Design load capacity is calculated by taking into account the total heat gain from all sources, such as people, lights, and equipment, and subtracting the total heat loss from the building envelope. This calculation is based on the building’s size, orientation, insulation, and other factors. The total heat gain is then divided by the total cooling capacity of the HVAC system to determine the design load capacity. This calculation should be done by a qualified HVAC technician to ensure accuracy and efficiency.