Hello.
A string of 10 solar panels on a roof are individually wired to a terminal strip, to individual fuses, then to a second terminal strip for series/parallel connections.

Panels are 72 cells each, rated 285 Watts each, fuse rating below shows 20A max. What would be the minimum ? 10 Amperes ?
Wires from panel to first terminal strip are 5m long, unifilar AWG18 in metal duct.

 

What would be the minimum ? 10 Amperes ?

That's my read of it, that the panel can supply just over 8A. Anything more than 10A would signal that something is wrong. That's assuming no reflector that might increase the insolation over the standard 1 sol.

 

Fuse ratings are not absolute things, and are dependent on temperature; a 10A fuse could fail with 8A through it on a hot sunny day. You don't want to be replacing fuses more often than necessary, both due to the inconvenience, and the expense of the solar-rated fuses (around $4 each). It's not safe to use common AGC or automotive fuses, since those are rated for 32V DC maximum.

 

You would only use a smaller fuse to protect wiring or other devices attached to the system that can't handle 20 amps. Size the fuse to the maximum that the wiring can handle. 18 AWG is small for the task and it seems strange to me that anyone used 18 AWG to wire it. Any chance you can upgrade the wiring?

I think the manufacturer is saying that, even though the panel can't output 20 amps, everything in the solar panels, the output wiring, internal connections, etc., is rated to 20 amps. Therefore they spec 20 amps as the maximum fuse size.

Lastly bassbindevil brings up a very good point, use a fuse rated for the maximum voltage of the system, especially if you are wiring them in series.