Background: I am modifying an existing circuit in preparation to install a 120V heat-pump water heater. The reason it is 120V is because I'm participating in a research study testing the viability of HPWH in houses where it would be too expensive or impossible to install 240V circuits (the fact that I already have 240V available is beside the point - this way I'm helping the cause and I get to ditch my 15 year old gas wasting water warmer for free :) ).
My question is - can I mix solid wire and stranded wire in the same circuit?
I have an empty surface-mount 4" box next to the water heater location with 2 capped-off solid black #10 THWN2 that go directly back to the main panel. Conduit is 3/4" EMT. I could install a 5-20R with the two conductors and a single-pole breaker, but having a taped black conductor as neutral is sort of janky and I don't think it is code-compliant, plus it limits my future options.
I happen to have enough white stranded #10 THWN2 left over from another project. My plan is to pull this wire in to provide the neutral so that I can install the NEMA 5-20R I need for the study (plus an extra if I configure it as a MWBC with a 2-pole) and I'll already have the wiring in place so if in the future I want to upgrade to a 240/120V 14-30R all I need to do is change the receptacle and breaker.
The only issue is that the two existing conductors are solid and the new one would be stranded. I'm not worried about terminations because the panel end will land on the neutral bar and the duplex 5-20R I have is spec grade and allows back-wiring (i.e. straight-in screw clamp, NOT back-stab).
So - is mixing solid and stranded in one circuit kosher? I'd like to be able to use the wire I already have instead of buying more.