So I am an electrical engineer and spend a lot of time working with high speed pulses and many different types of cable. So this topic interested me. I did a little bit of digging and found conflicting claims about the resistivity of solid vs stranded wire. I looked at some wire tables that showed the resistivity for solid vs stranding (many different strand counts) all for the same AWG. According to the wire tables, for the most part the stranded versions are slightly more resistive than the solid. What I didn't find was much in the way of explanation for why that might be true. The first important fact is that for a given gauge, the cross-sectional area of copper is the same or very close to the same. However, with stranded wire, the strands are helical so in a 1 foot length of wire the strands are more than 1 foot. On the other had, all the strands touch each other. The current will always take the path of least resistance so the current has to "decide" whether it is easier flow down the longer strand or transfer from strand to strand. For that to happen, the surface condition (oxidation) and contact pressure between the strands is important. I think that in the end, the difference in resistance is pretty small, especially for the lengths of wire in our typical cars. There is probably more stray resistance is poor solder joints and in any connectors we use.

So the skin depth at 4 kHz is about 1mm and that is already less than the wire radius for 10AWG and even 12 AWG. At 20 kHz, the skin depth is about .4mm. So skin effect is in play i.e. the current is not uniform across the wire. Bigger gauge wire always wins here because it has more surface area. One could argue that the stranded wire has more surface area, but as I mentioned above, the electrons have to transfer between strands which probably adds some resistance.


Bottom line is use whatever type of wire you like. I doubt that it makes any measurable difference in performance if all other things are equal. I do know from my own personal experience that 10 AWG wire can be pretty hard to solder because it conducts heat away from the joint so quickly. So I think the focus should be more on good soldering to motor/ESC tabs and connector pins.