How many amps will it be safe? I heard someone say 25 amp breaker doesn't mean it will handle 25amps continuously, rather, it means it can handle up to 25amps of instantaneous surge in power usage.
If by "safe" you mean "won't get tripped" then generally the I^n rating should be for continuous use. I'll assume that you've got at least 14awg on that circuit to be "safe" in the "won't burn my house down" sense of the term. ;-)
Edit: To be on the safe side I'd go with #12 or #10 wire
I personally ran a 6ga 240v line for my rigs...overkill until I hit 15 rigs but the cost was not much higher.