Choosing the correct circuit breaker is critical for electrical safety and system reliability. Interrupting capacity ensures breakers can safely interrupt fault currents without damage.
This article explores breaker selection by interrupting capacity per NEC guidelines, including formulas, tables, and real-world examples. Learn to calculate and select breakers confidently.
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- Calculate interrupting capacity for a 480V, 200A breaker with a fault current of 25kA.
- Determine minimum interrupting rating for a 120/208V, 100A panel with 10kA available fault current.
- Find breaker size and interrupting capacity for a 600V system with 35kA fault current.
- Evaluate if a 150A breaker with 22kA interrupting rating is suitable for a 240V system with 18kA fault current.
Common Interrupting Capacity Values for Circuit Breakers per NEC
| Voltage Rating (V) | Breaker Ampere Rating (A) | Typical Interrupting Capacity (kA RMS Symmetrical) | Common Applications |
|---|---|---|---|
| 120/240 | 15 – 100 | 10 – 22 | Residential, Light Commercial |
| 120/208 | 15 – 225 | 10 – 35 | Commercial, Office Buildings |
| 277/480 | 15 – 600 | 18 – 65 | Industrial, Large Commercial |
| 600 | 15 – 1200 | 25 – 100+ | Heavy Industrial, Manufacturing |
Additional Interrupting Capacity Ratings by Breaker Type
| Breaker Type | Interrupting Capacity Range (kA) | Typical Voltage Ratings (V) | Notes |
|---|---|---|---|
| Molded Case Circuit Breaker (MCCB) | 10 – 100+ | 120 – 600 | Wide range, used in industrial and commercial |
| Miniature Circuit Breaker (MCB) | 6 – 25 | 120/240 | Residential and light commercial |
| High-Voltage Circuit Breaker | 100 – 2000+ | >1000 | Utility and large industrial applications |
Fundamental Formulas for Breaker Selection by Interrupting Capacity
Understanding the formulas behind breaker interrupting capacity is essential for accurate selection and compliance with NEC standards.
1. Available Fault Current (AFC) Calculation
The Available Fault Current at the point of breaker installation is the maximum prospective short-circuit current the breaker must interrupt.
- AFC: Available Fault Current (Amperes, A)
- Voltage: Line-to-line voltage (Volts, V)
- Ztotal: Total system impedance (Ohms, Ω)
- √3: Square root of 3 (≈1.732), used for three-phase systems
This formula assumes a balanced three-phase system and calculates the symmetrical RMS fault current.
2. Interrupting Capacity (IC) Selection Criterion
The breaker’s interrupting capacity must be equal to or greater than the available fault current at its installation point.
- IC: Interrupting Capacity of the breaker (kA RMS symmetrical)
- AFC: Available Fault Current at breaker location (kA RMS symmetrical)
NEC Article 110.9 mandates that equipment must have an interrupting rating not less than the available fault current.
3. Short-Circuit Current Calculation from Transformer Data
When transformer data is available, the fault current can be estimated using the transformer’s kVA rating and impedance.
- Isc: Short-circuit current (Amperes, A)
- Transformer kVA: Transformer rating in kilovolt-amperes
- Voltage: Line-to-line voltage (Volts, V)
- %Z: Transformer impedance percentage (%)
This formula estimates the maximum fault current contribution from the transformer.
4. Total System Fault Current Calculation
When multiple sources contribute to fault current, total fault current is the vector sum of individual contributions.
- Itotal: Total available fault current (Amperes, A)
- I1, I2, …, In: Individual fault current contributions from sources
This root-sum-square method accounts for multiple sources such as utility and generators.
Real-World Application Examples
Example 1: Selecting a Breaker for a 480V Industrial Panel
A 480V three-phase panel has a calculated available fault current of 30,000 A (30 kA). The panel feeder breaker is rated at 250 A. Determine the minimum interrupting capacity required and select an appropriate breaker.
- Step 1: Identify available fault current (AFC) = 30 kA
- Step 2: Breaker ampere rating = 250 A
- Step 3: According to NEC 110.9, breaker interrupting capacity (IC) must be ≥ AFC
- Step 4: Select a breaker with IC ≥ 30 kA at 480V
From the interrupting capacity table, breakers rated 250 A at 480V commonly have interrupting capacities ranging from 35 kA to 50 kA. Therefore, a 250 A breaker with a 35 kA interrupting rating is suitable.
This ensures the breaker can safely interrupt the maximum fault current without damage or hazard.
Example 2: Calculating Interrupting Capacity for a 120/208V Commercial Panel
A commercial building has a 120/208V panel with a 100 A breaker. The transformer supplying the panel is rated 500 kVA with 5% impedance. Calculate the available fault current and determine the minimum interrupting capacity for the breaker.
- Step 1: Use transformer short-circuit current formula:
Calculate denominator:
Calculate short-circuit current:
- Step 2: Breaker rating = 100 A
- Step 3: Minimum interrupting capacity = 27.8 kA
- Step 4: Select a breaker with IC ≥ 27.8 kA at 120/208V
From the table, a 100 A breaker with a 35 kA interrupting rating is appropriate, providing a safety margin above the calculated fault current.
Expanded Technical Considerations for Breaker Selection
Beyond basic interrupting capacity, several factors influence breaker selection to ensure compliance and safety:
- NEC Article 110.9 Compliance: Equipment must have an interrupting rating not less than the available fault current.
- Voltage Rating: Breaker voltage rating must match or exceed system voltage.
- Breaker Type and Frame Size: Different breaker types (MCCB, MCB, HV breakers) have varying interrupting capacities and applications.
- Coordination and Selectivity: Proper coordination ensures selective tripping, minimizing system downtime.
- Temperature and Environmental Ratings: Ambient temperature and installation environment affect breaker performance.
- Manufacturer’s Data: Always consult manufacturer catalogs for exact interrupting ratings and breaker specifications.
Using software tools or AI calculators can streamline the selection process, reducing human error and improving accuracy.
Authoritative References and Standards
- National Electrical Code (NEC) – NFPA 70
- IEEE Standards for Electrical Safety and Equipment
- UL Standards for Circuit Breakers
- Eaton Breaker Selection Guides
Adhering to these standards ensures safe, reliable, and code-compliant electrical installations.