When to use: Estimate available fault current at the transformer secondary and at a downstream point (panel, MCC, or equipment location) using the industry-standard Point-to-Point Method, as published by fuse and breaker manufacturers (Bussmann/Eaton SPD guides). Use the result to verify that equipment interrupting/withstand (AIC) ratings meet or exceed the calculated available fault current.
Estimate available fault current at a transformer secondary and at a downstream point in a radial electrical system, using the Point-to-Point Method published by fuse and breaker manufacturers (Bussmann/Eaton SPD Selection Guides). Use the results to verify equipment interrupting (AIC) and withstand ratings meet or exceed the available fault current at each point in the system.
The transformer secondary available fault current, Isca, is first calculated assuming an infinite (unlimited) primary bus: Isca = (kVA × 100) / (√3 × V × %Z). This is the standard conservative simplification used when detailed utility source impedance is not being modeled — it always gives the higher, worst-case fault current. If a finite utility source is known (available fault current and voltage at the primary), its impedance is referred through the transformer turns ratio and added in series with the transformer impedance for a more refined estimate.
To find the fault current at a point downstream of the transformer (a panel, MCC, or piece of equipment), the impedance of the conductor run between the transformer and that point is added to the source impedance. The conductor impedance reduces the available fault current the farther downstream you go — this is expressed as a multiplier M = 1/(1+f), where f is the ratio of conductor impedance to source impedance. Available fault current at the point = Isca × M.
Manufacturer point-to-point guides typically use a published per-conductor-size "C constant" table lookup. Those tables differ by conductor material (copper vs. aluminum) and by conduit type (magnetic steel vs. non-magnetic), and small transcription errors are easy to introduce. Rather than presenting a numeric C-value table that has not been independently verified against the current published source, this tool asks for the conductor's own impedance in ohms per 1000 feet — sourced from the cable manufacturer's datasheet or from NEC Chapter 9, Table 9 for the correct conduit type. The underlying math is identical to the table-based method; only the input is more transparent and independently verifiable.
Compare the calculated available fault current at each point against the interrupting rating (AIC) of breakers and the short-circuit withstand rating of panelboards, switchgear, and busway at that point, per NEC 110.9 and 110.10. Equipment must be rated equal to or greater than the available fault current. This screening calculation is appropriate for early design and verification checks — final equipment selection on any real project should be confirmed by a complete short-circuit study.
It is a simplified, widely-taught method for estimating available short-circuit current at successive points along a radial electrical distribution system, published by fuse and breaker manufacturers such as Bussmann (Eaton) in their Selection Procedure/SPD guides. It starts from the transformer secondary fault current and steps downstream, adding conductor impedance at each segment.
Assuming the utility source has zero impedance (infinite available fault current) is the standard conservative simplification used whenever detailed utility source data isn't being modeled. It produces the highest — and therefore safest to design against — fault current, since any real utility source impedance would only reduce the available fault current.
No. This tool models a single-source radial system only. It does not include motor contribution to fault current, multiple parallel sources, or looped/networked topologies — all of which require a full study per IEEE 141/242 using dedicated software. Use this tool for preliminary screening, not final equipment certification.
Published conductor impedance ("C-value") tables vary meaningfully by conductor material and conduit type, and this project could not independently verify every table entry against the current official source with confidence. To avoid presenting unverified numbers as fact, you enter the conductor's impedance (ohms/1000 ft) directly from your cable manufacturer's datasheet or NEC Chapter 9, Table 9 — the calculation itself is unchanged.
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