I decided to tap the tool power inside the tool, not at the panel. Used an AC optosiolator (wired in parallel with the motor) into the arduino. The DC power control (I used an SSR instead of a relay) and microcontroller live in a box on the DC.
I also left extra inputs - you won't regret it. 
I added some ESD diodes, since I was running a pair of wires down 15' of hose that moves air at 1100CFM - could be some static, eh?
It's been running for a few years (the arduino-based circuit runs 24x7) with no problems at all.
cool project!
-j