Equipment is typically labeled with its voltage rating, and may advertise the amount of tolerance it will accept before degraded performance or damage can be expected. A unit that requires 230 V AC with a tolerance of 5% will not require a buck–boost transformer if the branch circuit (under load) is between 219 V AC and 241 V AC. Measurement should be made while the circuit is loaded, as the voltage can drop several volts compared to the open measurement. The transformer must be rated to carry the full load current or it may be damaged.