Compute available short-circuit current at any point in your distribution system. Combines the transformer's bolted fault current (from kVA and %Z) with cable impedance from source to fault. Use the result to verify NEC 110.9 interrupting ratings on every downstream protective device.
Typical: 2-6% for distribution transformers
A free calculator gets you in the ballpark. For permit-stamped, defensible work, ClarkTE delivers a PE-stamped short circuit and coordination study from a working engineer — typically within 48 hours of receiving your one-line and load data.
Short-circuit current (or fault current) is the current that flows when a short — phase-to-phase, phase-to-ground, or three-phase — closes the circuit through near-zero impedance. It is far higher than load current (often 10× to 100×) and is set by the source's short-circuit MVA, the transformer's percent impedance, and the cable impedance from source to fault.
Every protective device — breaker, fuse, switch — has an interrupting rating (kAIC) it can safely break without exploding. If the available short-circuit current at its location exceeds that rating, the device fails violently during a fault. NEC 110.9 requires the interrupting rating to equal or exceed the available fault current at every device location.
The transformer fault current calculator returns the secondary bolted fault current from %Z and assumes infinite primary source impedance — the worst case at the transformer terminals. This calculator extends that downstream: it adds cable impedance from transformer to the device location, returning the actual lower fault current at that point. Both numbers matter; the lower one is what the device sees, but the upper one is what NEC 110.9 sometimes requires you to design to.