Some links on selecting a proper fuse for a circuit -
Fuse Sizing - Learning Center - Powerlet Products
How to Determine the Fuse/Wire Size for YOUR Project | Top Forum Picks - Oznium Blog
Generally speaking, a 10A fuse should handle an 11A load for a much longer time than it could handle a 13A load without blowing. The extra time to blow prevents people from blowing fuses by accident. For example, an 18ga wire can handle a load of 15A, but for safety reasons, its practical current load is only around 8A. An 8A heating vest could be fused for 10A and could be wired with an 18ga wire, but the vest will produce more heat if the circuit uses a 16 or 14ga wire since there will be less voltage drop due to the wire's resistance. Think of it as a larger diameter hose allows more water to flow through it than a smaller diameter hose. Even though the 16 or 14 ga wire can handle a higher current load, the circuit in this case should still be fused for 10A since that is the closest fuse size for that load. The heavier wire reduces the chance of the circuit overheating and the 10A fuse will do a better job of protecting the heating vest since it only needs 8A power. Even though the wire size might support a larger current load, running a 15A fuse might mean the heating vest could be exposed to higher power levels than it is designed for. This why a circuit should be fused for an expected load, but the wiring should be sized for the next higher load as a safety precaution.
Circuit sizing is a subjective area, which might help explain why Polaris originally used a 20A relay "protected" by a 30A breaker to handle all 4 headlights ( 2 x 65W center headlights + 2 x 55W for the outer auxiliary headlights) which presented a cumulative load of 240W which is pretty much at the limit for a 20A circuit. Add in the extra power to keep the high beam shutter powered out of the way and the circuit is at the ragged edge of safety. If someone was running all 6 halogen headlights on the same circuit, they were definitely asking for trouble (LED bulbs have a much lower current draw and would have been within limits even with 6 normal strength LED bulbs).The bean-counters may have said everything was still within acceptable design constraints, but the real-world showed otherwise, so the circuit was redesigned. In my case, I switched to LED bulbs and rewired the outer auxiliary headlights to run off the Fog Light circuit, effectively giving me a separate circuit for each set of headlights, even when running the Canadian Headlights for a total of 6 headlights.
For more info, try Googling for "automotive wire size vs current load"
