What I Want
Fellow comes into the electrical section and asks for a 20 amp outlet, 14/3 wire, and a 30 amp 2-pole fuse. Apparently, he's hooking up new toy in his workshop. He's got coveralls and a painter's cap on, so he's certainly dressed the part.
Shame it's a costume.
I've seen electricity described as water running through a hose (the thicker wire/hose, the more current passes through), and that's a fair description if you want to understand current; but you also need to think of load (how much work you want the electricity to do). When you're talking about load, then electricity is much more like a rope: put too much load on a rope, and it snaps. In much the same way, if the draw of electricity through your wires is too high (the work load is too great) then your wires melt and burn down your house.
The right fuse stops that from happening by breaking the connection (it's also called a breaker) so electricity doesn't go through the wire any more - it "snaps" before the figurative rope does.
In some cases, 14 gauge wire can handle 20 amps of work load - something using a constant and predictable amount of energy, for instance. But most tools will have spikes in their energy needs, ones that are not predictable and obviously not constant. If you've ever used a circular saw, you know it works harder while you're cutting than when you're not, yes? That takes more energy, thus a spike. In these cases, 14 gauge wire may not be able to handle the spikes in power, burning your house down.
So if the tool you're setting up takes 20 amps to operate, do NOT put a 30 amp fuse on it and leave it at that! The breaker will let through as much electricity as the tool needs to operate, fine: but how's it getting there? If you guessed "Through wire too small for the work load", congratulations! Your house just burned down!
I wouldn't have minded so much if he hadn't mocked me for being concerned about his home, insisting that what he was taking was right. But it did do me a world of good to serve him again when he came back twenty minutes later to buy the right equipment.