Electrical Metering for Linemen and Techicians


Single Phase 3 Wire 120/208 Network 


(Form 12S Meter)


A three phase 4 wire Wye transformer bank provides this type of service.  The windings in these overhead transformers must be paralleled.  Notice from the previous diagram, that only two phases and the neutral are required to serve this load.  Typically, additional single phase and three phase services are taken from the transformer bank. All three phases have appropriate phase to ground voltage levels (120V) for use in lighting circuits; therefore, load can be balanced more easily than from a 4 wire Delta bank.

Key Points for Metering:

  1. Be certain the bottoms of the potential coils are connected to the neutral.
  2. Since the potential coils are connected from phase to ground, use a 120V meter.
  3. Specially designed single stator meters exist for metering this type of service.  Check with your metering manufacturer if you wish to weight the alternatives to using the form 12S meter.

Common Mistakes:

    1. Often, a form 2S 240V (house meter) meter is mistakenly used for this type of single-phase service.  Although this is a 3 wire single-phase service, the form 2S single stator meter is incapable of dealing with the phase angles provided by this three-phase transformer bank.  Approximately 75% registration will result during balanced loading conditions resulting in a loss of revenue.
    2. The phase to phase voltage for this service is 208 volts.  Customer equipment connected phase to phase must be appropriately rated for 208 volts not 240 volts as in the typical 3 wire single-phase service.
    3. Many linemen refer to the voltage level of 208 volts as a wild leg voltage, which is an inappropriate term for this service.  208 volts is simply the common phase to phase voltage for this service.  Wild leg is a term associated with a 4 wire Delta service.