Car Horn Relay

Hi guys, i am in the process of upgrading my horn, and I have hit a wall with trying to figure this out.
I have a 2009 nissan versa sedan 1.8S, the factory horn is a single tone "meep-meep" type that is a safety hazard. I bought myself a pair of PIAA sports horns. The new horns require a 15A fuse, while my car has a 10A fuse from the factory. I know that I need to add a relay. What type of relay do I need, a SPDT or a SPST? Can someone help me plan out the wiring specifics for this? Thanks for all the help!

SPDT

You are simply going to use the factory horn trigger (honk wire if you will) to triger your new relay that

will make your new horns honk. Look in the relay section of this site for some ideas. Most horns are neg. trigger.

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This vehicle should already have a horn relay, look at the underhood (engine bay) fuse box (es, used to have a couple of Maxis and they had separate relay boxes). It will be marked either horn or with a trumpet icon. Just up the fuse to 15amps and use the existing wiring. I've added air horns, 20amps to the Masximas and my current Galant doing exactly rthe same thing, the relay is rated for at least 20amps.

Yup! just as howard said! The relay will handle the additional draw, just up the amperage of the original fuse!

And for what its worth, I agree.

FYI:
In general, increasing the size of a fuse is not recommended.
This is because fusing is usually there to protect the downstream load - in this case the wiring (as opposed to fuses in equipment).
And wiring size is usually matched to the fuse, or rather - the fuse should be the max Amperage capacity of the cable (though often with a 70% or similar safety margin).
But sometimes smaller fuses may be used. EG - in this case, maybe the circuit's cable/wire handles 20A and the existing relay 15A, but the original horn is only 5-8 Amp. Would it not be safer with 10A fuse? (EG - horns are often like speakers - a coil of wire. But being subject to weather, heat, solvents & vibration their winding could begin to short, therefore lowering resistance therefore increasing current, hence heat - then fire. So let's limit its current to 10A instead of the circuit's capability of 15A (or 30A - whatever it may be).

But what if your wiring etc is only rated for 10A and you will run more through it?

Since it's a horn and thus has intermittent use, overheating is very unlikely to happen. (Unless perhaps you are a Cab driver, or on the road near me.)

Yet again - remember the heat test. Bad or undersized connections & cables will get warm to hot.
Fuses often run warm and can be hot enough to burn fingers etc. That's cool - bad pun - I mean - that's acceptable. They operate by melting. (But a constantly hot fuse should be investigated.)

And FYI#2, although I see many horn circuits with SPDT relays, many get by with SPST - they only need to be connected or not (whether the relay earths/grounds a "hot" horn, or supplies the "hot" +12V to a grounded horn).    

Exactly, in fact I just had another look at my car,I left one original horn in,pulled the second horn's wiring back and fed it to my air horn compressor, kept the SAME fuse size, no problems, no extra relay yet I live in greater London, I'm very impatient, especially with school mums in SUVs and oldies in Civics (keep quiet I'm only 63), no problems, no fuse blows.

Thanks for the help guys (Howie you are the man!) Everywhere I read, most peoples opinion is that I should use a seperate relay and avoid increasing fuse sizes just like that. I havent done any car electrical work, so I would rather be safe than sorry. Can you guys tell me how to correctly wire a horn relay using a 15A fuse? Thanks

Just as a PS if your replacement horns are electro-mechanical i.e.normal speaker type instead of air horns, you don't even need a relay!