What sort of device plugs in to this outlet?
That appears to be a NEMA-6 30 receptacle. According to the National Electric Code, it must be used only for a 30 amp circuit. Look for a 30 amp, 240 volt circuit breaker in you box. Unless you find another receptacle like that in the house that is connected to the same breaker, the entire 30 amps is available for use by whatever is plugged in there.