Estimates each paralleled generator's fault current contribution at a common bus using the standard subtransient-reactance method: Ifault = Irated ÷ Xd'' — then sums the individual contributions arithmetically, since generators in parallel simply add their contributions at a common bus to a first approximation.
Standards followed
Subtransient-reactance fault contribution method (standard generator short-circuit estimation)
What it can do
- Rated current and fault contribution for each generator, from its rated kVA and subtransient reactance (Xd'').
- Total fault current at the common bus.
Example problem
Two 500kVA generators at 415V, each Xd''=0.12pu.
- Rated current per generator = 500,000/(√3×415) ≈ 695.6A.
- Fault contribution per generator = 695.6/0.12 ≈ 5,797A ≈ 5.80kA.
- Total at bus = 5.80 × 2 ≈ 11.59kA.
≈11.59kA total fault contribution.
Want the full step-by-step version?
This ignores network/inter-machine impedance between the generators and the common bus, and doesn't account for fault current decay over time (subtransient vs transient vs steady-state) — it's a first-approximation screening tool, not a substitute for a full short-circuit study when sizing switchgear.