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Prospective Fault Current (PFC) Calculator | TMUK Group Ltd

PFC Fault Calculator

Calculate PSCC & PEFC for BS 7671 Compliance

Enter any two known values to automatically extract the third
Single Phase Ifc 0.00 kA 0 Amps
Three Phase Ifc 0.00 kA 0 Amps
PSCC: Prospective Short-Circuit Current (L-N or L-L)

PEFC: Prospective Earth Fault Current (L-E)

PFC: Enter the highest value recorded of either the PSCC or PEFC on your EIC/EICR.
Need the Maximum Zs limits? Use our Master Zs Engine to verify the earth loop impedance for your protective device ➔

Understanding Prospective Fault Current (PFC)

In electrical engineering and inspection, Prospective Fault Current (PFC) is the maximum current that could theoretically flow through an electrical installation during a fault condition (such as a short circuit or earth fault). Calculating and verifying this value is a mandatory requirement under BS 7671 (The IET Wiring Regulations) when completing an Electrical Installation Certificate (EIC) or an Electrical Installation Condition Report (EICR).

Why is the PFC Calculation Important?

Knowing your fault current is critical for selecting the correct electrical switchgear and protective devices. Every circuit breaker (MCB, RCBO) and fuse has a "breaking capacity" (measured in kA). For example, a standard domestic MCB is usually rated at 6 kA, while commercial breakers may be rated at 10 kA or higher.

If a short circuit occurs and the Prospective Fault Current exceeds the breaking capacity of your protective device, the device may fail to interrupt the fault. This can result in the breaker welding closed, exploding, or starting an electrical fire. The breaking capacity of the protective device must always be greater than the PFC at that point in the installation.

PEFC vs. PSCC: What's the Difference?

When testing an installation, engineers must measure two distinct types of fault current:

  • Prospective Earth Fault Current (PEFC): The current that will flow in the event of a fault between the Line (Live) conductor and the Earth/Protective conductor.
  • Prospective Short Circuit Current (PSCC): The current that will flow in the event of a fault between live conductors (Line to Neutral in single-phase, or Line to Line in three-phase).

The overall PFC entered on your electrical certification is simply the highest value recorded out of the two tests (PEFC and PSCC).

How to Calculate Fault Current (Ohms Law)

If you have measured the earth fault loop impedance (Zs or Ze) using an electrical tester, you can calculate the expected fault current using Ohm's Law (I = V / R).

For a standard single-phase 230 V UK supply:
Fault Current (Amps) = 230 V ÷ Impedance (Ω)

For three-phase systems (400 V), the fault current is significantly higher. As a rule of thumb in the field, engineers often double the single-phase fault current to get a safe three-phase estimate. However, the exact mathematical calculation used by our TMUK Group calculator is to multiply the single-phase current by √3 (approximately 1.732).

Trust the Experts at TMUK Group Ltd: Accuracy is everything when it comes to electrical safety and compliance. Whether you are conducting periodic testing or designing a new low-voltage distribution network, understanding fault currents ensures your protective devices will perform when you need them most. For professional, commercial-grade fixed wire testing and EICR inspections across the West Midlands, contact the NICEIC-approved experts at TMUK Group Ltd.