One should never use a power cord under 16 gauge in a stereo system. 20 gauge runs have no place in a stereo system. 20 gauge is fine for some low wattage lamps and LED lights, but not a stereo component. Yes, it is a potential fire risk. It will also blunt dynamics and bass performance. It will sound lean lacking body and proper weight.
Now you can use 20 gauge on 5,9 and 12 volt DC power cables. The ones used to power modems, ethernet switches and such off of a linear power supply
Solid core conductors should also not be used. They stress and break over time with handling and use. This is especially true of thin 28 gauge conductors.
Lastly, it is not safe to rely on the thin Teflon coating of these conductors as the sole material between the L, N and G runs. Any small nicks, breaks, cracks in the wire can cause shorts and possible electrocution.