You can calculate the amp draw by dividing the amount of wattage by the voltage. For example, a 1000-watt hair dryer running on 120 volts produces 10 amps of current draw. Ampacity, a term formed from the words ampere and capacity, is the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration. You can use amp draw to determine whether the load on a circuit is safe. As a rule of thumb, do not load a circuit over 80 percent. You should limit the draw on a 20-amp circuit to 16 amps. Most household circuits have a 20-amp capacity, although some are only 15 amps.