...So, if I went with a heavier wire (with fuse) the electrical component is not at risk?...
And one last question...when I decide on the wire guage, will it be clear which size fuze to get?
The part at [b]risk with inadequate wiring is the wiring itself, or really its insulation, which can overheat and fail.
A much less important effect is that undersized wiring will cause too much resistance, which causes too much [b]loss of voltage (as Darwin mentioned), which means less left to run whatever is using the power, which then might not work properly.
When the current is determined, the minimum wire gauge is known. When you pick a wire gauge, you have determined how much current can be safely carried; the [b]fuse (or circuit breaker) must be rated at a high enough current to allow the device (light, appliance, whatever...) to work, but always no higher than the wire is good for. The fuse can just be picked to match the least capable wire or connector in the circuit (see Joe's ratings, for instance), unless the device needs the fuse in the circuit to be no more than some maximum size.
For [b]example, if a refrigerator
is on its own circuit (to make this simple) and needs 10 amps, and you follow (for instance) Colin's guide and use 10-gauge wire (good for up to 30 amps), then the fuse must be at least 10 amps (so the refrigerator
can run without blowing the fuse) but no more than 30 amps (for safety). As long as the refrigerator has its own fuse, you can use up to 30 A for the circuit, but the manual might say something like "must be supplied by a circuit with a fuse of no more than 15A", or something like that.
It doesn't make sense to me to fuse for vastly higher currents than would reasonably be expected, even if the wire could handle it safely, but others may disagree.
Sorry about the [b]terminology confusion: since the thing which is using electricity determines how much current (or power) flows, some of us sometimes refer to that device as "drawing" current; it doesn't mean anything different from the battery
"sending" current, or "pushing" current through the device, it's just a different perspective.
In this discussion, we have really only been concerned with [b]protecting the wire - not the battery
- because no normal load in a trailer could need enough current to be too much for the battery. A big inverter (for instance) connected by large-gauge wire (think car battery cables...) to a small trailer battery could mean that the battery would be damaged before a fuse blew. Matching components is a good thing...