Technically, stranded wire has a slightly larger diameter than solid wire of the same gauge to account for air gaps between strands. However, the ampacity remains the same. A 12 AWG solid wire and a 12 AWG stranded wire can carry the same amount of current.
Below is a comparison chart converting AWG to Metric sizes, along with their typical ampacity (current-carrying capacity) for standard copper wires. cable size chart
Low voltage systems are extremely sensitive to voltage drop. Technically, stranded wire has a slightly larger diameter