Short circuit calculator

Calculate fault current and breaker ratings

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The Short Circuit Calculator helps electrical engineers determine short circuit current (fault current) at electrical panels and equipment. Calculate available fault current based on transformer size, impedance, and system voltage.

Short circuit current calculations are essential for selecting properly rated circuit breakers, fuses, and equipment. Equipment must be rated to withstand available fault current to prevent damage and ensure safe operation.

Enter transformer kVA rating, secondary voltage, and impedance percentage to calculate full load current and short circuit current. The calculator helps select appropriate breaker and equipment ratings that exceed available fault current for safe electrical system design.

Features

Everything you need

Short circuit current calculations
Full load current determination
Transformer impedance support
Multiple voltage system support
Breaker rating verification
Equipment rating checks
kA and ampere outputs

How it works

Simple workflow

1
Enter transformer kVA rating
2
Input secondary voltage
3
Specify impedance percentage (%Z)
4
Review calculated full load amps
5
See short circuit current in amps and kA

Use cases

Built for real-world scenarios

Breaker selection

Determine required circuit breaker interrupting ratings based on available fault current.

Equipment rating

Verify electrical equipment ratings exceed available short circuit current.

System design

Calculate fault current at different points in electrical distribution systems.

Code compliance

Ensure electrical systems meet NEC requirements for fault current protection.

FAQ

Frequently asked questions

SCA = FLA ÷ (%Z ÷ 100), where FLA = kVA × 1,000 ÷ (√3 × Voltage). For 500 kVA, 480V, 5%Z transformer: FLA = 601A, SCA = 601 ÷ 0.05 = 12,020A = 12 kA.

Impedance (%Z) limits short circuit current. Typical: 2-6% for distribution transformers, 5-6% most common. Lower impedance allows higher fault current, requiring higher-rated breakers.

Breaker interrupting rating must exceed available short circuit current. Common ratings: 10 kA, 18 kA, 22 kA, 42 kA, 65 kA, 100 kA. Always select rating above calculated fault current.

High fault current can damage equipment and create safety hazards. Proper breaker and equipment ratings ensure safe interruption of fault current and prevent equipment damage.

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