Available Fault Current (AFC) vs. System Impedance

2027 NFPA 70E Electrical Safety Training — Click any component to explore

AVAILABLE FAULT CURRENT (AFC) AT EACH POINT GENERATING PLANT UTILITY SOURCE HV Transmission TRANSFORMER Pad-Mounted, 1000 kVA HV BUSHINGS LV kVA OCPD AMPS M OL START STOP MCC PANEL Motor Control Center M 3Φ T-BOX INDUCTION MOTOR 3-Phase, 480V, 100 HP AVAILABLE AFC ~50,000 A AVAILABLE AFC ~22,000 A AVAILABLE AFC ~12,000 A AVAILABLE AFC ~4,000 A LOW Z IMPEDANCE (Z) ► ► ► INCREASES ► ► ► HIGH Z HIGH I FAULT CURRENT (Iₛ⁣) ◄ ◄ ◄ DECREASES ◄ ◄ ◄ LOW I
Component
Utility Source / Transmission Tower
High-Voltage Transmission & Generating Plant
Available Fault Current (AFC)
~50,000 A
Maximum available at the service entrance
Impedance Added Here
~0.3% Z
Utility source impedance only

⚡ AFC Calculation Example

Use the simplified formula: ISC = VLN / Ztotal  (single-phase approximation for illustration)

volts
%Z
%Z
%Z
%Z
Results will appear here…

* Simplified single-phase illustration. Real AFC calculations require three-phase methods per IEEE 1584 and involve source MVA, %Z, conductor lengths, and power factor corrections.