Which Gauge Wire / Amperage to use?

[Diesel]

How do I know which grade of wire and fuses to use for a certain device?

Is its rating marked somewhere on the casing?

And if it says "draws 15A" - do I put a 20A fuse on 20A/30A wire?

Cheers, Diesel

[Brock]

Yes

[ruSTyEB]

Devices max draw should be up to 75% of fuse.  This is because a fuse can still blow after time (due to heat factors) even if lower than rated fuse. Eg 95% load may blow in 30-60 mins depending on ambient temp.  100% in a minute or so.

You want your fuse rated lower than the wire rating, so if there is a fault, the fuse blows before the wire melts.

[saaz]

Check to see if the 'device' has a peak power draw followed by a more constant power draw (eg HIDs)