Check the listing for details. Square D Type DD Thermal Overload Relay Heater Units - Most Sizes!!. Condition: New – Open box. Listed at 10.99 USD. This Square DType DD Thermal Overload Relay Heater Unit is a top-quality electrical equipment that provides reliable overload protection for your heating system. With its high-quality construction and advanced features, this relay unit is perfect for businesses and industrial settings that require heavy-duty electrical equipment. The unit is available in most sizes, making it a versatile option for a variety of applications. It is designed to work with a range of electrical systems and is easy to install, providing you with a hassle-free experience. Whether you're looking to upgrade your heating system or seeking a replacement relay unit, this Square DType DD Thermal Overload Relay Heater Unit is a great choice.