Installing a doorbell transformer requires two basic steps. Finding a location and then setting the transformer. The second step is hooking up the wiring. Most of the time, you are going to replace a transformer that is already in place. This is the easiest installation.
Not sure if you need to replace the doorbell transformer? See the articles, 'How To Fix a Doorbell' and 'Troubleshooting Wired Doorbells' for more information.
Locating the Transformer
A doorbell transformer has to have a supply of the 110/115 volt power from your homes electrical power supply. It needs to be in a location where wiring can be run to it. Near the main panel, subpanel or junction box would be a likely choice. You will need to consult local building codes and have an understanding of how to add electrical wiring if this is a totally new installation.
The articles 'Basic House Wiring' and 'Electrical Safety Tips' can provide some useful information. Never work on wiring with the power on. Always double check with a tester before you disturb or alter any wiring. Consulting an electrician would be a wise choice if you are unfamiliar with electrical wiring.
The decision is much simpler if you already had a transformer and you are just replacing it. You can install the new transformer in the same location as the old one. Make sure the Power is Off!! before you remove the old transformer. Unhook the wires on either side of the transformer. You will have the two power wires (and possibly a ground), on one side. On the other side you will have two low voltage wires.
Hooking Up the Wires
With the transformer in place you can hook up the wires. First hook up the power supply wires and the ground wire if a lug is provided. Check the instructions that came with your doorbell kit to verify that you are using the right terminals. After the power supply is hooked up, you can install the low voltage wires. Generally, it does not matter which wire is hooked to which terminal. If the neutral and hot are indicated and the doorbell wire is color coded, such as 'red' and 'white'. You can hook the white wire to the neutral and the 'red' or colored wire to the hot lug.
Many times you will find that the doorbell wire is not color coded and the terminals are not marked. Check the instructions to make sure that wires can be generically installed. Hook one wire to each terminal or as indicated in the instructions.
Leave the power to the transformer turned off until the rest of the doorbell wiring is done.
The transformer is a simple device. All it does is reduce the regular household current down to a voltage that the door bell can use. If you already have one, replacing them is not that hard.