A 400-ampere electric service is typically metered with a combination of a watt-hour meter and a current transformer (CT) for accuracy and safety purposes.
The watt-hour meter measures the amount of electrical energy that is consumed by the customer and records it in kilowatt-hours (kWh) for billing purposes. The CT works in conjunction with the watt-hour meter to reduce the amount of electrical current flowing through the meter and to provide a safe and accurate measurement of the electrical load.
The CT is typically a transformer that is installed around one or more of the service conductors, and reduces the current flow in the secondary coil to a level that can be safely measured by the watt-hour meter. The watt-hour meter can then accurately record the amount of electrical energy being used by the customer, without being damaged by the high current levels that may be present in a 400-ampere electric service.