Back in the late 1950s when Bill Locklin invented the first 12-volt lighting system, he provided power to it with a low voltage AC transformer. Out of necessity, he built it himself. The transformers we use today are remarkably similar, and it’s useful to understand how they work.
A transformer does not transform anything. The way we use it is more like a power reducer. It functions like a pressure regulator on a drip system, which lowers the pressure but does not cut off the flow.
Transformers work on the principal of electromagnetic induction. There are two sides to a transformer: a primary side and a secondary side. The primary side accepts a higher current such as 120-volt. The secondary side is the low voltage side. This can produce 12-volt AC power or be a multi tap from 12- to 15- volt AC power. When the transformer is plugged into a 120-volt receptacle, a voltage is induced from the primary side to the secondary side. This means that the 120-volt side never comes in direct contact with the 12-volt side. A circuit breaker on the low voltage side protects the low voltage side. If a set of cables in the field should cross and create a direct short circuit, the breaker will trip. This will also happen should the transformer be overloaded.
When mounting a transformer, it is important to follow all of the manufacturer’s specifications.
Above-ground transformers must be installed a minimum of 12 inches above grade. This is measured from finished grade to the bottom of the transformer. Most transformers will be mounted outdoors on a wall. Always use the proper mounting hardware. Plug the transformer into a ground fault circuit interrupter protected outlet with a plastic bubble cover. Be sure to mount the transformer within 3 feet of the GFCI receptacle. Create a drip loop on the power cord. This will help keep the transformer dry.
Transformers can be mounted to a pressuretreated lumber post. Some manufacturers provide a prefabricated unit with a GFCI and bubble cover. Both should be set in a concrete footing. Often, a transformer is installed by the swimming pool equipment. There is normally a GFCI receptacle mounted to the side of the pool panel. Do not plug the transformer into this receptacle. This is the GFCI for the pool light. Have a licensed electrician add another circuit breaker and GFCI receptacle for landscape lighting.
Be sure you are using the correct conduit size for field wires you are installing into the transformer. This conduit should also include male adapters for the knockout size on the bottom of the transformer. To allow the cable to move through the sleeve more easily, 90-degree electrical sweeps should be installed on the bottom of the conduit. Using the conduit will protect your field wires and provide a tidy installation.
Before you install cable to the transformer terminals, make sure the transformer is unplugged from the 120- volt receptacle. It is especially important to have a solid mechanical connection on the terminals. Loose terminal connections can cause fires. Strip the insulation off your cable carefully and avoid cutting any strands. Open the terminal throat with the proper screwdriver. Place one side of the cable into the proper voltage tap and tighten it. Rotate the cable in a clockwise motion and tighten the terminal as much as possible. Repeat this process with the other side of the cable into the common terminal block. The block will normally be gray or white in color and marked “COM.”
When this is completed, plug in the transformer to the receptacle. Turn on the circuit breaker to test the system. The circuit breaker will need to be in an on position to provide power to the field. Follow all the manufacturer’s instructions for installing timer photocells or Wi-Fi smart plugs specifically for low voltage landscape lighting.
Transformers are the heart of any low voltage lighting systems. Be sure to select one with Underwriters Laboratories and or ETL certifications.
