Shunts are rated by the maximum current and voltage drop at that current. For example, a 500 A, 75 mV shunt would have a resistance of , a maximum allowable current of 500 amps and at that current the voltage drop would be 75 millivolts. By convention, most shunts are designed to drop 50 mV, 75 mV or 100 mV when operating at their full rated current and most ammeters consist of a shunt and a voltmeter with full-scale deflections of 50, 75, or 100 mV. All shunts have a derating factor for continuous (more than 2 minutes) use, 66% being the most common, so the example shunt should not be operated above 330  A (and 50  mV drop) longer than that.