Short-Circuit Current at Panel (Point-to-Point)
I_sca at transformer secondary, M-factor, and let-through fault current via the Bussmann point-to-point method.
Example
You enter
- Utility transformer kVA 1500
- Utility transformer %Z 5.75
- Secondary V 480
- Phase three
- Conductor C-value (from shard) 22185
- Run length (ft) 100
- Parallel sets 1
You get
- I_sca at secondary 34865 A
- I_sca at panel 22249 A
Checked against published worked examples from: NEMA / Bussmann (Cooper).
Details, formula, and sources
Formula or tableI_sca_secondary = (kVA × 1000) / (V × √phases × 0.9 × Z%/100), the 0.9 being Bussmann's worst-case allowance for the transformer impedance tolerance. f = (√3 × L × I_sca) / (n × C × V) for three-phase (use 2 instead of √3 for single-phase). M = 1 / (1 + f). I_sca_panel = I_sca_secondary × M.
Source and editionBussmann Point-to-Point Method (Eaton/Bussmann SPD electrical-safety publication) by name. C-values from Eaton/Bussmann published table.
Free accessBussmann SPD documents free at eaton.com/bussmann-spd. Free read-only access at nfpa.org/freeaccess.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesutility_kVA, utility_Z_pct, secondary_V, phase, C_value, length_ft, parallel_sets, I_sca_secondary_A, I_sca_panel_A
- C-value table data/electrical/conductor-c-values.json keyed to conductor class / size / racewayEaton/Bussmann SPD
- Three-phase factor √3 (use 2 for single-phase)Bussmann SPD
- Transformer %Z nameplate x 0.9 (worst-case impedance tolerance); infinite primary source; motor contribution not addedBussmann SPD point-to-point notes