Enter the fault current and system voltage into the calculator to determine the AIC (Ampere Interrupting Capacity) Rating. The AIC rating defines the maximum fault current a protective device can safely interrupt, and selecting the correct rating is a core requirement of NEC 110.9.
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What is an AIC Rating?
AIC (Ampere Interrupting Capacity) is the maximum fault current, expressed in RMS symmetrical amperes, that a circuit protective device can safely clear without sustaining damage, welding contacts, or exposing live parts. It is stamped directly on circuit breakers and fuses by the manufacturer and tested under standardized short-circuit conditions. The AIC rating of any installed device must be equal to or greater than the available fault current (AFC) at the exact point of installation. This requirement is codified in NEC Section 110.9. Unless a breaker nameplate states otherwise, the NEC assigns a default interrupting rating of 5,000 A to circuit breakers and 10,000 A to fuses.
AIC Rating Formula
The AIC rating of a device is a nameplate specification. The available fault current (AFC) that determines the minimum required AIC at any installation point is calculated from transformer characteristics. For a three-phase system, the bolted fault current at the transformer secondary is:
AFC_{3\phi} = \frac{kVA \times 1000}{V_{LL} \times 1.732 \times \%Z}For a single-phase system:
AFC_{1\phi} = \frac{kVA \times 1000}{V \times \%Z}- AFC = Available Fault Current at transformer secondary (amperes)
- kVA = Transformer rated capacity
- V or VLL = Secondary voltage (V); use line-to-line for three-phase
- %Z = Transformer impedance as a decimal (5% = 0.05)
The required AIC of any device at that location must be greater than or equal to this AFC value. Conductor impedance between the transformer and a downstream panel reduces AFC at that panel, so the most conservative (highest) fault current always occurs directly at the transformer secondary terminals.
Typical Available Fault Currents by Transformer Size
The table below shows calculated AFC values at the transformer secondary under bolted-fault conditions using typical nameplate impedance values. These represent the ceiling fault current that protective devices at the service entrance must be rated to interrupt.
| Transformer | Secondary Voltage | Typical %Z | AFC at Secondary | Minimum AIC Tier |
|---|---|---|---|---|
| 25 kVA, 1-phase | 240 V | 2.5% | 4.2 kA | 5 kA |
| 50 kVA, 3-phase | 208 V | 2.5% | 5.6 kA | 10 kA |
| 112.5 kVA, 3-phase | 480 V | 3.5% | 3.9 kA | 10 kA |
| 300 kVA, 3-phase | 480 V | 5.0% | 7.2 kA | 10 kA |
| 500 kVA, 3-phase | 480 V | 5.0% | 12.0 kA | 22 kA |
| 1000 kVA, 3-phase | 480 V | 5.75% | 20.9 kA | 22 kA |
| 1500 kVA, 3-phase | 480 V | 5.75% | 31.3 kA | 42 kA |
| 2500 kVA, 3-phase | 480 V | 5.75% | 52.2 kA | 65 kA |
Utility source impedance upstream of the transformer further increases fault current beyond these values. In dense urban grids where the utility delivers infinite bus fault current, AFC at large transformer secondaries can exceed 100 kA, pushing device selection into 100 kA or 200 kA AIC tiers used in primary switchgear.
Standard AIC Rating Tiers
Manufacturers produce circuit breakers and fuses in standardized AIC tiers. Residential molded-case breakers commonly carry 10 kA ratings, which covers the vast majority of single-family service entrances fed by distribution transformers under 167 kVA. Commercial and light industrial applications at 208 V or 240 V typically require 22 kA devices. Industrial 480 V switchgear fed by large unit substations routinely requires 65 kA or 100 kA rated breakers, and main switchgear at major facilities may specify 200 kA. The full ANSI/IEEE standard tier progression is: 5 kA, 10 kA, 14 kA, 18 kA, 22 kA, 25 kA, 30 kA, 42 kA, 65 kA, 85 kA, 100 kA, and 200 kA.
AIC Rating vs. SCCR
AIC and SCCR are related but apply to different objects. AIC is a property of a single protective interrupting device, specifically the maximum current that device can clear. SCCR (Short-Circuit Current Rating) is a property of an entire assembly, such as an industrial control panel or motor control center, describing the maximum fault current the assembly can withstand without unacceptable damage. A panel can carry an SCCR of 65 kA even if its individual branch breakers are rated only 10 kA, provided a series-rated or current-limiting protective device at the line side provides coordination. NEC Article 409 governs SCCR marking for industrial control panels, while NEC 110.9 governs AIC compliance at each individual device location.
NEC 110.9 Compliance and Consequences of an Undersized AIC Rating
NEC Section 110.9 requires that equipment intended to interrupt current at fault levels must have an interrupting rating sufficient for the current available at its line terminals. Installing a breaker with an AIC rating below the available fault current is a code violation and a genuine safety hazard. When fault current exceeds the breaker’s interrupting capacity, the device cannot extinguish the arc. The result is a sustained arc blast, which can rupture the breaker enclosure, ignite surrounding materials, and expose personnel to severe burns and blast overpressure. The failure mode is not a quiet trip but an explosive destruction of the device. NEC 110.24 additionally requires that the available fault current be field-marked at the service equipment in commercial and industrial occupancies, creating a documented reference that plan reviewers and inspectors use to verify AIC adequacy throughout the downstream distribution system.
Voltage Dependence of AIC Ratings
A single breaker model may carry different AIC ratings at different voltages. Higher voltage makes arc interruption more difficult because the arc reignites more readily after each current zero. A breaker listed at 65 kA at 240 V may be derated to 42 kA at 480 V and 25 kA at 600 V. The nameplate or manufacturer datasheet specifies the interrupting capacity at each applicable voltage. Using the 240 V AIC figure for a 480 V system is a common specification error that results in an undersized installation even when the numbers appear to exceed AFC on paper.