I have a couple of questions regarding this install.
I have a Mini-soundbomb, which draws too many amps for the original Horn circuit. Works on direct wire to battery.
Therefore I need to install a Relay.
Im not paying for a kit so will make my own ( 
)
I need ;-
30 amp in line fuse.
relay,
connectors,
wire.
Q. Is relay required, a, Normally Open?
Q. What gauge of wire is require? A. (found) 12 gauge.
Although this is a 30 amp fuse the use is infrequent and the draw is only a little over 5 amps.
Q. Is this suitable?
Puzzled why it would use a 30 amp fuse, if it is only drawing circa 5 amp. I would ask them why.
Perhaps there’s a big initial current surge as it starts up?
I don’t know if ‘anti-surge’ fuses exist in the range of automotive fuses which would let you use a lower rating. Either way, you’d probably want a beefy relay that could handle the initial surge.
But what rating is the relay? Could it cope with 30 amp?
The one shown in the original post has ‘30 Amp’ in the title, so I guess it’s designed for 30 Amp 
Though having said that, it’s an E-Bay listing so I assume it’s some random Chinese item that may or may not be what it claims to be. Though for this use case, there’s probably no harm that could result if it wasn’t what it said.
I get this cynicism from a youtube channel I watch that often takes apart electrical items bought from E-Bay that are downright dangerous and/or not what they claim, so I personally wouldn’t buy anything from there.
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From what I gather, the fuse is there to protect the bike, not the horn. You need to run straight off the battery for the relay, if this shorts with no fuse it could damage some of the sensitive stuff. I think.
You should keep the fuse as close to the battery as possible
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Indeed. Fuses are designed to protect against the high current from faults and short circuits that otherwise might cause excessive heat/fire in the device or melt wiring.
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