Guitar cabs are not usually 8 ohms any more, but 4. So 100 watts (DC, ignoring capacitance effects for the moment) will be the square root of 25 - ie 5 amps.

Many guitar leads will act as fuses at that current rating!

Many bass amps are nowadays up to 1KW - so at 4 ohms that will be the square root of 250 - ie about 16 amps.

However, a halfway grunty PA rig will have about 3Kw on the subs, and the subs bridged to run at 2 ohms, so that is going to be about the square root of 1500 - ie nearly 40 amps!

Twin and earth is also not a good idea since the "twin" is usually single core. Nice and free from capacitance efects for installation work, but it tends to fatigue and break if frequently uncoiled and re-coiled (particularly the way many musicians do it).