Calculator · Electrical · Cooper Bussmann · IEEE 141 · NEC 110.9

Short circuit current calculator

Cooper Bussmann point-to-point method. Computes SCA at the transformer secondary, derated by cable run to the fault point, with optional motor contribution. Recommends the minimum AIC rating for the breaker at that location. Reviewed by a licensed PE.

Use the short circuit calculator

Enter your transformer kVA, secondary voltage, and %Z (or pick a NEMA preset), then optionally add the cable run length to the fault point and motor contribution. The calculator outputs SCA at the transformer secondary, the derating along the cable, the total fault current at the fault, and the recommended breaker AIC.

CALC.005 Short Circuit · Point-to-Point · NEMA + IEEE 141

Transformer kVA[kVA]

kVA

Impedance %Z[%Z]

Or pick a typical NEMA size[Std]

Secondary voltage (line-to-line)[V]

Primary fault (∞ if blank)[kA]

Cable material[Mat]

Size standard[Std]

Cable size[A]

Cable length to fault[L]

Parallel sets[Par]

Motor contribution (A)[I_m]

Standard NEMA practice: motor contribution ≈ 4× connected motor FLA. Set length 0 for fault at the transformer secondary terminals (worst case).

Short-circuit current at fault point

— kA

SCA at transformer terminals; cable derating reduces it downstream.

SCA at transformer secondary: — A
Cable derating factor M: —
SCA at fault (no motor): — A
Motor contribution: — A
Total fault current: — A
Recommended AIC: — kA

PASS · 22 kA AIC SUFFICIENT

FORMULA · I_sec = (kVA × 1000) / (√3 × V × %Z/100) SOURCE · COOPER BUSSMANN · IEEE 141

Figure 1 — Short circuit current waveform: asymmetrical peak at fault, then DC decay to symmetrical I_sym

Short circuit current — at a glance

Short circuit definition: An unintended low-impedance path between conductors that lets fault current rise to many times the normal load — often 10–65 kA in fractions of a second.
Short circuit current formula (3-phase): I_sc = (kVA × 1000) / (√3 × V_LL × %Z/100) at the transformer secondary, assuming an infinite primary bus.
Point-to-point method: Compute SCA at the transformer terminals, then derate downstream by M = Z_xfmr / (Z_xfmr + Z_cable) for each cable run to the fault point.
Motor contribution: Running motors back-feed the fault for the first half-cycle. Standard NEMA practice: add 4 × Σ FLA of all motors at or downstream of the fault point.
Bolted vs arcing fault: A bolted fault is a metal-to-metal short with negligible arc impedance — used for AIC sizing. An arcing fault has plasma impedance and current is typically 0.4–0.8× the bolted value.
Required AIC ratings (NEC 110.9): Common breaker AIC: 10 kA residential, 22 kA light commercial, 65 kA industrial, 100 kA motor control centres.
500 kVA, 480 V, 4.5%Z example: I_sec = 500 000 / (√3 × 480 × 0.045) = 13.4 kA at the secondary terminals before any cable derating.
Symmetrical vs asymmetrical: Symmetrical RMS is the steady-state AC value used for breaker AIC. Asymmetrical peak (first half-cycle) can reach 1.5–2.7× I_sym due to DC offset.

The short circuit current formula

The Cooper Bussmann point-to-point method evaluates fault current in two stages: at the transformer secondary terminals (worst case), then derated downstream by the impedance of each cable run between transformer and fault.

Eq. 01 — SCA at transformer secondary (3-phase, infinite primary bus) SI · Cooper Bussmann SPD

I_{sec} = \frac{kVA \cdot 1000}{\sqrt{3} \cdot V_{LL} \cdot \frac{\%Z}{100}}

I_sec: symmetrical RMS short-circuit current at xfmr terminals, A
kVA: transformer rating, kVA
V_LL: secondary line-to-line voltage, V
%Z: transformer impedance from nameplate, %

Eq. 02 — Cable derating multiplier (point-to-point) SI · Per-unit impedance method

M = \frac{Z_{xfmr}}{Z_{xfmr} + Z_{cable}} \qquad I_{fault} = M \cdot I_{sec}

M: derating multiplier (0 < M ≤ 1), —
Z_xfmr: transformer impedance referred to secondary, Ω
Z_cable: cable resistance to fault point = ρ·L/A, Ω

Eq. 03 — Single-phase variant SI · Cooper Bussmann SPD

I_{sec, 1\phi} = \frac{kVA \cdot 1000}{V \cdot \frac{\%Z}{100}}

I_sec, 1φ: single-phase SCA — no √3 factor, A
V: secondary voltage (line-to-neutral or line-to-line as appropriate), V