I am wondering, if I have a multi tap ballast and want to run it on 240v would I use the COM wire as a hot/load along with the 240v wire hot/LOAD then green as always as earth?

I honestly don't know.
Made a step up transformer out of a old 400w metal halide ballast to run my 3 phase fluroscent ballast.

That will work fine ! It's exactly how I'm running the Wide-Lite .

Yes, you can as the "Common" on 120 or 240 Volts AC is really a hot half of the time, and an common the other half of the time. It`s Alternating Current you know!

Yes, you can as the "Common" on 120 or 240 Volts AC is really a hot half of the time, and an common the other half of the time. It`s Alternating Current you know!

Yes, but with it alternating between the two so rapidly, you should always treat it as 'hot' 
Trust me, I've found out the hard way of not doing so... 

The neutral wire is bonded to earth at some point,  be it the nearest supply transformer, a rod in the ground or whatever, so unless there is a fault somewhere, it should never be 'hot' (although it's good practice to always treat it as such). The live (hot) wire isn't, so regardless of which way the AC current is flowing at that time, it's still 240V or whatever above ground potential.

Yes, assuming split phase 240V common in the US, one hot lead goes to common, and the other goes to 240V tap. This means the lamp base is live at 120V while the lamp is operating, so beware.

I believe that is why the Canadian code requires CWI ballasts for 240V applications.

I believe that is why the Canadian code requires CWI ballasts for 240V applications.

Indeed, same goes for the other line-line voltages like 208v, 480v, and 600v. It's also the reason why a lot of "multi tap" HID ballasts sold here are 120v/277v/347v instead of 120v/208v/240v/277v.

Yes, but with it alternating between the two so rapidly, you should always treat it as 'hot' 
Trust me, I've found out the hard way of not doing so... 

Right. I treat the both wires as hot on both 120V and 240V.

That's why I like the ballasts we have here, with terminal blocks and not trailing wires with bare ends. We don't usually get multi-tap ballasts in the same way you do as our voltages are limited to 220-240V as a rule, but some do have taps for 220/230/240/250V so there can be unused connections at times.

Right. I treat the both wires as hot on both 120V and 240V.

I got myself shocked at work wiring up a 1000w MH high bay to 277v. The junction box had 3 different lighting circuits inside . We had turned off the breaker for the circuit I was working on but unbeknownst to me , all 3 circuits shared the same neutral !! I grabbed the neutral wire thinking it was dead and got a shocking surprise !!

Eeeek    Good job you didn't end up deaded 

277v HURTS !! The best part is I was 35' in the air at the time in a boom lift ! Fortunately , I only had contact for a split second as I pulled away VERY quickly !

277v HURTS !! The best part is I was 35' in the air at the time in a boom lift ! Fortunately , I only had contact for a split second as I pulled away VERY quickly !

I`ve never gotten shocked by more than 120v supply, but I have gotten shocked by a compressor capasitor. What supply does 277v come from?