TrueFault: Fault Current Calc

509 Electric

About this app

TrueFault is a fast, offline available fault current calculator for working electricians. It walks you from the utility source down the feeder to the point you care about — and shows the inputs, assumptions, and warnings behind every number, so you can check the result instead of just trusting it.

WHAT IT DOES
- Guided 5-step workflow: System, primary fault current, transformer, path to the fault, result.
- Point-to-point short-circuit method — conductor and bus-run impedances are accumulated in order along the feeder to your selected point of fault.
- Source modeling — enter the utility available fault current (AFC) and primary voltage, or assume an infinite bus for a worst-case check.
- Transformer step-down — kVA, secondary voltage, and percent impedance (%Z), with impedance-tolerance handling.
- Conductor runs — copper or aluminum, #12 through 500 kcmil, run length, and parallel sets. Impedance is from NEC Chapter 9, Table 9 (75 C, steel conduit — the conservative, lowest-AFC assumption, disclosed on the report).
- Bus runs — 400 / 800 / 1200 / 2000 A busway and panel-bus impedance.
- Generator and motor contribution — tie a generator (kVA/kW, power factor, subtransient reactance Xd) or a downstream motor (full-load amps x locked-rotor multiplier) to a chosen point.
- Read available fault current in kA at any point on the path, with source/secondary fault current, total and path impedance in ohms, and generator/motor contribution in amps.
- Visual one-line diagram and system-component summary so you can review the whole path before you rely on the result.
- Field label and a traceable calculation report — method notes, input basis, warning codes, and validation issues.
- Fully offline — run a check in a switchgear room, a basement, or a job trailer with no signal.

WHO IT'S FOR
Electricians, apprentices, estimators, panel builders, and inspectors who need a quick available fault current / short-circuit check — to sanity-test equipment interrupting rating (AIC) and short-circuit current rating (SCCR), or to support available-fault-current field marking under NEC 110.24.

ACCURACY
Conductor impedance is based on NEC Chapter 9, Table 9, and benchmark cases are cross-checked against published transformer, conductor, generator, and motor examples. TrueFault is a field aid for qualified professionals, not a stamped engineering study — results are drafts to verify against code review, engineering judgment, utility documentation, and manufacturer/nameplate data. Always verify against the NEC edition adopted in your jurisdiction.

PRICE
TrueFault is free to download, and the single-phase calculator never expires. A one-time Pro Unlock ($6.99 — no subscription, no renewals) adds three-phase calculations, motor-contribution inputs, PDF-ready labels and reports, saved projects, saved default settings, and local report history. Buy once, own it forever.

PRIVACY
Every calculation runs on your device. TrueFault collects no personal data, requires no account, and works fully offline. Saved projects, settings, and report history stay in local storage on your phone and are never transmitted.

Built by 509 Electric, a licensed Washington electrical contractor (EL01) in Yakima — field tools made by people who wire buildings for a living. Part of the True suite of NEC field calculators for electricians.

What’s new

Updated to target Android 16 (API level 36) to meet Google Play's target API requirements. No functional changes to the fault current calculations.