What feature must dry-type transformers have when installed outdoors?

When dry-type transformers are installed outdoors, they must have a weatherproof enclosure. This requirement is crucial because outdoor environments can expose transformers to a range of weather conditions, including rain, snow, wind, and temperature fluctuations. A weatherproof enclosure ensures that the transformer is protected from moisture and debris, which can lead to electrical failures or reduced efficiency.

The integrity of the enclosure helps maintain the proper operation of the transformer and prolongs its lifespan by preventing corrosion and other environmental damage. In contrast, a standard enclosure might not offer the necessary protection against the elements, and while a steel enclosure could provide durability, it may not be inherently weatherproof without additional protective features. An insulated enclosure is important for electrical safety, but it does not specifically address the need to protect against weather conditions, which is the primary concern for outdoor installations.