A Guide to Transformers For Lighting

Transformers are used to reduce the standard line-voltages to a low-voltage such as 12VAC. VA (Volt-Amps) indicates your power supply/transformer requirement.  All transformers are sized to a rating of Volt-Amps.  There are two different styles of transformers to choose from, either remote or surface mount transformers. Transformers can then be broken down into two types: magnetic or electronic transformers.

Low-voltage systems, or 12v, are generally more flexible and suited for residential environments.  Line-voltage systems, or 120v, are more suited for permanent commercial and institutional applications.  Review the advantages and disadvantages to 12v and 120v lighting systems below.

12v Lighting System:

  • Cable can be re-positioned, so there is flexibility when installing or relocating
  • Designed for damp conditions and operates safely in wet conditions
  • Lamps available in low wattage and various beam spreads
  • Can be plugged into existing outdoor receptacles
  • Transformer required
  • Voltage drops can occur if installed improperly

120v Lighting System:

  • Wire must be buried 18 inches and placed in a protective conduit; it is easy to install if area is pre-wired
  • All fixtures and connectors must be waterproof
  • Lamps and fixtures are larger and less fixtures are required for a complete lighting plan
  • Higher watt lamps and one or two beam spreads produce high light levels and can be wire into home’s electrical supply
  • Must always be installed by a licensed electrical contractor
  • No transformer required; voltage-drop not as critical

Low-Voltage Wire Size Table

Use this table to help determine the correct wire size for a recommended 3 percent drop in voltage at the specified wattage.  The length shown is the length of wire from the transformer to the power feed for 12 VOLT or 24 VOLT.

12 VOLT 5 ft. 6-15ft 6-20 ft. 21-40 ft. 41-60 ft. 61-90 ft.
150 watt #12 GA #8 GA #6 GA #4 GA #2 GA #1 GA
300 watt #10 GA #6 GA #4 GA #1 GA 1/0 3/0
24 VOLT 5 ft. 6-15ft 6-20 ft. 21-40 ft. 41-60 ft. 61-90 ft.
300 watt #14 GA #12 GA #10 GA #6 GA #6 GA #4 GA
600 watt #12 GA #8 GA #6 GA #4 GA #2 GA #1 GA

Remote Transformers are installed at a distance from the light fixture, which allows the transformer to be hidden. Please note that the electrician installing the fixture must be aware of the voltage drop.

As a general rule of thumb, to achieve a minimum voltage drop of 3% from the output of the remote transformer to the power feed canopy, use the appropriate wire gauge specified in our voltage drop table shown below.

Surface Mount Transformers are installed close the actual lighting fixture’s junction box in the ceiling.  Since surface mount transformers are visible, they are typically designed in a sleek canopy.  A benefit of using surface mount transformers  is that they will not experience drops in voltage the way remote transformers do and do not have boost traps.

Magnetic Transformers operate at standard low frequencies (50/60 Hz).  They are both very durable and reliable.  They experience much less voltage drop over long distances compared to high frequency electronic transformers.

Electronic Transformers are very compact and much smaller than their magnetic counterparts. They provide built in protection against electrical shorts applied at the outputs and to the lighting system.  In order for electronic transformers to operate properly, the output must be loaded to a minimum of 50% of the rated wattage of the transformer.  Any output load below 50% may result in lamps flickering and bulbs to burn out prematurely.  Since electronic transformers operate at much higher frequencies, most standard volt amp meters intended for 60hz type measurements cannot be used to accurately measure the outputs of these transformers.

Electronic transformers may interfere with nearby radios or televisions.  If you experience an issue, a line filter may be installed at the transformer source or the television/radio.  An alternate option is to use a magnetic transformer, which operates at a lower frequency and will not cause any interference.

Landscape Transformers.

hinkley transformer

Hinkley Lighting Landscape Transformer

Under cabinet transformers.

kichler lighting transformer

Kichler Lighting Transformer

Track Lighting Transformers.

Tech Lighting Transformer

Tech Lighting Transformer

To properly size your transformer, add up the wattage for all incandescent lamps and VA (Volt-Amps) for all LED lamps and fixtures.  Both incandescent and LED can be on the same tap/wire. Your energy cost is measured by watts.