Equipment data for short circuit calculations

The first step in analyzing a power system is to get the data for the power available at the site, the utility data. This data can be obtained from the power company. When calling the power company, explain the type of information you need and ask for the engineering department. It may be helpful to explain why you need the information.

The power company will be able to supply this information for the point in the power system where their responsibility for the power system ends and the customer's responsibility starts. A common location for this point is the secondary of a pole or pad mounted transformer. If the customer is responsible for the transformer, the transition point would be the primary of the transformer. Sometimes a pole mounted disconnect will be the transition point. The power company will specify where in the system their responsibility ends.

The data needed is the line to line voltage (V_LL), short-circuit current (SC KA), and X/R. Obtaining the voltage is simple enough. SC KA is the short circuit current available at a bolted three phase fault. "Bolted fault" means that all three phases are connected together with no added impedance. X/R is the ratio of reactance to resistance in the supply. SC KA and X/R may need to be derived from other data.

Power factor (PF) is sometimes specified instead of X/R. This must be the short circuit power factor. Power factor is defined as the cosine of the angle between voltage and current. X/R is the tangent of this same angle. X/R can be found from power factor by taking the tangent of the inverse cosine of the power factor.

This is all the data needed for the utility.

Transformers are specified by output voltage (V), KVA rating, percent impedance (%Z), and X/R ratio. The X/R ratio is the ratio of reactance to resistance. This information, with the exception of X/R, is usually on the transformer nameplate.

If X/R is not specified on the nameplate, the transformer manufacturer may be able to supply this. When contacting the transformer manufacturer, it may be helpful to have the transformer serial number. If X/R cannot be obtained, check the SCA V1.0 program Help menu for reference values that can be used in the calculations.

Data needed for cables is resistance, reactance and length. Cable resistance and reactance data is available in cable standards and manufacturer's specifications. The program will automatically determine resistance and reactance values per unit of length based on the selected cable gauge, material, configuration, temperature rating and conduit type, or let you enter numeric values for cable resistance and reactance into the corresponding date entry fields. Lengths may be measured, paced off and estimated.

Motors are a source of short-circuit current. They will act like generators when a short-circuit occurs. At a minimum, motors must be specified by horsepower which will be listed on the motor nameplate. Motor specifications are as follows:

kV: Motor rated kV.

HP: Motor horsepower. Motors may be represented individually or as a lumped group.

FLA: The Full Load Amps.

PF: Motor operating power factor. This is used with the Efficiency to determine kVA. Motor groups should use an average value of power factor.

EFF: Motor operating efficiency. This is used with the Power Factor field to determine kVA. Motor groups should use an average value of efficiency.

kVA/HP: The motor or motor group kVA rating per horsepower. This is a simple way of defining the kVA when full load amperes, or efficiency and power factor are not known. Typical values for standard efficiency motors are listed below:

Motor FLA Calculations

There are three ways to calculate motor Full Load Ampere (FLA) rating. The FLA calculation methods are shown below in priority order:

If the kVA is known, the motor FLA should be determined by the following equation:

If the motor KVA is not available but motor HP and (kVA/HP) values are known, the motor FLA should be determined from the following equation:

If both the KVA and the (kVA/HP) are not available, the motor FLA should be determined from the following equation:

Motor Impedance and X/R Ratio for Short Circuit Analysis

Reactance to resistance ratio X/R and impedance for short circuit analysis %Z can be read off from the motor nameplate. If motor X/R and/or %Z is not specified on the nameplate, the motor manufacturer may be able to supply this. When contacting the motor manufacturer, it may be helpful to have the motor serial number. If X/R or %Z cannot be obtained, check the SCA V1.0 program Help menu for reference values that can be used in the calculations.