I'm attempting to connect a relatively small electric water heater in my pole barn. It requires a 60amp breaker, so my plan was to run 6/3 NM from the main panel (only required 6/2 but I decided to just get the 6/3 in case the next owner wanted to do something different) through the attic to the other side of the building (pole barn garage), come down through the ceiling and down the wall a few inches, and splice/convert to MC inside a massive junction box that is already connected to a piece of unistrut up near the ceiling -- primarily so I don't have exposed NM running 4-5 feet down the wall to the disconnect.
Once the splice from the 6/3 NM to the MC cable was made inside the junction, it would go from the junction box as MC to the disconnect switch, where it would then go to the water heater itself. In retrospect, I should have just run 6/3 MC from the main panel, through the attic, and directly to the disconnect. I wanted to save money with 6/3 NM for most of the run and here I am.
The problem I'm encountering is that the 6/3 NM from the electrical supply house came with 10awg solid copper ground, and 6awg stranded conductors. The 6/3 MC cable I ordered from an online supply house (local only had rolls of 500'+) has 6awg stranded conductors and 8awg stranded ground according to the person I talked to. It is my understanding (I could be mistaken) that you should not splice different-sized wires together.
Is it safe/to code to splice 10awg solid copper ground to 8awg stranded copper ground? If not, is there any solution other than running 6/3 MC from the main panel all the way to the disconnect switch? I've already run the 6/3 NM through the attic, secured it at proper intervals, etc. so ideally I would love not to have to take it all down.
EDIT: Wanted to add this is a 11kW water heater, solely to heat water for a reverse osmosis system to get the stated GPD rate. The stated GPD rate assumes 65 PSI inlet, 77F water, no more than 500ppm. The correction factor is .52 if you have 50F inlet water, which I do from my well.
Also, I have completely separate service to my pole barn (different transformer, meter, panels, etc.) than my house, and have done load calcs and have the room for 60amps without a problem.