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Version 6.10
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Utility Transformer Characterization and Chart

The utility transformer characterization is the key component for establishing the starting available short circuit current (SCA or AIC) of any distribution system. Volts' Utility Transformer Characterization module offers a complete format to enter basic values while obtaining a complete and accurate characterization and beginning short circuit current values.

Key Features:

  • Beginning SCA or AIC ampacity from base kVA rating, secondary voltage and percentage of impedance using either known or estimated values.
  • Ability to include percent of impedance 7.5% tolerance in the computations.
  • Establish or compute the base line power in MVA.
  • Establish or use the utility transformer primary voltage as the base voltage.
  • Establish or use the utility transformer power (kVA) as the base power.
  • Establish or use the utility transformer XR ratio as the base XR ratio.
  • Select the exact transformer Delta / Wye configuration.
  • Establishment of the cable derating impedance value based on your actual ambient temperature.


Knowing that only a certain amount of information is given concerning the utility transformer's characterization, Volts is equipped with a chart format of IEEE Std 141 national standards transformer values to help "fill in" the blanks of unknown values. Although this chart lists all the transformer values, it is important to use any specific values supplied by the utility company. Therefore, Volts will not overwrite any entered values upon posting the chart values to the characterization screen. The chart lists both single and three phase transformer values for:

  • Impedance (%Z)
  • % Reactance (&X)
  • % Resistance (%R)
  • Reactance / Resistance Ratio (X/R)

  These utility transformer values begin the high accuracy of Volts' Short Circuit and Arc Flash analysis reports.

Accuracy In, Accuracy Out


It simply cannot be any easier or more accurate to obtain the correct information the first time.


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Reprinted with permission from NFPA 70 2008, NEC® copyright© 2008, National Fire Protection Association. This is not the complete and official position of the NFPA on the referenced subject which is represented only by the standard in its entirety. The title National Electric Code® and the acronym NEC® are registered trademarks of the National Fire Protection Association (NFPA).

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