7,500W / 240V = 31.25A, rounds to 32A, which for continuous use (like a heater) gets a 40A breaker. 8 AWG is standard for 40A. So everything matches perfectly.
Which means, there is something going wrong. That could be:
- Heater drawing more power than rated.
- Wiring problem resulting in extra resistance.
- Bad breaker.
While 6 AWG wire is lower resistance than 8 AWG wire, a 30' run shouldn't be a problem at all.
A bad breaker is unlikely, though not impossible.
A brand new heater going bad is unlikely, but a little more plausible than a bad breaker. While the heater is basically a giant toaster, there are internal connections for the controls, fan, etc. and if any of those is bad then it could produce quite a bit of extra heat in the wrong place, or it may result in a partial short circuit. (A full short circuit would simply be an instant breaker trip.)
But my money is on a wiring problem. A bad connection could result in high resistance at one spot, which would waste energy ($), create heat (fire) and (fortunately!) trip the breaker if it is bad enough. A bad connection could also result in a partial short circuit, which would increase the total current used. Actually, mildly bad (turning 32A into 40A) would be the worst because it would take a long time to trip the breaker. My guess is you are significantly over 40A, perhaps 50A. If your heater/wiring pulled 100A then it would trip much faster than "a couple of minutes".
Turn off the breaker. Carefully check, and recheck, every splice, wire nut, screw connection, etc. from the breaker all the way to the heater. All it takes is one bad connection to ruin everything. Wire nuts are cheap - check the size and replace them if they are the wrong size or show any signs of melting/arcing/burning. All connections, except ground wires, should have no bare wire exposed - stripped wire should be entirely inside wire nuts or under screws.
If that doesn't solve the problem, use a clamp meter to measure current on the hot wires when the heater is running. If you show something close to 32A then you may actually have a bad breaker. But I bet you'll find much higher current and the problem is either the heater itself or the wire connections.