Cadet 120v vs 240v Baseboard Heaters (Installed heater wont heat up fully. Basic wiring.) 

I mistakenly installed a 240 volt heater on a 12 gauge (yellow casing) dedicated circuit with a 20amp breaker. I should have installed the 120 volt heater. The result was a 240 volt heater that barely put out heat.
For the 240 you need a two pole breaker where both the white wire AND the black wire lead to the power breaker. Make sure you mark the white wire with black tape to show it is HOT.
From there you follow the excellent instructions in these videos:
For 120 there's this one for a SINGLE POLE on-board thermostat: • How to install a Singl...
For 240, there's this video for the DOUBLE POLE breaker where both wires are hot: • How to install a Doubl...