additional load, regardless of the temperature rating of the wire. The wire rating is how much the temperature of the wire rises above the outside ambient air temperature when carrying the full rated current. If you are not carrying 20A in the wire or you live in a really hot place like Las Vegas, the temperature of the wire gets nowhere near its melting point. Which is what determines the wire rating temperature rating which is based on a 30 degree Centigrade ambient air temperature. Even if the wire is 60C wire, it is tough to raise the wire temperature that much by current flow. Additionally, old wire was designed conservatively. What was called 60C in the 1950's is more like 75C now. Except for the few previously listed caveats, you are fine.
