Final Answer:
A transformer must be derated if the ambient temperature exceeds 40ºF.
Step-by-step explanation:
Transformers are sensitive to temperature variations, and their efficiency can be affected by high ambient temperatures. The derating of a transformer becomes necessary when the ambient temperature exceeds a specified threshold. In this case, the threshold is 40ºF. Derating involves reducing the transformer's nominal capacity to compensate for the impact of elevated temperatures on its performance.
When transformers are exposed to high ambient temperatures, the insulation and cooling systems may experience challenges, leading to a decrease in efficiency. Derating helps prevent overheating and ensures that the transformer operates within safe temperature limits. It is crucial to follow manufacturer specifications and industry standards to determine the appropriate derating factors based on ambient temperature conditions.