Cooper Bussmann Short Circuit Calculator

Calculator form

Formula used

This calculator follows a point-to-point short-circuit workflow. It can start at transformer secondary terminals or at a second transformer with known primary fault current.

• Three-phase transformer full-load current = (kVA × 1000) ÷ (V × 1.732)
• Single-phase transformer full-load current = (kVA × 1000) ÷ V
• Transformer multiplier = 100 ÷ %Z
• Terminal fault current = full-load current × transformer multiplier
• Second transformer f = (I primary × V primary × 1.732 × %Z) ÷ (100000 × kVA) for three phase
• Second transformer M = 1 ÷ (1 + f)
• Second transformer secondary fault = (V primary ÷ V secondary) × M × I primary
• Run f = (1.732 × L × I) ÷ (C × n × E) for three phase
• Run f = (2 × L × I) ÷ (C × n × E) for single-phase line faults
• Run M = 1 ÷ (1 + f)
• End-of-run fault current = starting current × M
• Estimated motor contribution = total motor FLA × selected multiplier
• Total available fault current = end-of-run fault + motor contribution

How to use this calculator

1. Choose whether you want transformer terminal mode or second transformer mode.
2. Select three-phase or single-phase operation.
3. Enter transformer kVA, voltage values, and impedance.
4. For second transformer mode, enter the known primary fault current.
5. Enter conductor length, runs per phase, and fault voltage for the downstream point.
6. Select conductor material, arrangement, conduit type, and size, or enter your own C value.
7. Add total motor full-load amps if you want an estimated motor contribution.
8. Enter equipment SCCR in kA when you want a simple pass or fail check.
9. Press the calculate button and review the result block above the form.
10. Use the CSV and PDF buttons to save the output.

Example data table

This example uses a three-phase transformer with a downstream conductor run.

Frequently asked questions

1. What does this calculator estimate?

It estimates available short-circuit current at transformer terminals or at a downstream point after conductor impedance and optional motor contribution are considered.

2. What does the C value represent?

The C value is the reciprocal impedance constant used in the point-to-point conductor run formula. Different materials, conductor groupings, and conduit types change this value.

3. When should I use second transformer mode?

Use it when upstream available fault current at the transformer primary is already known and you need the estimated available fault current at the downstream transformer secondary.

4. Why is motor contribution optional?

Motors can feed fault current briefly during a short circuit. This calculator allows a practical estimate by multiplying total motor full-load current by a selected factor.

5. What is the SCCR check doing?

It compares the calculated available fault current against the equipment short-circuit current rating. A pass means the entered SCCR is not below the calculated available current.

6. Should I enter a custom C value?

Enter a custom value when your conductor setup is not covered by the built-in options, or when your design standard requires a different tabulated value.

7. Is this enough for final protection design?

No. It is a planning and checking tool. Final device selection, selective coordination, and field verification should follow project requirements and current manufacturer data.

8. Why might field values differ from this result?

Real systems vary by utility strength, exact transformer impedance, conductor characteristics, operating voltage, motor presence, and the actual location of the fault point.