How to Wire A Low-Voltage Transformer

May 20, 2020 August 10th, 2020 No Comments

Transformer Instructions “Quick Start”


  • The transformer must be plugged into a GFCI (ground-fault circuit-interrupter) circuit (either a GFCI receptacle or GFCI breaker at the circuit panel)
  • Do not use extension cords with transformers.
  • The transformer must be mounted in a vertical position.
  • The bottom of the transformer must be at least 12″ from the ground.
  • A transformer may become hot; do not mount on vinyl.

On the secondary side (low voltage taps) of the transformer, each circuit has its own magnetic circuit breaker (toggle switch) that is connected to a Common Tap (marked with a label, “COM”). These Common Taps can accommodate up to 300W (25A). The exception to that is when the transformer capacity is less than 300W; in that case, the capacity will be equal to the transformer capacity.

Even though the capacity of each Common Tap is 300W (or transformer capacity if less than 300W), the National Electric Code specifies that a circuit can only be loaded up to 80% of its maximum. That means that the working capacity is 240W (or 80% of the transformer capacity is smaller than 300W). Furthermore, it is important to account for the loss of current through the wire, which reduces the capacity by an additional 10%. This means that the true optimal working capacity of a transformer is about 70% of the transformer’s total capacity.

Generally, it is a good practice to connect each of your home run lines directly to the transformer, spreading them evenly among the available commons. Ideally, if two or more home run wires share a voltage tap, then connect them to the same common.

Connecting lighting runs to the transformer:

The home run cable (e.g. 10/2 or 12/2) has two wires. One wire gets connected to the Common Terminal (labeled “COM”), the other wire connects to a Voltage Terminal (the colored labels that say 12, 13, 14, etc). The correct Voltage Terminal is the one that results in an optimal voltage at the fixtures connected to that run. This is a Multi-Tap transformer, which means you can output at a variety of voltages to compensate for Voltage Drop (see our learning area for more information on layouts and voltage drop). For example, if you have a long run of 100 feet you may need to use the 15v Voltage Terminal in order to have 12v of power all the way out at the far away fixtures.

You can hook up multiple home runs to each and any of the terminals. For example, you may have 6 home runs, in which case you would have 6 wires in the same COM terminal and the other half of the 10-2 wires go into the colored terminals. The Voltage Terminals can have multiple runs/wires in each tap as well. To connect a home-run wire to the terminals, split the paired wire down the middle to separate the two wires and strip half an inch of insulation off each wire. One end goes to the COM terminal, the other into a voltage terminal. Loosen the terminal screws (small flat head screwdriver) and insert the wire into the terminal opening which is located at the bottom (90 degrees from the terminal screw facing the bottom of the transformer). Tighten screw to secure wire(s).


Low voltage wire does not have polarity, it does not matter which of the two wires connects to the Common Terminal and which to the Voltage Terminal, as long as one wire goes to each. Optional timers and photocells plug right into the transformer ports labeled and located inside the cabinet. It is easier to see and insert the wires into their terminals when the transformer is lying on the ground. Affix all your wires BEFORE mounting the transformer.

VOLT Outdoor Lighting Transformer Troubleshooting


  1. First, check that there is power at the GFI outlet. Do not assume there is power. Check it with a voltmeter and make sure there is 120v AC. If you have a timer please unplug/remove it. They are often a source of failure.
  2. Test the Timer Outlet inside the transformer cabinet with a voltmeter. You should have 120v AC there as well. If you do, go ahead and plug the timer receptacle cord back into the timer receptacle outlet (the short cord inside the transformer cabinet needs to be plugged into the socket inside the cabinet) to complete the circuit or the transformer will not operate. If you have a photocell installed, please remove it.
  3. Then put the jumper cable back into the photocell receptacle port to complete the circuit or the transformer will not operate. Be sure the jumper is pushed down tight. If you have a photocell in the transformer will not turn on when it is light out. Even if you cover up the photocell there is a delay (the photocell needs to be covered for about 1 minute before it will allow the transformer to turn on).
  4. Make sure all the toggle switches are in the “ON” position (UP). The toggle switches are circuit breakers and can trip if there is a surge, overload or short. Sometimes the circuit breakers themselves can fail.
  5. Now it’s time to test the secondary side (low voltage taps/ multi-tap). Do not use the screws facing forward to test for voltage, put a voltmeter in the garage door style opening that holds the cable (on the underneath side of the outdoor transformer). Put one voltmeter prong on the common, the other on a tap (eg 12v tap or 15v tap). You should read anywhere from 12v AC to 22V AC depending on which tap you are testing. If you do not get any voltage, remove any wires (home run wires) from the common and the taps you are testing and try again without any load.

Contact Us

Service Line: 813-978-3700

Mon - Fri 8 AM - 8 PM ET

Sat - Sun 10 AM - 6 PM ET

Local Pickup (Lutz, FL)

Mon - Fri 8 AM - 6 PM ET

Quick and Easy FAQs

Get answers when you need them.

Are You a Contractor?

Sign up for benefits. Apply for a Wholesale Account.