Harris Institute of Technical Training

I was wanting to know what kind of reading you would get if you set a 3 wire meter in a 2 wire service? Since the new AMI meters require 240 at the source side to register, I'm assuming they wont work. But what would an old analog cl200 read on this old service?

Thank you for your question. It is one that I have been asked often.

From the Meterman’s Handbook 9th edition Pg. 139 – (this information is missing in the 10th edition)
“With the improved voltage compensation on modern meters, some utilities use the standard three-wire, 240 volt, single stator meter (Form 2S) on two-wire, 120 volt services in place of the standard two-wire (Form1S) meter.…. By connecting the two current coils in series, the meter Kh constant and registration are not changed and the voltage compensation provides good performance at the 50 percent voltage operation.” [120 volts on a 240 volt potential coil (the metered accuracy of the connected load is close to 100% of the actual)]

This connection is detailed in Figure 12-14 on page 378 of the 9th edition and in Figure 12-19 on page 421 in the 10th edition. Where an A-base meter socket adapter is used to arrange the correct connection for the Form 2S meter (series connected current coils and the opposite potential for the meter potential coil connected directly to the non-polarity side of the coil). With this connection the load current of the connected customer is seen twice by the two ½ (half) current coils in the Form 2S meter and the 240 volt potential coil has 120 volt on it which leads to correct metering accuracy.

There are several scenarios where a Form 2S three-wire meter has been installed in error on to a two-wire, 120 volt service. With all of these scenarios there are return current path issues that will make it so the metered accuracy is truly unpredictable. With some of these scenarios the accuracy might be predicted to be close to 100% but this is mostly on paper and not in the real world. The best way to prove the error of the 2S when compared to the correctly installed 1S is to review the bill after the (1S meter) has been installed.

Please let me know if you have any further questions/comments. If you don't have access to a Meterman's Handbook, send me an e-mail and I will get you a diagram.