Short Circuit Calculations: Basic Calculations and Transformer Sizing

Short Circuit Capacity: Basic Calculations and Transformer Sizing

Short circuit capacity calculation is used for many applications: sizing of transformers,

selecting the interrupting capacity ratings of circuit breakers and fuses, determining if a

line reactor is required for use with a variable frequency drive, etc.

The purpose of the presentation is to gain a basic understanding of short circuit capacity.

The application example utilizes transformer sizing for motor loads.

Conductor impedances and their associated voltage drop are ignored not only to present a

simplified illustration, but also to provide a method of approximation by which a plant

engineer, electrician or production manager will be able to either evaluate a new application

or review an existing application problem and resolve the matter quickly.

Literature containing a detailed discussion of short circuit capacity calculations are

available within the electrical power transmission industry. [1]

The following calculations will determine the extra kVA capacity required for a three

phase transformer that is used to feed a single three phase motor that is started with

full voltage applied to its terminals, or, "across-the-line."

Two transformers will be discussed, the first having an unlimited short circuit kVA

capacity available at its primary terminals, and the second having a much lower input

short circuit capacity available.

kVA of a single phase transformer = V x A

kVA of a three phase transformer = V x A x 1.732, where 1.732 = the square root of 3.

The square root of 3 is introduced for the reason that, in a three phase system,

the phases are 120 degrees apart and, therefore, can not be added arithmetically.

They will add algebraically.

Transformer Connected To Utility Power Line

The first transformer is rated 1000 kVA, 480 secondary volts, 5.75% impedance.

Rated full load amp output of the transformer is

1000...