I have tested this a LOT and I did not use the relay! Stock circuit is fused for 5 AMPS. Bad Boy takes 15 on start up and drops to 8. I have laid on the horn for a full minute and did not blow the fuse nor did the wires or anything get hot!
Save yourself the trouble, you do not need the relay. Do it if it makes you feel good
Bob
Bob,
The 2013 RT's and newer may be a different story. I don't doubt that with a dedicated fuse for the horn circuit, and wiring of reasonable thickness, you wouldn't need a relay. However, on my 2013 there is a 20a fuse that is marked "Load Shedding". I discovered by looking at the wiring schematic that that one fuse feeds the horn PLUS a "Load Shedding Relay" which goes on to feed the the driver's heated hand grips, the passenger heated grab bars, the fog lights, the entire ASC (suspension) system, and the cruise control. Between the four heated grips, the fog lights, and the ASC pump, that's a fair bit of amp requirement, should they be all on at the same time. (For the 2014 model, I noticed they raised that fuse to 25a, but I don't know if they added any additional load on the "Load Shedding Relay".)
If I were going to add an air horn, I maybe could get by with swapping out that 20a fuse for a 25a one and not using a horn relay - especially since I never use the heated passenger grips. BUT on the 2013's and newer, the battery is right there near where you would mount the horn, so adding a relay for the horn would be a very simple matter.
As for the argument that the air horn needs a relay to work at full volume, thats a bit hard to reconcile with what I know of electronics. Any accessory is going to draw the current that it needs to work. As you say, if the circuit can't handle the current, what would happen is that wires would get hot and/or the fuse would blow.
Bottom line though.... given the circuit design of the 2013's and newer, and the fact that the battery is right there, its probably best to go ahead and use a horn relay, IMO.