You can look up the National Electrical Code (NEC) for free to search for wire and conductor ratings. The NEC has tables with wire ratings, including 310.15(B)(16).

Table 310.15(B)(16) gives you temperature and ampacity (size) ratings for different conductors. For example, number 10 wire at a 140-degree-Fahrenheit insulation rating is rated for 30 amps. However, when you have a 167-degree-Fahrenheit insulation rating with copper conductors, it’s rated for 35 amps.

We sometimes use non-metallic cable, which generally has a 140-degree-Fahrenheit insulation rating, so that’s why we say that number-10 wire can carry 30 amps.

However, there are certain types of wires and assemblies that have specific application concerns, and they may not be able to go into damp environments. Overall, we have to understand that ratings are NOT always what they seem; we have to look at applications and derating factors to make sure we’re using the correct wiring.