You'd usually have multiple wires and split the current up. Also, if you design carefully you can let your wires get hotter than what would be recommended for i

You'd usually have multiple wires and split the current up. Also, if you design carefully you can let your wires get hotter than what would be recommended for in-wall wiring (what the calculators usually figure for). If you know you'll have airflow, and the insulation is rated better, you could run them hot.