You have 6 AWG THHN wire rated at 75°C with a base ampacity of 65 amps. You are installing it in a boiler room where the ambient temperature reaches 104°F (40°C).
When wires are bundled together (in a conduit, a cable tray, or even a single jacket like Romex/NM cable), they radiate heat onto each other. The more wires you pack, the less heat can escape. derating wire
Common wire types: