Ask three electricians how to size wire and you’ll get the same first answer: match the gauge to the circuit. But when you’re the one standing at the counter with a project in front of you, that answer isn’t much help.
Every spool of wire has a string of letters printed on the jacket, THHN, XHHW-2, NM-B, and a handful of others. They’re not random. Each one is a code that tells you exactly.
Pick up almost any spool of building wire meant for conduit and you’ll see two markings printed side by side on the jacket: THHN and THWN-2. They’re not two different products they’re two ratings the same conductor has earned.
If you’ve spent any time comparing wire and cable, you’ve run into the same question over and over: copper or aluminum? Both are legitimate, code-approved conductors. Both show up constantly in service entrance cable.