I'm installing a 240 volt electric baseboard heating system that has a total watts of 5,000. A 2500 and a 1500 watt unit will each have their own wall thermostat. Then there will be two 500 watt units each with their own unit mounted thermostat. I plan on using a 30 amp breaker and 10-2 wire. My question is, the units are spread out over quit a distance and I wondering if I can run 10-2 wiring to a central location and then branch off to each unit or thermostat with 12-2. Will this overload any wires and is it OK to put a junction box in the circuit like this?

Thanks

Yes you can splice the #10-2 at a junction box, and take off in two directions to the heaters. The total amps on the feeder to the box will be about 20-21 amps, the branches from the box will be lower, no fear of overloading.

Just be sure not to bury the junction box, as it will always need to be accessible.

Use very good splice connectors that will splice the #10 wires very good. The large blue "scotch-loks" made by 3M will work good for this. Loose connections here will heat up quickly due to the load.