AAll of our AC ammeters can be used to measure 120V/400Hz current up to their maximum full-scale rating. This is the current that passes through their built-in CT's primary circuit (or the current passing through an external CT connected to our ammeters). However, the ac voltage that is used to supply operating power to the ammeter must be in the 50-60Hz range as specified in their product data sheet, under the 'Power Supply Voltage' section. Our AC ammeters can measure 400Hz currents, but they cannot be powered from 400Hz sources.