Harris Institute of Technical Training

Have a general question I can't seem to answer adequately . Can anyone explain how a Form 2S meter with 2 120 volt phases used as the potential input, can accurately measure 120 volt loads? thanks in advance

Thanks for your question Mark. The Form 2S meter has a single potential coil and two half current coils. As you say, the meter potential coil is connected across the two 120 volt phases or hot legs that serve the customer making it so the voltage across this coil is 240 volts. The current coils in the meter are half current coils, which means that each coil provides half the meter disk torque that a full current coil would. This is to say that if you have 10 amps flowing in one or the other of the 2S meter (½) current coils you would only get 5 amps of meter disk torque from the actual 10 amps flowing through either meter current coil. In theory, a half current coil will have half as many actual physical turns when compared to the turns on a full current coil.

So now to answer your question, let’s start with an 1800 watt load (a space heater) that is connected at 120 volts. When you have this 120 volt load being metered by a Form 2S meter the full current (15 amps) flows through only one of the meter current coils (the neutral conductor completes its path). The full 15 amps flow in the half current coil but the torque that the meter disk is receiving from this current coil is only 7.5 amps.
The connected space heater load is 120 volts multiplied by 15 amps of current = 1800 watts
7.5 amps of torque multiplied by the 240 volt potential coil (full torque) = 1800 watts
If you have additional questions please let me know and I will do my best answer them.

On a related topic: The Single Phase Proofs and Truths book has diagrams and explanations that prove how a 2S meter measures 120 volt loads. It also details and proves other single phase metered loads on various self-contained and instrument rated single phase meters. This book and the metering proofs it provides is the basis for all advanced meter measurement theory and will increase your understanding of three phase metered circuits.