63 x 5. 28 inches and the install process follows the same steps: after choosing the rooms in which you’re going to install the Nest Protect at least in every room which has a fuel burning appliance, you need to make sure there is a minimum of 10 feet from any cooking appliance and avoid placing it on a corner you can choose either a wall or the ceiling. Then, you need to take the provided backplate and fix it to the wall/ceiling using the four screws and afterwards, just connect the smoke detector to the backplate using a circular motion if you have the wire powered version, the installing process also includes connecting the device to a power source, but, it’s ideal that this is done by a qualified person, to make sure the installation is made in accordance with the country or state’s regulations. The way to differentiate the first and the second generation of Nest Protect is by verifying if it has a battery door the Nest Protect 1 does not have one, verify the back plate the second generation has a round backplate, while the first has a square plate and the easiest one is to look at the edges of the device the second generation has rounded corners all around the case, while the first gen has straight edges. Right now, on the market, there are three types of smoke detectors: those that use ionization sensors, those that use photoelectric sensors and lastly, those that use both type of sensors also called dual sensor smoke alarms. The Nest Protect, theoretically, falls into the last category, but things are a bit more complicated because it doesn’t have a ionization sensor, but it does use the Split Spectrum Sensor which nullifies the need of the radioactive ionization sensor as you know, it detects smoke produced by flaming fires which burn at a fast rate so, if the ionized air is disrupted by smoke, it triggers the alarm.