257 In lieu of the oven construction described in 25.5 and 25.6 the oven may be a commercially manufactured oven having still-air convection heating.The heaters are to be located below the floor of the oven compartment,and are to be thermostatically controlled to provide the required oven temperature regardless of the heat gain from the device under test.Glowing coils shall not radiate directly onto the ballast enclosure.The volume of the oven compartment is to be between 5-1/2 ft3(0.156m3)and 8 ft3(0.226m3)