Low-Voltage Lighting Transformers

By Constantine Pergantis, Nite Lites, North Potomac, MD

Transformers are the heart of any low-voltage lighting system. A transformer can convert the 120V line voltage electricity down to a safe 12 volts.

Question 1: Number of lights on one transformer

Question 2: Entire rated capacity of the transformer

Question 3: Auto transformer or isolation transformer

Question 4: Advantage of a two-circuit transformer

Question 1: How many lights can I put on one transformer?

Pergantis: Transformers are rated by their capacity in watts. Transformers are typically available in sizes 100, 200, 300 and 600 watts. You must determine how many lights/watts your system will have initially and in the future.

For example, if you start out with five path lights at 20 watts and four spotlights at 50 watts, you need a 300-watt transformer. However, if you expect to add more lights in the future, you probably are better off with a 600-watt transformer.

Question 2: Can I use the entire rated capacity of the transformer?

Pergantis: You sure can. Just know the capacity of your transformer and don't exceed it. Most transformers are equipped with a circuit breaker to protect them from overloading.

There has been a misconception over the years that you can use only 80 percent of a transformer's capacity. The only reason to buy a transformer larger than you need is to provide capacity for adding more lights in the future.

Question 3: Should I use an auto transformer or an isolation transformer?

Pergantis: Isolation transformers are the safest. They may cost more, but safety always comes first.

Auto transformers have one copper winding inside. If the winding fails, the full 120 line voltage could travel through the transformer and harm someone. Isolation transformers have two copper windings and operate magnetically to eliminate the risk of electrocution.

Question 4: What is the advantage of a two-circuit transformer?

Pergantis: Two-circuit transformers offer more efficient power distribution. They allow longer cable runs and increase light output by reducing voltage drop. The total wattage of the transformer can be used over one long cable run or can be equally split among two shorter runs.

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