Reliably, 10-12 amps is about right. Although the specifications for the ZW state 275 watts, that's the maximum INPUT power, the efficiency of the transformer is probably at best 80% or so, and less at lower voltages. Remember, PW transformers were rated by input power, not output power. Realistically, you're looking at around 220 watts or so output power at full voltage. If you are running less than full voltage, don't expect the same wattage. If you think you can get 220 watts at 10 volts, for instance, that would be 22 amps, and the breaker trips at 15 amps. I'd probably assume the transformer is good for 10 amps at any selected voltage, you won't go far wrong with that value.