Primary Substation % Impedance
Secondary Substation Impedance Data
Copper Conductor Cable Reactance and Resistance Data
Aluminum Conductor Cable Reactance and Resistance Data
Square D Bus Duct Reactance and Resistance Data
Rotating machine KVA and reactances for short circuit calculation

These values were taken from several sources including Square D "Procedure for determining short circuit values in secondary electrical distribution systes" by Russell Ohlson. The values below can be easily revised to any values that the user desires.

* - generally pertains to all voltages 600V and below

Note: NEMA industry standards allow a maximum manufacturing tolerance of +/- 7.5% of the specified transformer impedance value. However, any tolerance is usually ignored in short circuit calculations on the assumption that plus tolerance offers an extra margin of safety and minus tolerance is adequately compensated for by miscellaneous circuit impedances not specifically included in the calculations. But if desired, multiplying the specified impedance, reactance and resistance values by 0.925 will compensate for even the most adverse tolerance conditions.

Motor Specifications

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:

1.0 kVA/HP - Induction < 100 HP and 0.8 PF Syn Motors
0.95 kVA/HP - Induction 100 < 999 HP
0.90 kVA/HP - Induction > 1000 HP
0.8 kVA/HP - Synchronous 1.0 PF

Motor KVA Calculations

There are three ways to calculate motor kVA. Below the kVA calculation methods are shown in priority order:

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

KVA = (1.73)(FLA)(kV)

If the FLA is not available and the kVA/HP is known, the motor kVA should be determined from the following equation:

KVA = (HP)(kVA/HP)

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

KVA = (HP)(0.746)/(EFF)(PF)

Motor Short Circuit MVA Contribution Calculation

Table below displays ANSI Standard impedances and interrupting duty multipliers. Code numbers are chosen according to the motor types, sizes and modeling method. Using the ANSI Code is the recommended method to determine motor impedances used for ANSI Standard short circuit calculations:



Note: X" for induction motor groups >50 HP and <50 HP are typically assumed equal to 16.7%. Using the impedance multipliers, this corresponds to an equivalent motor contribution of 3.6 to 4.8 times the full load current.

X/R: The reactance to resistance ratio from the motor nameplate or manufacturer spec sheet [typical data].

X"dv: Subtransient reactance in percent on the motor HP base. Normally this is 16.7% for induction motors. Average subtransient reactances for three phase synchronous machines are:

0.09 for 2-pole turbine generators
0.2 for salient pole generators and motors
0.24 for air cooled condensers



Example: The short circuit MVA momentary contribution of the 15 MVA X"d=0.2 synchronous motor is equal to it's own MVA base (KVA time 1000) divided by it's own per unit impedance time the ANSI code multiplier from the ANSI Code table above:

SC MVA = (KVA)(1000)/(X"dv)(ANSI CODE MULTIPLYER) = 15 MVA / ( 0.2 x 1.0)
    = 75 SC MVA
X/R = 15

Arc Flash Study &
Labels per NFPA 70E

AFA V3.0 , V2.0

Arc Flash Label
Printing Services

Generator
Transfer Switches