Prospective Fault Current (Ipf) is the general term used for the highest amount of current that will flow under fault conditions.
On a single phase installation fault conditions can either be Prospective Earth Fault Current (PEFC) Line to Earth or Prospective Short Circuit Current (PSSC) Line to Neutral. Whichever one is the highest would be the overall PFC for that circuit / installation.
On a 3 Phase installation the highest fault current will occur during a short circuit between L1, L2, and L3 and as such we need to allow for this by either measuring the Line to Line PFC which most metres don’t do or to calculate it by using sq root of 3 (1.74) x PSCC. As a rule of thumb we simply double the PSSC as this gives us a bit of wiggle room.
There’s no formula for this listed in the 17th Edition BS7671 as they don’t like putting anything too useful in the book as they won’t sell other publications.
There’s a couple of mentions of it in the On-Site Guide 10.3.7 ‘last paragraph’ page 108.
Heads up: The PFC will always be the highest at the origin of the installation as the impedance/resistance is always the lowest there. So as a rule if it’s not too high at the mains it will be okay everywhere else. This is because it will decrease due to the increase in resistance as we move further away from the origin. (ohms law I = V/R).