Air horn compressor current?

[Juzz976]

Fuses are selected as Circuit Protective Devices, thus includes A: Wiring and B: Connected Load.

If your using a 20A fuse to protect 20A Cable and your connect load will catch fire at 15A continous then you havn't acheived much.
Also a fuse has a fusing factor...how much fault current required for an instant blow or how long it will take to blow in overload conditions.
A 20A fuse wont blow at 20A.

Also if your connect your fuse in parallel to the main fuse (off Battery) you can either cook your alternator, trip alt fuse or drain battery if combined load exceeds the alternator output.

If you use a fuse where its current rating is high enough and the current drawn through it on top of the total current drawn from other sub circuits you'll blow ya fuse a few mins up the road.

Also an overload or sub ciruit fault can potentially blow the main fuse if there isn't descrimination between upstream devices. Ie a down stream device of the wrong trip/fusing factor and current rating for the upstream device supplying it.

Breaking capacities/Fault ratings don't come into it as there is veryvery little prospective short circuit current available.

In summary wiring it whack may cause you some greif, either today tmrw or 6months down the track....
I'd hate to be out of town on a ride press the horn and the whole damn bike goes dead, not cool.

[{.bLanK}G_o_D]

Fuses are selected as Circuit Protective Devices, thus includes A: Wiring and B: Connected Load.

Correct.
All those who said a fuse is only there to protect the wiring, you are sew wrong. Tisk tisk
Please never touch any high voltage equipment, I have seen so much dodgy wiring in my time it's scary. I've even seen questionable to dodgy stuff done by registered electricians.

[{.bLanK}G_o_D]

I think I might look into this further. Assuming a 12VDC load at 20A (the fuse size), what should the AWG size of the wire be?

How long is a piece of string? Or wire in this case.
Resistence in a conductor increases over distance.
Possably not relevent in the case of an air hone on a bike, your cables arn't going to be that long. But anybody working with electricity should know that.

[davereid]

Fuses are selected as Circuit Protective Devices, thus includes A: Wiring and B: Connected Load..

Absolutely true.

But the man wanted a simple fix to a simple problem, and he couldn't measure the current draw. Experience suggests that the horns will draw 15-18 amps. If the cable is rated at well over 20 amps and the fuse is located close to the battery, a 20 amp fuse is ideal.

Its quite true to suggest that a fault in the load may cause a fire at loads less than 20 amps. Such is the real world. The load can be disconnected by taking a finger off a button. The rider may crash, the fuel pipe may fall off, a tree may fall on your head, the emperor (most likely) has no clothes.

I would absolutely buy your switchboard. But I'd stick cable safe for 30A and a 20A fuse on my tooter, and sleep soundly.