High Interrupting Capacity (AIC) Breaker Calculator – NEC

Understanding the High Interrupting Capacity (AIC) breaker calculation is critical for electrical safety and compliance. This calculation ensures circuit breakers can safely interrupt fault currents without damage.

This article explores NEC guidelines, formulas, tables, and practical examples for accurately determining AIC ratings. It equips professionals with essential tools for safe electrical system design.

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• Calculate AIC for a 480V, 3-phase system with 1000A fault current.
• Determine breaker AIC rating for 208V single-phase, 500A load.
• Find minimum AIC for 600V, 3-phase, 2000A short circuit current.
• Evaluate AIC requirements for 240V residential panel with 10,000A fault current.

Comprehensive Tables of Common High Interrupting Capacity (AIC) Breaker Ratings per NEC

Key Formulas for High Interrupting Capacity (AIC) Breaker Calculations According to NEC

Calculating the required AIC rating for a circuit breaker involves understanding the maximum prospective short-circuit current at the point of installation. The NEC mandates that the breaker’s interrupting rating must be equal to or greater than this prospective current to ensure safe interruption.

1. Prospective Short-Circuit Current (Isc)

The prospective short-circuit current is the maximum current that can flow during a fault condition. It is calculated as:

• Isc = Prospective short-circuit current (Amperes, A)
• V = System voltage (Volts, V)
• Zs = Total system impedance to the fault point (Ohms, Ω)

The impedance Zs includes transformer impedance, conductor impedance, and any other series impedances between the source and fault location.

2. Interrupting Capacity (AIC) Requirement

The breaker’s interrupting capacity must satisfy:

• AIC = Breaker interrupting capacity (Amperes, A or kiloamperes, kA)
• Isc = Prospective short-circuit current (Amperes, A or kiloamperes, kA)

NEC Article 110.9 requires that equipment be rated to safely interrupt the available fault current without damage.

3. Calculating System Impedance (Zs)

System impedance is often derived from transformer and conductor data:

• Zt = Transformer impedance (Ω)
• Zc = Conductor impedance (Ω)

Transformer impedance is typically given as a percentage on the nameplate and converted to ohms:

• V_rated = Transformer rated voltage (Volts, V)
• %Z = Transformer impedance percentage (%)
• S_rated = Transformer rated power (VA or kVA)

4. Conductor Impedance Calculation

Conductor impedance depends on length, size, and material:

• R = Resistance of conductor (Ω)
• X = Reactance of conductor (Ω)

For short circuit calculations, the reactance is often approximated or neglected depending on accuracy requirements.

Detailed Real-World Examples of High Interrupting Capacity (AIC) Breaker Calculations

Example 1: Calculating AIC for a 480V Industrial Panel

An industrial facility has a 480V, 3-phase panel fed from a 1500 kVA transformer with 5% impedance. The conductor length from transformer to panel is 50 feet with a resistance of 0.05 Ω and reactance of 0.02 Ω. Determine the minimum AIC rating for the panel’s main breaker.

Step 1: Calculate Transformer Impedance (Zt)

Using the formula:

Where:

• V_rated = 480 V
• %Z = 5%
• S_rated = 1500 kVA = 1,500,000 VA

Calculate:

Step 2: Calculate Total System Impedance (Zs)

Conductor impedance Zc = R + jX = 0.05 + j0.02 Ω

Magnitude of Zc:

Total impedance:

Step 3: Calculate Prospective Short-Circuit Current (Isc)

Using:

Note: For 3-phase systems, line-to-line voltage and √3 factor apply.

Calculate denominator:

Calculate Isc:

Step 4: Determine Minimum AIC Rating

The breaker must have an interrupting capacity ≥ 4,498 A. Standard breaker ratings are in kiloamperes (kA). Convert:

Choose the next standard breaker AIC rating above 4.5 kA, typically 10 kA for 480V industrial panels.

Example 2: Residential Panel AIC Calculation at 240V

A residential panel is supplied at 240V single-phase from a 200 kVA transformer with 4% impedance. The conductor length is 100 feet with resistance 0.1 Ω and negligible reactance. Calculate the minimum AIC rating for the main breaker.

Step 1: Calculate Transformer Impedance (Zt)

• V_rated = 240 V
• %Z = 4%
• S_rated = 200,000 VA

Calculate:

Step 2: Calculate Total System Impedance (Zs)

Conductor impedance Zc = 0.1 Ω (resistance only)

Total impedance:

Step 3: Calculate Prospective Short-Circuit Current (Isc)

For single-phase:

Step 4: Determine Minimum AIC Rating

Convert to kA:

Standard residential breakers typically have 10 kA AIC ratings, which is sufficient.

Additional Technical Considerations for High Interrupting Capacity Breakers

• NEC Article 110.9: Requires equipment to have an interrupting rating not less than the available fault current.
• Coordination: Proper AIC rating ensures selective coordination, preventing unnecessary outages.
• Breaker Types: Molded Case Circuit Breakers (MCCB) and Air Circuit Breakers (ACB) have different interrupting capacities.
• Voltage Ratings: Higher voltage systems require breakers with higher AIC ratings due to increased fault energy.
• Manufacturer Data: Always verify breaker AIC ratings from manufacturer catalogs and UL listings.

Summary of NEC Guidelines for AIC Ratings

References and Further Reading

• National Electrical Code (NEC) – NFPA
• UL Standards for Circuit Breakers
• Schneider Electric Breaker Catalogs and Technical Guides
• Eaton Electrical Breaker Specifications