I bought the safety horn kit that comes with the viair 275c compressor/tank combo. i need a switched on power source. I would rather not splice into any wires. I would like to use a power tap that goes into the fuse box or a cigarette lighter power adapter. How many amps would i need?
Be very careful about wiring high amp loads into a fuse box. Fuse taps are great for smaller stuff but I wouldn’t ever wire a compressor that way. Run a wire from your battery terminal to where you have the compressor. Put a fuse in-line to that wire (there should be one supplied). Make sure you have an equally good path and wire back to the anode (i.e. negative terminal). If you tap into the fuse panel using a circuit not designed for at least 20 amps, you’re going to melt or fry something.
Thanks. I have decided to go to a local customs shop that does stereo installs. I don’t wanna go screwing anything up. I contacted them and they said they would do it.
Never mind they want to much money lol. I think I understand what im doing a lot better now. My question should of said my pressure switch needs an ignition on source. my compressor is going to a relay that goes to the battery. I just wanted to use a fuse tap for the pressure switch that needs an ignition source. What did you use as an ignition on source?
Anyone?
I’ll chime in here, How I did it on both my car and my dads car was I went to the auto parts store and bought whats called an “Add a Circuit” I looked for a fuse that would be switched on only when the ignition was turned to the “On” position my case I chose the “Cruise Control” Fuse and removed it then popped the fuse onto the “add a circuit” and installed it onto where the cruise control fuse was. From there I ran it to a toggle switch, then, to the “Ignition On” signal or “switched” on the pressure switch/relay combo which I assume you have. I then ran a power wire with an inline 30 amp fuse directly from the battery to the “+ from battery” wire, then ran a wire from " To Compressor" directly to the compressor. I grounded the compressor and relay/pressure switch combo to a good ground (Usually metal part of car that is unpainted). It may seem a bit intimidating but if you ever had a hobby doing a car stereo install, this is very similar to that. 
edit, pics!!!
My power wire for one of my compressors…
the “Add A Circuit”
what it looks like…
switch when ignition is “Off” or “ACC”
switch when the ignition is turned to the “ON” or “Run” position.
Pressure switch/relay combo
The power wire that you seen earlier from the battery
The wire that runs from my Toggle switch and the “Add a Circuit”
Wire that runs to the compressor from the “Compressor +”
Random pic of my compressors…
And How I grounded everything…
Hope this gives you an Idea on how to wire it up 
OK great I’m on the right track. I got everything wired just like that. Just need to do an ignition on source. Thanks for the answer.
I popped some pics up there to help also ^^^^^^^^ Welcome
So did you use your OEM horn button or put your own button in?
I wired it to a toggle switch on the oe horn. That way i can switch between the oe horn or the train horn.
How many amps is your add a circuit? I read that it doesn’t take many amps just to be the relay trigger. My add a circuit is 1-10 amps. Is this enough?
It takes very little amps to trigger a relay, really does not matter what amp fuse you tap into as long as that is all you are doing.