Instrument transformers, commonly known as current transformers (CT's) and potential transformers (PT's), are special "metering transformers" used to step down the voltage or current to levels a normal watt-hour meter can handle. Standard self-contained watt-hour meters can generally handle currents up to 200 amps and voltages up to 480 volts. It can still be used if the customers service requirements are larger than this, but it must be installed in conjunction with instrument transformers .
Instrument transformers are typically found in metering installations where larger commercial or industrial customers are served. Instrument transformers are also commonly used in substation metering for measuring electricity flowing back and forth between two utilities. In short, any service with currents or voltages which exceed the current and voltage rating of self-contained meters, usually 200 amps and 480 volts, must use instrument transformers.