Transformer and System Inputs
Formula Used
Three-phase full-load current
IFL = (kVA × 1000) ÷ (√3 × VLL)
Single-phase full-load current
IFL = (kVA × 1000) ÷ V
Simplified transformer-terminal fault current
ISC = IFL ÷ Zpu
Finite-source mode adds utility impedance on the transformer base. Parallel identical transformers contribute fault current together. Feeder impedance then reduces downstream current.
Motor and generator currents are added at the selected fault point. The safety margin increases the recommended interrupting rating. Formal studies still require verified system data.
How to Use This Calculator
- Enter the transformer kVA, phase, voltage, and nameplate impedance.
- Select maximum, nominal, or minimum fault-current conditions.
- Choose infinite-source or finite-source utility data.
- Add feeder length, conductor size, material, and temperature.
- Include motor or generator contribution when applicable.
- Enter the existing breaker, fuse, panel, or switchboard rating.
- Calculate and review the pass, marginal, or fail result.
Use verified nameplate and utility values whenever possible. Estimated conductor data supports early planning only. Final equipment selection needs professional engineering review.
300 kVA Example Data
The table assumes three-phase service and 5.75% transformer impedance. It excludes utility, feeder, motor, and generator impedance. Values show conservative terminal fault estimates.
| Secondary voltage | Full-load current | Estimated fault current | Fault current |
|---|---|---|---|
| 208 V | 832.7 A | 14,482 A | 14.48 kA |
| 240 V | 721.7 A | 12,551 A | 12.55 kA |
| 400 V | 433.0 A | 7,531 A | 7.53 kA |
| 415 V | 417.4 A | 7,258 A | 7.26 kA |
| 480 V | 360.8 A | 6,276 A | 6.28 kA |
| 600 V | 288.7 A | 5,020 A | 5.02 kA |
Important Electrical Terms
Available fault current is the current a system can deliver during a fault. It depends on source and circuit impedance. It changes at different system locations.
AIC or AIR describes a protective device interrupting rating. The rating must exceed available fault current. Device markings must match system conditions.
SCCR describes equipment assembly short-circuit current capability. Panels and control equipment need adequate SCCR. Series ratings require documented tested combinations.
Frequently Asked Questions
What is transformer interrupting capacity?
Transformers do not usually carry breaker AIC ratings. The term normally means required downstream equipment capacity. That rating must exceed available fault current.
Is 300 kVA enough information?
No. Voltage and percentage impedance are essential. Utility and feeder impedance also affect results.
Why does lower impedance increase current?
Lower impedance offers less opposition during a fault. More current can then flow. Equipment ratings may need substantial increases.
What is infinite-source mode?
It assumes the utility source has negligible impedance. Transformer impedance becomes the main limit. This often gives a conservative terminal value.
Should feeder impedance be included?
Yes, for downstream equipment studies. Cable resistance and reactance reduce fault current. Accurate conductor data improves the estimate.
How are motors handled?
Running motors briefly feed current into faults. The calculator estimates this from motor load. Manual current can also be entered.
Can parallel transformers be calculated?
Yes, when transformers are identical and share the bus. Their available currents are added. Unequal units need a detailed impedance model.
What rating should a breaker have?
Its interrupting rating must exceed available fault current. A planning margin is also useful. Applicable codes and studies control final selection.
Is peak current the same as RMS current?
No. RMS current supports interrupting comparisons. Peak current supports making and withstand considerations.
Does this replace a short-circuit study?
No. This calculator supports screening and early design. Final work needs verified data and qualified review.
Safety Notice
This calculator provides planning estimates only. Ground faults, arcing faults, sequence networks, device let-through, and series ratings need specialised analysis. Follow local codes and qualified engineering guidance.