Trev, the point I was trying to make is that when you double the voltage, you increase the power dissipated by four. Chrisf1 is concerned that his new 6v horn might be damaged by going to 12 volts. I got sideways with the math but if Chris wants to protect his horn, he'll need to put an equal resistance in series with it; if it's a 0.6 ohm horn, the power rating of the resistor should also be equal and that calculates to 60 watts.

Without that resistor, the horn's going to draw 20 amps and dissipate 240 watts. I believe it's capable of that for short bursts, but "short" is a relative term.

David