Circuit breaker derating refers to reducing the rated current of an already installed circuit breaker to ensure the safe operation of electrical equipment. This is done by replacing fuses or adjusting parameters when it's found that the breaker's rated current exceeds the actual current used by the electrical system. This adjustment makes the circuit breaker suitable for the actual needs of the electrical system. There are several reasons for derating circuit breakers, with the most significant being to ensure the safe operation of electrical equipment when its actual load is less than the breaker's rated current. If the breaker's rated current is too high under such circumstances, it might fail to protect the equipment properly, potentially causing outages and affecting the equipment's normal operation. Here's a deeper look into why and when circuit breakers need to be derated and by how much.

Why Derate Circuit Breakers?

Circuit breakers may need to be derated primarily when the operating ambient temperature exceeds the specified value, which is generally set at 40°C (or over 65°C for motor protection).

When is Derating Necessary?

When distribution cabinets are installed outdoors, and the summer temperature is high; the ambient temperature exceeds 40°C.

When installed indoors, but with poor ventilation and heat dissipation conditions, leading to temperatures inside the distribution cabinet exceeding 40°C, even if the room temperature does not.

Even if the distribution box is installed in an air-conditioned room, but contains many circuit breakers resulting in poor heat dissipation, derating should be considered.

In high-altitude areas, where the thin air reduces air density and weakens convective cooling, causing temperature rises. Although higher altitudes lead to lower air temperatures, partially compensating for the effects of altitude on electrical heating, this compensation is not enough to offset the increase in temperature rise, necessitating product derating.

By How Much Should You Derate?

Derating should be based on the installation location and load factor, but it's challenging to specify an exact figure. Generally, if the environmental temperature is high and cooling is inadequate, a derating factor of 0.8 is used; in very stringent conditions, a factor of 0.7 might be applied. Over-derating can lead to unnecessarily large cable specifications, significantly affecting costs and potentially impacting short-circuit protection sensitivity. Therefore, excessive derating should be avoided.