Is the heat divided into more than 1 circuit? For 5 kw of heat the amp draw is approx 22 amps. My guess is you have 5 kw hooked to the fan circuit and the other 10 kw is on its own circuit. If that is the case you need a 60 amp breaker for the 10 kw and would need wire size of 6
I have been informed by building codes that I will require a 4 guage 4/3 to complete this task... That a 6 guage is not recommended to carry the load safely. Thank you.