Fuse size does not dictate how much current a radio or any other device uses, but it limits the current that can flow in a circuit.

A 20A fuse will 'blow' (actually a metal filament or strip melts when it gets too hot from too much current) if more than 20 amperes flow in the circuit, and generally a 20A fuse will be used to protect a circuit that draws less than 20 amperes, say 15-16 amperes.

The wire is sized for acceptable voltage drop with this current. If the circuit draws more current, there is more more voltage loss and more heating of the wire. In extreme cases, say a 50A fuse and 10 ampere wire, it could get hot enough to start a fire and the fuse would not blow.

If nothing goes wrong in the circuit, a 20A, 30A, or 50A fuse won't matter. If something does go wrong, the larger fuse could allow dangerous overheating, and as pointed out, could allow a fire to start.

If you have a choice between being stranded, or using a larger fuse temporarily to get home, use the larger fuse, then go back to the correct size as soon as you can.

Oh, and fuses only 'blow' when too much current causes the strip to melt. If a fuse has blown, there has been an overload. Replacing the fuse may mean the new fuse will also blow, unless the overload has been removed.

M-B fuses can also corrode, usually the contact ends, so they don't pass current either, but that's a different story.


Best Regards,
Jim