Not sure about amp rating for relay in horn circuit.

Discussion in 'Automotive Electronics' started by 11487209, Sep 21, 2016.

  1. 11487209

    Thread Starter New Member

    Sep 21, 2016
    I am wiring horn circuit into a Yamaha Big Bear 400. 12V battery and a 1.5A horn. I would like to know what size relay and fuse to use in the circuit. Wire gauge sizes are 20 to 26.

    I am also curious how this is computed. Is it just based on load amperage?

    Any help is appreciated.
    Thank you.
  2. #12


    Nov 30, 2010
    Yeah, it's just amperage. It would be hard to buy an automotive relay that won't carry 1.5 amps. The last horn I wired used 10 amps.
    There are internet charts for machine wiring but I'm not that good. I just use the house wiring specs I know. If a 12 gauge wire can be fused at 20 amps, a 22 gauge wire can be fused at 2 amps. Still, that's pretty skinny wire to survive vibration on a vehicle. I don't think I'd do that, just for mechanical reasons.
  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    The horn is probably electromagnetic and so inductive in nature. This puts more stress on the relay contacts, so to extend the life of the relay contacts put a snubber circuit across the contacts. You should be using a relay intended for automotive use. Unless space is at a premium be generous with the contact current rating, perhaps 10A .

    Having said all that you (or at least, I) don't use the horn very often so contact life may not be a problem.
    I bought a new car a few years back and it was nearly a year before I discovered that the horn didn't work!
  4. 11487209

    Thread Starter New Member

    Sep 21, 2016
    Thank you. I will probably stick with the 22 gauge wire. It will be run through conduit, zip tied, and all connections soldered. It sounds like 10A will be ok for the fuse and relay. Thank you again for the help.
  5. bwilliams60

    Well-Known Member

    Nov 18, 2012
    I think the lowesr wire gauge we use in automotive industry is 18 gauge. I wouldnt go any smaller than that. I would run 16 gauge and be safe, fuse it with 10 amp and use a 20 amp standard Bosch relay with internal diode suppression.
    Good luck with your circuit.
    shortbus likes this.
  6. MrSoftware

    Senior Member

    Oct 29, 2013
  7. tcmtech

    Distinguished Member

    Nov 4, 2013
    No need to overkill the design. :rolleyes:

    Any 12 volt 2 amp or higher rated switch or push button will die of weathering and old age before you horn wears out the contacts. Any standard 2 amp automotive fuse is fine.