Lots of good comments here so far, but a couple of notes:
When I first plug in my model 3 it closes the contactor in the Wall Connector and shows the voltage on the screen before it ramps up the charging current. So if you look immediately after plugging in you can get the unloaded circuit voltage and then watch as it ramps to see where it stabilizes out. If you are getting a lot of drop there (which five volts is not really a lot at 240v) then the question is where are those being lost? They could be lost upstream if your main panel... A poor connection somewhere on the link back to the service transformer or undersized wiring coke cause this (outside of your control).
As others have mentioned, utility voltage fluctuates during the day. It all depends on how far you are from the substation you are and how good your utility is. My utility is fantastic and I am close to the substation, so I see very little voltage fluctuation, but it still does happen!
Fun fact: The utility transmission grid varies a bunch in voltage due to load and generation on the grid. We depend heavily on “online tap changers” or other voltage control units at the serving substation to bring the voltage into a good usable range. If you ever look at voltage graphs of delivered voltage from the grid you will see them operating constantly during the day. They typically change in two volt increments up or down.
Nice job on the install. I would not be worried! (Just to verify: You used copper wore right?)
Yes copper 444 with a 6 ground
