The rule of thumb I use is simple. For speaker wire, buy the fattest / thickest wire you can afford, that is to say the cable with the greatest cross-sectional area ( usually shown in square millimetres ( such...
So you think that you connect your expensive amplifier to your expensive loudspeakers with a couple of bits of wire found in a box in the shed. Or do you take the advice of the rather nice chap in the HiFi...