Meter forms, or the form number on an electric meter are the numbers that help us as meter techs determine what type of meter should go in a particular service. There are a variety of form numbers that are used. There are also basically two types of services that metering installation will have. The most common is self-contained. The other is transformer-rated. The difference between the two is that that the transformer-rated service uses CT’s whereas the self-contained service does not. Blondel’s Theorem is typically used to determine what type of meter to install in each different type of service.
Here are the most common and the most common service types that they are used in.
Form 1s. A form 1s meter a meter that was commonly used in single phase, 120v two wire services. While there are still many 1s meters out in the field and there are utilities who still buy them, they are not nearly as common as they once were. The reason being that many homes that used this type of service converted to 240v either because they upgraded or because they were forced to by their utility. Utilities wanted to make the switch because it is so easy to put a 240v form 2s meter in a 1s socket. The two meters, to the naked eye, look identical. Without reading the nameplate it is hard to distinguish the difference. The meter base for a 1s is also wired differently than a 2s. However, an untrained eye may not catch this difference and install a form 2s meter by accident. Also, many utilities decided to convert many of the 120v services to 240v just so they could install a 2s meter. There is no difference for the customer as they still see 120v. There are just no wires connected to one of the load side terminals of the meter base. The most common use for the 1s meter now is for sign services which only require 120v.
Form 2s. The form 2s is by far the most common meter form out there. This meter is most commonly used in a 240v, single phase three wire service. This is the meter that most people will find on their homes. This form is also used for many small businesses as well. Click here for a form 2s meter wiring diagram.
Form 3s. Form 3s meters are typically used for single phase two wire services where the service is so large that it requires CT’s. They can also be used for three wire single phase services as well as single phase primary metering. A form 3s meter uses one CT. This is a transformer-rated meter and will not interrupt the customer’s service when pulled.
Form 4s. Form 4s meters are used for single phase three wire services with two CT’s. This is a transformer-rated meter that would be used when the service is too large to put in a form 2s meter. Here is a wiring diagram for the form 4s meter.
Form 5s. Form 5s meters are transformer-rated meters that can be used in a variety of different service types. They can be used in anything from a 4 wire wye service to a 3 wire delta service. The most common use for this meter and the only one that I recommend using for new installations is in a 3 wire delta service. The reason that I recommend using this meter for only one service type is that when you go through your billing system and start looking for errors, you will be able to tell based on the multiplier what service type is in the field as well as have a clue about the voltage. This can help you troubleshoot problems that come from the billing or meter reading department without even leaving the office. For instance, if I had an order that said that the display was out on the meter and the multiplier was 160, I would go ahead and take a 4:1 PT out in the field with me because more than likely it has gone bad.
Form 9s. The form 9s is most commonly used in a 4 wire wye service. This meter is also used in a 4 wire delta service, also known as a high-leg service. Here is a wiring diagram for the form 9s meter.
Form 12s. Form 12s meters are self-contained meters that can be used on a few different services. They can be used on 3 wire delta three phase services as well as single phase services that are pulled off of a three phase transformer. As an example, if you had a 120/208 4 wire wye transformer feeding a building and you had a customer that only wanted single phase, you could pull two legs and the neutral off the the transformer to get the single phase. This is also known as a network service. Here is a form 12s wiring diagram.
Form 16s. Form 16s meters are self-contained meters that are most commonly used to meter 4 wire wye services.
These are the most common meter forms that are used. Also, I would like to add that with the invention of the solid state meter came a feature known as auto-ranging. This feature allows you to use a form 9s meter in a 4 wire delta service. Formerly you would have used an 8s meter because of the high-leg. The same applies to the form 16s. Before auto-ranging a 15s meter was used to properly meter a 4 wire delta service. Now, the meter does the calculations internally and you only need one meter for each socket type. You no longer have to carry an 8s meter and a 9s meter or a 15s meter and a 16s meter. Furthermore, you no longer need to to carry a 120v 12s and a 240 12s meter.