Fuse and wire size for added horn.

[DanFelix]

I’m adding an additional horn to my ‘18 JL Sport 3.6l. I would like it to always work, even when the Jeep is not running, just like the current horn. I am unclear as to what size fuse and wiring I should use. The horn is a 12V DC horn. How do I calculate the fuse and wire size? Thanks.

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[LDSSILLS]

Look at the manufacturer's specifications. It should tell you the amperage draw of your new horn. .

Match that to your fuse size. Be realistic if the current requirement is 1lets say 14-amp you would use a 15-amp fuse as a 10amp fuse may not carry the current load. If the current is 15.8 amps you would still use a 15-amp fuse. Try to stay within the nearest fuse value to your horns current draw. Because if you increase to let's, say 20amp fuse for a 15.8 current draw, you could run the risk of a fire in the wires.

Found this on the net and it seems appropriate for wire gauge vs length of wire. See below for why this important.
https://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/

Many folks don't realize that current drops over a length of wire because of wire resistance. So not only do you have to consider correct wire size for your current draw (Component Amperage requirements) you must make sure the source of the current is the same at component. )

Another thing folks don't realize is insulation is not just for "assurance the wire won't short out" Insulation also deters fire because if the fuse fails and the current applied to the wire causes heat. To much heat and you have a wire short and a fire, potentially a big one. Depending on how the wire is routed. Good news as wire size increases or decreases so does the insulation type and size.

Granpa Larry

 

[1Evil55]

I want to see video of the train horn

 

[DanFelix]

Look at the manufacturer's specifications. It should tell you the amperage draw of your new horn. .

Match that to your fuse size. Be realistic if the current requirement is 1lets say 14-amp you would use a 15-amp fuse as a 10amp fuse may not carry the current load. If the current is 15.8 amps you would still use a 15-amp fuse. Try to stay within the nearest fuse value to your horns current draw. Because if you increase to let's, say 20amp fuse for a 15.8 current draw, you could run the risk of a fire in the wires.

Found this on the net and it seems appropriate for wire gauge vs length of wire. See below for why this important.
https://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/

Many folks don't realize that current drops over a length of wire because of wire resistance. So not only do you have to consider correct wire size for your current draw (Component Amperage requirements) you must make sure the source of the current is the same at component. )

Another thing folks don't realize is insulation is not just for "assurance the wire won't short out" Insulation also deters fire because if the fuse fails and the current applied to the wire causes heat. To much heat and you have a wire short and a fire, potentially a big one. Depending on how the wire is routed. Good news as wire size increases or decreases so does the insulation type and size.

Granpa Larry

Excellent. Thank you. Unfortunately, I cannot find any amp info on the horn or its packaging. Will examine further. Again, thanks.

 

[THAW]

Excellent. Thank you. Unfortunately, I cannot find any amp info on the horn or its packaging. Will examine further. Again, thanks.

The rule I use:
Size the fuse to protect the wire; size the wire to minimize voltage drop for given device current over required wire length.

In your case (not knowing device Amp draw), match your added wire to the (inner) gauge of the wires already on the horn. Use a wire gauge chart (I like the Blue Sea version) to size the fuse and ensure your wiring run to the battery isn't too long for the wire gauge.

 

[DanFelix]

The rule I use:
Size the fuse to protect the wire; size the wire to minimize voltage drop for given device current over required wire length.

In your case (not knowing device Amp draw), match your added wire to the (inner) gauge of the wires already on the horn. Use a wire gauge chart (I like the Blue Sea version) to size the fuse and ensure your wiring run to the battery isn't too long for the wire gauge.

Very helpful, particularly in the absence of a listed amperage. Thanks.

 

[Mguy]

I’m adding an additional horn to my ‘18 JL Sport 3.6l. I would like it to always work, even when the Jeep is not running, just like the current horn. I am unclear as to what size fuse and wiring I should use. The horn is a 12V DC horn. How do I calculate the fuse and wire size? Thanks.

There are tests that can be done before permanently installing an electrical device of unknown amperage (that is, of unknown current draw). For a device believed to be of relatively low amperage, the easiest one is,

Take a wire of moderate gauge, say 16, at a total length of the circuit you're creating, and cut in half. With engine off but the switch on so that system voltage on the dash appears, hook up device (with the now two wires) directly to the main battery. Observe voltage drop. if a few tenths or less use wire gauge cut for the test, or perhaps test again with something a little smaller. If more than a few tenths voltage drop, test again with a larger wire.

A practical consideration, wire of too small a gauge can be difficult to work with. Smaller than 18 can be tedious.

An electrical point, wires of too high a gauge or too long a run do not create "current drop" on circuits. It's voltage drop which is of concern. Current capacity is a rating for wire to avoid resistance that can cause dangerous heat in the wire itself. Consider also, too small a wire might be safe and allow the device to work, but it's resistance may cause, in addition to voltage drop, increased amperage in the circuit--which unnecessarily drains the power source. Note, post #2 above has this wrong, but it seems the poster got it from the link cited, which also has it wrong.

Edit 2/1/2025 The article on the web page from the link in post #2 no longer has references to "current drop." Undoubtedly AI at work. The article has old comments from readers, who apparently were not as smart.

 

[Rogersocal]

There are tests that can be done before permanently installing an electrical device of unknown amperage (that is, of unknown current draw). For a device believed to be of relatively low amperage, the easiest one is,

Take a wire of moderate gauge, say 16, at a total length of the circuit you're creating, and cut in half. With engine off but the switch on so that system voltage on the dash appears, hook up device (with the now two wires) directly to the main battery. Observe voltage drop. if a few tenths or less use wire gauge cut for the test, or perhaps test again with something a little smaller. If more than a few tenths voltage drop, test again with a larger wire.

A practical consideration, wire of too small a gauge can be difficult to work with. Smaller than 18 can be tedious.

An electrical point, wires of too high a gauge or too long a run do not create "current drop" on circuits. It's voltage drop which is of concern. Current capacity is a rating for wire to avoid resistance that can cause dangerous heat in the wire itself. Consider also, too small a wire might be safe and allow the device to work, but it's resistance may cause, in addition to voltage drop, increased amperage in the circuit--which unnecessarily drains the power source. Note, post #2 above has this wrong, but it seems the poster got it from the link cited, which also has it wrong.

I agree with you.

If its a 15 amp horn rating, install a fuse and wire that is rated for 20 amps(length of run isn't usually an issue in smaller cars but can be depending on the way its installed. Hot engine areas typically need one AWG size bigger depending on actual temp and length.. blah blah blah.). A 15 amp horn on a 15 amp fuse will cause it to blow for no reason. But again, Fuse should match or be lower than the wires rated capacity to make sure the circuit and wiring is adequately protected.

As far as tedious.. Unfortunately I work with 2 to 28awg, twisted pair, shielded and double shielded 8-cond, Can-bus and more on a daily practically. I do love the smaller gage, detailed work and especially when working smt and circuit board stuff.

 

[Flip]

Excellent. Thank you. Unfortunately, I cannot find any amp info on the horn or its packaging. Will examine further. Again, thanks.

You could contact the manufacturer to find out the correct amperage. I've done this a few times.

 

[Mguy]

I agree with you.

If its a 15 amp horn rating, install a fuse and wire that is rated for 20 amps(length of run isn't usually an issue in smaller cars but can be depending on the way its installed. Hot engine areas typically need one AWG size bigger depending on actual temp and length.. blah blah blah.). A 15 amp horn on a 15 amp fuse will cause it to blow for no reason. But again, Fuse should match or be lower than the wires rated capacity to make sure the circuit and wiring is adequately protected.

As far as tedious.. Unfortunately I work with 2 to 28awg, twisted pair, shielded and double shielded 8-cond, Can-bus and more on a daily practically. I do love the smaller gage, detailed work and especially when working smt and circuit board stuff.

I do try to deal with wire protection separately from device protection. Sometimes with device fusing it's trial and error, hopefully blowing lower rated fuses until finding a higher rated one that lets the device work as needed. Typically motor devices, like pumps, benefit from that kind of operator patience.

And about patience, I just don't have it for work that requires a magnifying glass. My hat's off to you!

 

[LDSSILLS]

zooming down the road yesterday I had thoughts about your horn. Here is what you can do and it's safe. Wire it for 20 amps, 12AWG wire should do it over a 20-foot length. Then just use a 10-amp fuse. If that fuse does not blow, you're safe. If it does, then go to a 15-amp fuse. same thing. If the 15 amp blows go to a 20. If that blows you have a circuit problem.

I am willing to bet on a horn single wire circuit application you will be protected with a 10-amp fuse.

I googled amperage for big diesel semi-truck horn for worst case application. Answer was 10-20 amps. Really high powered was 30! Which you won't have. I am sure what I mentioned with the 12awg and fuse will work.

There yah go.

Grandpa Larry

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