Available Fault Current (AFC) calculations are most often performed by using fault current calculating software or spreadsheets that automatically calculate the final amount of AFC after entering specific values. Long before this type of software became available, the calculation was done manually with a few basic formulas that are commonly found in many of the electrician’s pocket guides like the “Ugly’s Electrical References” guide. These types of guides provide the most common electrical references and formulas used by the electrician. It is important to understand how the basic formula works even if the plan is to use software for the calculation.

Some formulas will calculate the AFC on the secondary side of the transformer and other formulas will calculate the AFC at the end of a run of conductor. Below is a 3-step formula to calculate three-phase AFC, also called the available short circuit current (ISC) at the end of a run of wire. In order to use the calculation, several factors must be known:

• The length of conductor
• The amount of AFC where the conductor originates
• The size of the conductor
• The type of conductor (copper or aluminum)
• If the conductor is part of a cable assembly or installed in a raceway
• If a raceway is used, is it plastic or ferrous metal (magnetic)
• Line to line voltage of the circuit

Step 1:

 F = (1.732 X L X I) ÷ (C X E_(L_L))

• 1.732 represents the square root of 3 since this is a 3-phase equation
• L represents the length of the conductor
• I represents the available fault current where the conductor originates
• C represents a combination of items and is most easily found using a table in the Ugly’s guide (see image). The value of C is determined using the type and size of conductor along with the raceway properties.
• E_{L_L} represents the line to line voltage of the electrical circuit being calculated

Step 2:

 Multiplier (M) = 1 ÷ (1 + F)

Step 3:

 Available Short-Circuit Current (ISC) = I X M