The definition of 'Available Fault Current' (AFC) is the maximum current that can flow in a circuit in a short-circuit condition. The magnitude of the AFC, measured in Amperes, is determined by the size of the electrical distribution systems and any connected electric motors or generators.    

The general rule of thumb to determine the contribution from the motors is to add the total full-load current of all motors and multiply by 4 

Fault availble at the secondary terminals on the transformer is based on secondary voltage and current, transformer size and transformer impedance.

The basic formula is          AFC = 100xFull Load Current / % impedance

These values can be found on the manufacturer's nameplate.
Information for transformers owned by the Local Distribution Company must be provided by them.

As stated in OESC 14-014 breakers in a load panel supplying branch circuits may have a lower AFC rating than the acual AFC provided the main breaker is rated for the maximum AFC and the breakers are tested and approved by the manufacter for series rated combinations.

Other relevant rules from the OESC include:

14-012 -  minimum rating of overcurrent devices (fuses and breakers):
100A, 250V or less = 5000A
above 100A, 250V = 10,000A

14-114 - supplementry overcurrent protection shall not be used as replacement for branch circuit overcurrent protection.

2-034 - epuipment must be marked to withstand AFC



Other factors affecting AFC include size, length and material of conductors between transformer and panelboard.
But motor loads will be added to AFC at panelboard.

The Electrical Safety Authority offers training workshops to further understand AFC and other topics.
Click here for more info ESAsafe
 

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