I am installing a D/pole thermostat to a 5000w 240v Garage heater using 10/3 wire and a 30amp d/pole breaker.. question is.. where do hook up the neutral (white) wire too in the thermostat box
A standard 240v heater with no electronics dosent require a neutral the load is pure 240v. All you need are the red, blacks (hots) and the equipment ground. If you had 120v controls you would need the neutral but your thermostat dosent require the neutral so I would cap it at both ends and label it in the panel in case you want to add a fancy controller in the future.
That type of heater rarely requires a neutral. Double pole thermostats do not provide the capability to control the neutral wire. In the event your heater does actually need a neutral, there is no need to break it. It would not provide a service hazard at the unit. Just pass it through the stat box.