Calculation sequence
Step-by-step method
- Calculate balanced transformer full-load secondary current.
- Divide 100 by nameplate percent impedance.
- Multiply and convert amperes to kA.
ElectricalAdvancedShort-Circuit Screening
Transformer Secondary Fault-Current Screening Calculator
Estimate transformer-limited balanced three-phase secondary fault current from rating, voltage, and nameplate impedance.
Why it matters: The simple transformer limit is useful for orientation but is not a short-circuit, coordination, or arc-flash study.
Calculated result
Engineering evaluation
Transformer-limited fault-current screen (Iˢᶜ)10.459 kA
- Full-load secondary current (Iᶠˡ)
- 601.407 A
- Transformer fault multiplier (Mᶠ)
- 17.391 ratio
Engineering interpretation
The transformer-only ideal screen is 10.45924 kA at the secondary terminals under the stated assumptions.
Important limitations
- Transformer-limited screening only. Never use this result as an interrupting rating, coordination result, incident energy, arc-flash result, available downstream fault current, or code-compliance determination.
- A formal study must include source and conductor impedance, motors, parallel sources, X/R, tolerances, grounding, and protective devices.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Iᶠˡ = 1000Sᵀ/(√3·Vᴸᴸ)
Mᶠ = 100/Z%
Iˢᶜ = Iᶠˡ·Mᶠ/1000
Current substitution
Iᶠˡ = 1000×500/(√3×480) = 601.4065 A; Mᶠ = 100/5.75 = 17.3913; Iˢᶜ = 10.45924 kA.
Equation legend
Inputs and calculated quantities
- Sᵀ
- Transformer rating — kVA
- Vᴸᴸ
- Secondary line voltage — V
- Z%
- Transformer impedance — %
- Iˢᶜ
- Transformer-limited fault-current screen — kA
- Iᶠˡ
- Full-load secondary current — A
- Mᶠ
- Transformer fault multiplier — ratio
Applications
How this calculation is used
Building design
Screen the magnitude of transformer-limited current before a formal study.
Construction
Check nameplate kVA, secondary voltage, and percent impedance entries.
Commissioning
Document the transformer-only theoretical screen without using it for acceptance.
Quality controls
Assumptions and limitations
Assumptions
- Balanced bolted three-phase fault at the transformer secondary terminals.
- Infinite primary source and transformer impedance as the only limiting impedance.
Limitations
- Transformer-limited screening only; never an interrupting-rating, coordination, incident-energy, arc-flash, or code-compliance result.
- A formal study must include upstream and conductor impedance, motor and parallel-source contribution, X/R, tolerances, grounding, and protective devices.
Field use
Verification procedure
- Follow the approved electrical-safety program and use correctly rated instruments.
- Confirm wiring, ratios, units, operating mode, and measurement boundaries.
- Document simultaneous readings and compare with approved project and manufacturer data.
Common engineering mistakes
- Using the result as available fault current at downstream equipment.
- Treating this screen as an arc-flash or protective-device study.
Technical basis
References and source standards
- IEEE Standards AssociationPower quantity, harmonic, transformer, motor, battery, and metering standards; verify the current applicable edition during professional review.
- NFPA 70, National Electrical CodeThe legally adopted edition and local amendments govern final electrical work; this library does not reproduce protected tables or establish compliance.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement