First of all, when the operating environment temperature of the circuit breaker is greater than the specified value, it needs to be derated. It is generally specified as 40°C (the temperature exceeds 65°C during motor protection).
Under what circumstances do you need to reduce the capacity?
1. The power distribution cabinet is installed outdoors, where the summer temperature is relatively high; the ambient temperature exceeds 40°C.
2. Installed indoors, but the indoor ventilation and heat dissipation conditions are poor. Although the indoor temperature does not exceed 40°C, the temperature in the power distribution cabinet has exceeded 40°C.
3. Even if the distribution box is installed in an air-conditioned room, but there are more circuit breakers in the distribution cabinet and poor heat dissipation, derating should be considered.
4. In high-altitude areas, because the air is thin in high-altitude areas, the reduced air density reduces the convection effect of heat dissipation, and the temperature rise will increase. Although the increase in altitude will also cause the temperature to drop, which partially compensates for the impact of the increase in the altitude on the temperature rise of electricity, this effect is not enough to offset the increase in temperature rise, and the product has to be reduced in capacity.
The capacity is derated according to the installation position and full load rate, but the specific figures are difficult to give. Under normal circumstances, when it is judged that the ambient temperature is high and the heat dissipation is not good, I will take a derating factor of 0.8, and 0.7 for extremely harsh conditions. If the circuit breaker is too large, the cable specification will increase, which will have a great impact on the cost, and will affect the sensitivity of short-circuit protection. Therefore, we do not use excessive derating.