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Fault Current Calculation
Transformer Fault Current (Infinite‑Bus Method)
The infinite‑bus method assumes the upstream utility is so strong that the transformer's own impedance limits the fault current. This gives the maximum worst‑case figure at the secondary terminals — exactly what you need to rate the switchboard directly below the transformer.
Three‑Phase
Full‑Load Current
Ifl = (kVA × 1000) ÷ (√3 × V)
Then divide by the per‑unit impedance: Isc = Ifl ÷ Zpu
Single‑Phase
Full‑Load Current
Ifl = (kVA × 1000) ÷ V
Same impedance division: Isc = Ifl ÷ Zpu
Impedance
Per‑Unit & Tolerance
Transformer impedance (Zpu) is typically stated as a percentage (e.g., 5.75%). Convert to decimal for division.
ANSI ±7.5% • UL ±10% — apply tolerance for worst‑case fault current.
Worked Example & Adjustments
A 750 kVA transformer at 480 V with 5.75% impedance has a full‑load current of about 902 A. Dividing by 0.0575 gives a fault current of roughly 15,700 A — about 15.7 kA at the secondary terminals.
Two adjustments keep the result conservative:
- Impedance tolerance: For the worst‑case (highest) fault current, multiply the nameplate impedance by 0.9 (ANSI) or 0.9 (UL conservative).
- Voltage variation: Multiply the final figure by 1.1 where a stiff utility supply is expected.
So the example becomes: 15.7 kA × 1.1 ÷ 0.9 ≈ 19.2 kA for a conservative switchgear rating.
Key Points
Why the Infinite‑Bus Method Works
01
Worst‑Case Conservative
Assumes infinite upstream capacity, so the transformer is the only limiting factor — ideal for switchgear withstand ratings.
02
Simple and Direct
Only requires transformer kVA, voltage, and per‑unit impedance — no complex utility data needed.
03
Tolerance and Voltage
Apply ±10% impedance tolerance and 1.1 voltage factor to account for real‑world variations and ensure a robust design.
04
Generator Contribution
When a generator is present, its fault current adds to the transformer contribution — use the combined value for total switchgear sizing.
Short Circuit Current Tool
How to Calculate Short Circuit Current
The rigorous method is defined by IEC 60909, which models every impedance between the fault source and the fault point. The calculator also runs the simplified infinite‑bus transformer method for fast checks — both methods are available in the tabs below.
IEC 60909 Fault Current
The central formula is: Ik″ = c × Un ÷ (√3 × Zk)
Where c is the voltage factor, Un is the nominal voltage, and Zk is the total fault‑loop impedance.
Volts (V)
Voltage factor per IEC 60909
Ohms (Ω)
Transformer Infinite‑Bus Method
For fast checks, the fault current at the secondary terminals is: Isc = Ifl ÷ Zpu
Where Ifl is the full‑load current and Zpu is the per‑unit impedance (%Z ÷ 100).
Typical: 2–8%
Fault Current Contribution
Generator Fault Current: What Your Generator Contributes
Here is where this tool differs from most fault calculators, which treat the utility or transformer as the only source. A generator contributes fault current too, and ignoring it underrates your switchgear.
A generator's fault contribution is limited by its subtransient reactance, written Xd″. For a typical synchronous generator, Xd″ falls between 0.12 and 0.25 per‑unit. The initial symmetrical fault current a generator delivers is therefore:
Ik″ ≈ Irated ÷ Xd″
Where Irated is the generator's full‑load current
With Xd″ at 0.2, a generator delivers roughly five times its rated current at the fault instant. In practice, expect four to eight times rated current from a synchronous generator, decaying rapidly as the fault moves from the subtransient to the transient to the synchronous state.
This matters for one practical reason: the circuit breakers and switchgear between the generator and the load must be rated for the generator's fault contribution, not just the utility's. A generator feeding a switchboard raises the available fault current at that board. Motors do the same, contributing roughly four to six times their rated current at the moment of a fault.
If your facility runs on generator power — or transfers to it during an outage — the fault study must include the generator. This short circuit current calculator gives you that contribution in the same step as the transformer method.
Integrated Design
Sizing the Generator Behind This System
Fault current, switchgear, and generator sizing are three parts of one design. The fault study tells you what the switchgear must withstand; the load list tells you what the generator must supply. The two share the same inputs — connected load, voltage, and power factor — which is why they belong in one place.
Once you know your fault current, the next question is whether the generator behind the system is correctly sized. An undersized generator cannot hold voltage when a large motor starts; an oversized one wastes fuel and risks wet‑stacking at light load.
Size the Generator Behind This System
Use the Generator Sizing Calculator to find the right kVA and kW for your load profile — based on the same inputs that drive your fault study.
About the Manufacturer
Why Choose Shandong Huali for Your Power System
A short‑circuit study is only as good as the equipment it rates. The generator in that study is where Shandong Huali manufactures everything in‑house.
01
One‑Stop Power‑Systems Manufacturer
We build diesel generator sets from 8 kVA to 4,000 kVA across silent, open, trailer, container, and high‑voltage configurations — alongside dust‑suppression and industrial automation equipment.
02
Factory‑Direct Pricing
Buying direct removes intermediaries, so you pay for engineering and materials rather than markup.
03
OEM/ODM Customization
Our team of 80+ engineers designs to your specification — voltage, frequency, enclosure, control system, and engine choice — so the generator integrates with your switchgear and protection scheme.
04
Certified, Tested Quality
Every unit is 100% load‑tested in our national‑standard testing centre before delivery, under ISO9001, CE, CCC, and ISO14001 systems. With 25+ years of manufacturing experience and exports to 20+ countries, we stand behind every generator we ship.
05
Proven Component Partners
We integrate Cummins, Perkins, Weichai, and Yuchai engines with Stamford alternators, matched to your duty cycle and budget.
Let's talk
Get a Factory-Direct Generator Quote
Request a factory-direct generator quote today. Tell us your required kVA, application, and location, and a power-systems engineer will review your design and respond with a tailored specification — no obligation, no hard sell.
Frequently Ask Questions
Q: How do you calculate short circuit current?
A: For a transformer source, divide the full-load current by the per-unit impedance: Isc = Ifl ÷ Zpu. For a full IEC 60909 study, use Ik″ = c × Un ÷ (√3 × Zk), summing every impedance from source to fault point. This short circuit current calculator runs both.
Q: What is available fault current?
A: Available fault current is the maximum short-circuit current a system can deliver at a given point, assuming the source voltage is at its maximum and the impedance is at its minimum. It sets the interrupting rating every breaker at that point must meet.
Q: How do I calculate transformer fault current?
A: Calculate the transformer full-load current, then divide by the per-unit impedance. A 750 kVA transformer at 480 V with 5.75% impedance produces about 15.7 kA at its secondary terminals.
Q: How much fault current does a generator contribute?
A: A synchronous generator contributes roughly four to eight times its rated current at the instant of a fault, limited by its subtransient reactance Xd″ (typically 0.12–0.25 per-unit). This contribution decays rapidly over the first few cycles.
Q: What is subtransient reactance (Xd″)?
A: Subtransient reactance is the generator’s impedance during the first few cycles of a fault, before the machine’s field and damper windings respond. It is the smallest of the machine reactances, which is why it produces the largest initial fault current.
Q: How do I size a breaker for fault current?
A: Select a breaker whose interrupting rating (kAIC) exceeds the maximum available fault current at its location. Include the contribution of generators and motors in parallel with the utility, not just the utility source alone.