There's no difference in resistance between solid/stranded of a given AWG.i have a bunch of 100 foot 12ga cords, and i cant run my heavy shit off them. Anything that pulls more than 12A is no go.
i think its the stranded, and the insulation doesnt help, because I can obviously pull 20A through 12/2 UF no problem.
I can actually pull 40A if i want to..... MWBC, but i try not to.