The meter installation type refers to how the meter is wired into the electric system and what other auxiliary equipment is necessary for the meter to work properly. There are two types we will discuss: self-contained and transformer-rated.
Self-contained
Smaller residential and commercial electrical services typically make use of a self-contained meter installation, which refers to a meter that is installed directly into the meter socket or base in series with the customer’s electrical service. Self-contained installations are also sometimes called a series installation since all the power used by the customer goes directly through the meter. In this case, there is no additional ancillary equipment needed to accurately measure the customer’s usage.
Because the meter in a self-contained installation is installed in series with the customer’s service, the electrical service is de-energized when the meter is removed. Before the advent of smart meters, utilities had to send someone to the property to disconnect a customer by removing the self-contained meter and placing a cover over the opening in the meter base. When a new customer moved into the building (or an overdue bill was paid), a return trip to the site was required to re-install the meter in the meter base to re-energize the service. With a smart meter, both the disconnect and reconnecting function can be accomplished remotely.
Self-contained installations are typically used for single-phase services of 200 amps or smaller and three-phase services of 320 amps or smaller and for single-phase voltages of 240 V or less and three-phase voltages of 600 V or less.
Transformer-rated
For customers with loads that exceed the rating of standard meters, additional ancillary equipment is required to accurately and reliably measure the customer’s electrical usage. This situation is common for large commercial and industrial customers as well as for components of the distribution and transmission system such as substations and generation switchyards. In a transformer-rated installation, the meter is not installed in series with the service since the customer's usage exceeds the level that would destroy the meter.
Because all the amps or electricity cannot be run through the meter, a special transformer is required to measure the customer's usage. These instrument transformers can be either current transformers, CTs, or potential transformers, PTs. The purpose of the instrument transformer is to reduce the current or voltage of the power flowing through the meter. The meter then calculates actual power usage or demand using ratios. For example, if an instrument transformer reduces the current or voltage flowing through a meter to 10% of actual flow, the meter would then multiply the reading by 10 to get an accurate number.
In a transformer-rated installation, the meter and associated ancillary equipment are usually installed in a special cabinet near the service transformer. Unlike a self-contained installation, a service with a transformer-rated installation will still deliver power to the customer when the meter is removed. So, this installation requires a lock-out device to allow the utility to lock out service when the meter is removed.