GlennD wrote:The ground and neutral can remain #8.
So it's not actually clear to me whether the neutral on a branch circuit is allowed to be undersized relative to the ungrounded conductors. Certainly it can be on a feeder, and certainly it often makes sense from a loading perspective. However, the NEC has specific language regarding downsizing the neutral on a feeder, and I'm not aware of any corresponding language on branch circuits.
As to the EGC (ground), a #10 copper is good up to 60 amps (Table 250.122).
GlennD wrote:It also makes sense to run a neutral wire even though it is not needed for an EVSE.
I agree if installing a 14-50 receptacle, however it makes sense to me to install a 6-50 receptacle and skip the neutral.
GlennD wrote:A 50A outlet should be wired for 50A.
I'm going to have to disagree, as a corollary would be that you can't use a receptacle for a 40 amp circuit, i.e. all 40 amp circuited EVSEs would need to be hardwired. This makes less sense to me than simply understanding that when you see a 50 amp receptacle, it may not be on a 50 amp circuit.
BTW, if using any 75C insulated wiring method (basically anything other than NM), then #8 Cu is good for 50 amps, in which case it would not be required to rerun the wiring should a 50 amp circuit be required in the future.
Do you happen to know if there are any internal differences between the HCS-50 and the HCS-40, other than the pilot signal? Is the HCS-50 of beefier construction? If so, that would be an argument for using a 40 amp breaker with the HCS-40, in that it will provide a greater level of safety against a failure in a car charger causing a current draw in excess of the limit advertised via the pilot signal. Not so likely, of course.