Wiring a garage heater?
Your heater is 7500W, which is 31.25 amps, and because it's a heater, it must be derated to 125% or 9375W or 38.8 amps. 

That is too much for a 30A wire and breaker, but is "just right" for 40A wire and breaker.  That is designed that way on purpose.  

So you need to use #8 cable, e.g. 8/2 Romex.  And a 40A breaker.  The 30A/10AWG wires are not suitable for this heater.  

If you want a heater able to exploit the #10 cable and 30A breaker, pick one whose output is 5300-5500 watts (22-23 amps).  That, multiplied by 125%, will fall just under 30A.  

If you are connecting it via cord-and-plug, Code requires you use a 50A receptacle.  You can either use a NEMA 6-50, which does not include neutral, or a NEMA 14-50  receptacle (but then you need to run 8/3 cable). 

If you also want to plug in actual 50A loads, such as an RV or electric vehicle charger, then run 6/3 cable to a NEMA 14-50 and use a 50A breaker.