Electrical Metering for Linemen and Techicians


Single Phase 240V 3 Wire


(Form 2S Meter)


This meter is designed to meter the standard 3 wire 120/240V residential service.  The form 2S meter does not conform to Blondel’s Theorem.  However, the two half-current coils along with a potential coil connected phase to phase, allow the meter to perform very well.

Note: Simply stated, half-current coils have half as many turns through the laminated iron core as a full current coil would have. 

Key Points for Metering:

  1. Each 120V (phase to ground reference) conductor must pass through the half-current coils.
  2. Although the neutral does not actually connect to the meter’s potential or current coils, as always, the meter socket should be securely connected to the meter socket for safety.
  3. Pay particular attention to the way the half-current coils are drawn in the preceding metering diagram.  You can never run two different phases through any metering device from the same direction.  This could be compared to clipping an ammeter around an extension cord to a lamp that was turned on.  You can’t get a reading because there is an equal amount of current flowing to and from the lamp at any moment in time.  Therefore, the ammeter can not detect a magnetic field.  This same principal applies to all metering devices.  Once again, never run two different phases through any metering device from the same direction!   One of the form 2S current coils passes through the stator’s laminated iron core clockwise while the other one passes through counterclockwise.

Common Mistakes:

  1. The form 2S meter is often miss-applied as a 3 wire Network meter.  Networking refers to using 2 phases and a neutral from a 4 wire Wye three phase bank to provide 3 wire single phase service.  The form 2S meter will only record 75% of the energy use under balanced loading conditions and percent of registration will vary as the load balance varies.  This meter can not deal with the three-phase phase angles present in this type of service.  The form 12S meter is required for self-contained network metering.   (See the 3 wire Single Phase Network diagram).
  2. This meter should not be used to supply 2 wire single phase service without appropriately re-wiring the meter socket.  (See Single Phase 2 wire w/form 2S meter).
  3. Energy theft is often accomplished by plugging this meter in upside down. The form 2S meter will run backwards if current flow is fed through the meter from the bottom of the current coils to the top.  In addition, the line and load side of a top load meter loop must be kept straight.  The term “Top Load Meter Loop” refers to a meter loop in which the current enters a weather head above the meter, flows down through the meter, back up the meter loop, out the weather head, and overhead to the customer’s load.  If line and load conductors are reversed, the form 2S meter will run backwards.