Form 12s Meter Wiring Diagram

The form 12s meter can be one of the more confusing meter forms out there. So, here I want to provide a form 12s meter wiring diagram in two different configurations. One is the 120/208 network service.  The other form 12s meter wiring diagram is the form 12s meter in a delta service.

Form 12s Meter Wiring Diagram Network Service

The form 12s meter can be used with the network service. The network service comes from taking two legs off of a three phase wye transformer and using the neutral. So, if you measure voltage from each phase to ground you should get 120v. Also, if you measure voltage from phase to phase you should get 208v.

This service is typically found in businesses that are located in a downtown area or businesses that are located in an industrial park. The reason for this is that many of these businesses do not require a full three phase service. So, the utility will pull two legs and a neutral off of the nearest wye transformer and call it a day.

Looking at the diagram you notice that there are two yellow terminals. These are either or terminals. Meaning that you connect the wire to one or the other in the meter socket. Also, some meter sockets do not come with this 5th terminal pre-installed. You have to purchase a 5th terminal separately and install it yourself.

With some form 2s meter sockets there are provisions to install a 5th terminal. The form 12s is also known as a self-contained meter. This means that the meter is installed in series with the load. Pulling the meter will also turn off the power.

Form 12s Meter Delta Wiring Diagram

For the form 12s delta wiring diagram we are going to talk about the service. This is the diagram for using a 12s with a delta service. Note that this is a three phase service. If you notice there is no difference in how it is actually wired. I used different colors here to denote the difference but it is actually physically wired the same way.

This could be a 120v, 240, or 480v delta service. My recommendation, as always, is against using a self-contained meter for voltages above 240v however. In the center of this diagram following the blue wire again this is an either or connection. You can connect the wire to the left center terminal or the bottom center terminal. Also note that the stab on the back of the meter is interchangeable.

Since this is a three phase service when measuring voltage across any of the three phases you should get the same voltage.

As always remember that the colors in the diagram are for illustration purposes only. You should always use the colors your utility uses.


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  1. I test electric meters for Cooperatives and city utilities.
    I have a customer that has a service that is coming from a wye service. It is 120v on each leg and 208 across lugs in a 4 llug meter socket.
    They have a class 2s 240 volt meter in that meter socket.
    In my opinion and after reading this article, this meter would not meter properly!
    It does not have a place to put a 5th neutral lug. I’m suggesting to get a 12s 120v meter with a 5fh lug. They could get a wire and connect to 5th lug and connect wire to the neutral or I call ground terminal in meter socket .
    I have done this before to others and seems to work fine.
    Please give me your opinion.

    • You are correct. You need to have a form 12s meter in that installation. A fifth terminal needs to be somehow added to the meter base whether the customer has an electrician put it in or the utility does it.

  2. A 12s meter would also meter a single phase 120/240 service correctly even though its not the most economical metering installation.

    • You are correct sir. A form 12s meter will correctly meter a 120/240v single phase service correctly even though it is not the industry standard.

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