During the initial design, and periodically in the life of the facility, current that results from a short-circuit should be calculated to ensure the short-circuit rating of the electrical equipment is not exceeded. 

Calculation during design achieves the following:

• The specified electrical equipment in the facility can withstand and interrupt short circuit currents imposed on it. A safety margin is included in the specification to accommodate error, and upgrades which may increase short circuit currents.
• Excessive equipment cost resulting from higher-than-necessary short circuit ratings is avoided.

Calculations during facility operation may be necessary in the following situations:

• Short circuit currents were not calculated during the design stage.
• A significant change to the source (utility or facility generator).
• Addition of large motors at the facility.
• As part of an Arc-Flash Hazard Analysis