Fault Current / AIC Calculator

Calculate available short-circuit fault current (AFC) and Ampere Interrupting Capacity (AIC) at service panels using the point-to-point method per NEC §110.24 and §110.9.

NEC edition

Inputs

kVA
%
ft
sets
A

Running motors feed the fault. Conventionally counted at 4× full-load current.

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⚡ Test your knowledge
NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?
  • A. Art. 100
  • B. Art. 120
  • C. Art. 200
  • D. Art. 230
💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.
Energy Management Systems were moved from Article 750 to which article in NEC 2026?
  • A. Art. 100
  • B. Art. 125
  • C. Art. 130
  • D. Art. 150
💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.
Which NEC standard size overcurrent device comes after 90A?
  • A. 95A
  • B. 100A
  • C. 110A
  • D. 105A
💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

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How it works & the NEC rules behind it

Under NEC §110.9 and §110.24, electrical equipment (panelboards, switchboards, meter enclosures, transfer switches) must possess an Ampere Interrupting Capacity (AIC) rating equal to or exceeding the maximum available symmetrical short-circuit current available at its line terminals.

Why Fault Current Calculation is Mandatory (§110.24):

  • Service Field Marking (§110.24(A)): Service equipment in other-than-dwelling occupancies must be legibly field-marked with the maximum available fault current and the calculation date.
  • Equipment Catastrophic Failure: If a circuit breaker with a standard 10,000 AIC rating is installed at a location where 22,000 Amps of short-circuit current is available, an electrical fault will cause the breaker to explode rather than safely clear the circuit.

Formulas & equations

Transformer Terminal Fault Current (Isc) = (kVA × 1,000) / (√3 × V_sec × (%Z / 100)) f-Factor (Point-to-Point) = (1.732 × L × I_fault) / (C × n × V_line) Multiplier (M) = 1 / (1 + f) Available Fault at Panel = I_fault × M

NEC reference table

Standard Commercial Panel AIC Ratings Typical Installation Application Standard Breakers
10 kAIC (10,000 Amps)Residential main panels, subpanels far from utility transformerStandard 1" breakers
22 kAIC (22,000 Amps)Commercial 120/208V panels close to 75–150 kVA transformersCommercial bolt-on
42 kAIC (42,000 Amps)480V distribution boards, heavy industrial servicesHigh-interrupting frame
65 kAIC (65,000 Amps)Heavy commercial main switchboards (500–1500 kVA services)High AIC molded case
100 kAIC (100,000 Amps)Direct utility vaults, industrial substationsCurrent-limiting fuses / breakers

Worked example, step by step

Example Calculation: 75 kVA 480V–208V Transformer Feeder to Subpanel

Scenario: 75 kVA transformer with 5.75% impedance feeding a subpanel 50 feet away using #3/0 AWG Copper conductors in steel conduit at 208V 3-Phase.

  • Step 1 (Transformer Secondary Isc): 75,000 / (1.732 × 208 × 0.0575) = 3,619 Amps.
  • Step 2 (Point-to-Point Reduction): Conductor impedance over 50 ft reduces peak current to ~3,200 Amps.
  • Step 3 (AIC Adequacy Check): A standard 10 kAIC rated panelboard (10,000A capacity) is fully compliant and passes inspection (§110.9).

Frequently asked questions

**Ampacity** is the continuous operating current a wire or breaker can carry without overheating (e.g. 200A). **AIC (Ampere Interrupting Capacity)** is the maximum catastrophic short-circuit fault current (e.g. 22,000A) a breaker can safely interrupt without blowing apart.

Yes. The electrical resistance and reactance of circuit conductors between the transformer and electrical panel add impedance, significantly attenuating and reducing available fault current as distance increases.

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