Available Fault Current — Point to Point

It is the current that would flow if a bolted short circuit occurred at a given point. It matters because NEC 110.9 requires every device intended to interrupt a fault to have an interrupting rating at least equal to what is available there. A 10 kA loadcenter on a 22 kA supply can fail catastrophically rather than clearing the fault. 110.24 additionally requires the available fault current to be field-marked at a service.

Because it is the conservative assumption and it needs no data from the utility. Treating the primary as infinite means all the impedance limiting the fault is the transformer's own, so the answer is the highest the transformer can produce. Including the real utility and primary-conductor impedance always lowers the result — so if the equipment passes on the infinite-source figure, it passes.

Because transformer impedance is a nameplate value with a manufacturing tolerance of about ±10%, and a transformer at the low end of that band produces more fault current. For an interrupting-rating check the worst case is the low-impedance unit, so the calculator can multiply the nameplate impedance by 0.90. This is standard practice in a short-circuit study.

You take the fault current at the start of a run, add the run's own impedance, and get the reduced current at the far end. The classic form is M = 1 ÷ (1 + f), where f is the run impedance divided by the source impedance. This calculator computes the source impedance implied by the upstream current and adds the run impedance from NEC Chapter 9 Tables 8 and 9 — algebraically identical, but every number traces to the code book rather than to proprietary conductor constants.

Yes, and it is the main reason a residential loadcenter is usually adequate. On the default 300 kVA example the current falls from 11,455 A at the transformer to 8,626 A after only 100 ft of 4/0 copper — a 25% reduction. Length is why fault current is highest at the service and drops steadily downstream, and why the equipment nearest the transformer needs the highest rating.

Residential loadcenters and their breakers are commonly rated 10 kA. Commercial panelboards run 22 kA and 65 kA, with 100 kA and 200 kA available on switchgear and current-limiting fuses. A series rating is also permitted, where a tested upstream device protects a lower-rated downstream one — but only in the exact combination the manufacturer tested and listed, and 240.86 requires it to be field-marked.

No. This is a planning tool for a single transformer and one feeder run. A real coordination study models motor contribution, multiple sources, cable and bus impedance throughout, asymmetrical peak currents, and arc-flash incident energy — and where equipment ratings are marginal or arc-flash labelling is required, that study needs a licensed engineer.