Keep in mind that we are mixing "code" and "safe." Code covers many scenarios (wire in a well insulated wall, house in a very hot climate, etc.) There is margin built in for safety. A 60 amp draw on a 6 gauge romex is 9% over its rating. This means ~19% more heat in the wire. Is this to code? No. Is this safe? That's a different debate. Since the vast majority of the members here aren't qualified to judge electrical safety, I think it's best to keep to code. As far as the car preventing a fire by noting the voltage drop, that is not something I'd rely on. It will happily charge at 48 amps with a 5 volt drop. That's 240 watts being dissipated in the system. If that heat is spread out across a long wire, no big deal. If that heat is in a small area, it is more than enough to light a fire. A 30 watt soldering iron can easily start a fire. Edit: I just noticed that Johnny Vector covered some of this while I was typing my reply.
NMD90 is simply Non Metallic Dry locations rated for 90 degrees C NM-B is simply Non Metallic and the b stands for a rating of 90 degrees C, NM-B is designed for Dry locations Both have the same type jacket and insulation and 6 gauge copper is 6 gauge copper so what is the difference if they are not the same? Personally I cannot see any difference in the specs except that the NM-B is not CSA certified therefor not acceptable in Canada and visa versa for the NMD90
NM-B is rated for 60C in the USA, not 90C. Although it is rated at a 90C conductor temp, its ampacity is limited to 60°C according to the NEC. From what I read, the 90C is for derating calculations, fixture terminations, etc. Regardless of that, when all of the calculations are done, you cannot exceed the 60C ampacity rating. For our Tesla HPWC to operate at its 48 amp max, it's simple. 60A breaker on a 4 gauge 60C wire (e.g. NM-B) or 6 gauge 90C wire (e.g. THHN in conduit, MC). No need to over complicate it or push the limits with a high current circuit.