Circuit Breaker Derating Calculations

What is Circuit Breaker Derating?

Circuit breaker derating refers to the process of reducing the rated current capacity of a circuit breaker to ensure safe operation under specific conditions. This is necessary when the circuit breaker is exposed to elevated temperatures, high altitudes, or other environmental factors that could affect its performance.

The primary purpose of derating is to prevent the circuit breaker from tripping prematurely or failing to protect the circuit when needed. Proper derating calculations help electrical engineers select the appropriate circuit breaker for a given application while maintaining safety standards.

Factors Affecting Derating

Several factors influence the derating of circuit breakers:

• Ambient temperature: Higher ambient temperatures reduce the circuit breaker's current-carrying capacity.
• Altitude: Higher altitudes result in lower air density, which can affect the circuit breaker's performance.
• Enclosure conditions: Circuit breakers in enclosed spaces may need derating due to heat buildup.
• Voltage level: Higher voltage levels may require derating to maintain safety margins.
• Circuit configuration: Complex circuit configurations may require additional derating factors.

Understanding these factors is crucial for performing accurate derating calculations and selecting the appropriate circuit breaker for any given application.

Calculation Method

The derating factor for a circuit breaker can be calculated using the following formula:

Where:

• Rated Current is the nominal current capacity of the circuit breaker.
• Temperature Rise is the expected temperature increase above ambient.
• Altitude Factor accounts for the reduction in current capacity at higher altitudes.

For most applications, the altitude factor can be approximated as 0.002 per 1000 feet above sea level. The temperature rise factor is typically based on the expected operating conditions of the equipment being protected.

Example Calculation

Let's consider a 100A circuit breaker operating at an altitude of 5000 feet with an expected temperature rise of 20°C above ambient.

1. Calculate the altitude factor: 5000 feet × 0.002 = 0.01
2. Calculate the temperature factor: 20°C / 75°C = 0.2667
3. Apply the derating factors: 100A × (1 - 0.2667) × (1 - 0.01) = 100 × 0.7333 × 0.99 = 72.5967A

The derated current capacity is approximately 72.6A. This means the circuit breaker should be rated for at least 72.6A to safely protect the circuit under these conditions.