H3 Heating and Air Conditioning (except boilers) below 15 tons. This license includes all single-
family residences (other than boilers) regardless of tonnage. This also includes commercial work with units up to and including 15 tons on buildings that do not exceed 15 tons.  The State of North Carolina defines it as the following:

87-21. (a) Definitions. – For the purpose of this Article: (4). The phrase "heating, group number three" shall be deemed and held to be a direct heating or cooling system of a building that raises or lowers the temperature of the space within the building for the purpose of comfort in which electric heating elements or products of combustion exchange heat either directly with the building supply air or indirectly through a heat exchanger using an air distribution system of ducts and having a mechanical refrigeration capacity of 15 tons or less. A heating system requiring air distribution ducts and supplied by ground water or utilizing a coil supplied by water from a domestic hot water heater not exceeding 150 degrees Fahrenheit requires either plumbing or heating group number one license to extend piping from valved connections in the domestic hot water system to the heating coil and requires either heating group number one or heating group number three license for installation of coil, duct work, controls, drains and related appurtenances.

Class I is allowed to perform work on all structures that do not exceed 15 tons.