Re: L5-30 wire for 220V?
IF this outlet is the only thing on the circuit, IF this circuit is served from the service origination where the neutral and ground are bonded, and IF the device is expecting a ground and not a split phase service to derive 120 volts plus 208/240, then you have a situation that isn't dangerous (though definitely wrong and illegal), - it's a semantics issue if the wire in question is a neutral or a gound since electrically they would be the same thing.
If any of the above aren't true, you are connecting your safety ground to a current-carrying conductor, which causes several bad things. The least bad situation is that there will be a voltage potential difference between the chassis of this device and other equipment that is correctly grounded. The much more bad situation is if a neutral wire fault develops somewhere in the chain, the chassis of your gear will suddenly be energized with anywhere between 0 and 208/240 volts, depending on the load distribution between legs. This kind of thing is a great way to fry ministers in baptistries.
Safety grounds can't be current carrying conductors.
Most of the 3-wire outlets that are of the hot/hot/neutral configuration were for appliances that had external grounding means - water pipes, etc. Mercifully these are a lot less common now.
