Where do the isolation transformers go?
Below ground, close to the light they serve. An inset light on a deep base can carries its transformer in that can beneath it (on a shallow base, in an adjacent can or pit); an elevated light or a sign has its transformer in a separate can or pit beside it or, on some designs, in the base can it stands on. The series loop passes through, and a short secondary lead feeds the light.
Each light or sign on a series circuit normally has its own isolation transformer, and it is installed below ground, as close as practical to the light it feeds. Which can it sits in depends on the fixture. An inset light on a deep base can has its transformer in that can under the fixture (on a shallow base, in an adjacent can or pit); an elevated light or a sign usually has a separate can or pit beside it, though an elevated light mounted on a base can may carry the transformer in that can. In every case the primary loop cable passes through the can and connects to the transformer’s primary connectors.
Keeping the transformer below ground keeps heavy equipment off the frangible mast and protects it from weather, jet blast and traffic. Keeping it close keeps the low-voltage secondary lead short, so lamp power is not lost in cable. Cans are fitted with a load-rated cover at finished ground level and drained, and spare cable is left so a transformer can be lifted out for testing, with the circuit isolated, without disturbing the cable runs. Record each location on the as-built drawings against its circuit and fixture reference.
Frequently Asked Questions
Q1Where is the isolation transformer installed in an AGL circuit?
Below ground and as close as practical to the light it serves. For an inset light on a deep base can it sits in that can, directly under the fixture; on a shallow base it sits in an adjacent transformer can or pit. For an elevated light or a sign it usually sits in a separate can or pit beside the fixture, or in the base can the elevated light stands on, with the primary loop cable passing through it.
Q2Why is the transformer kept close to the light?
The secondary side carries the lamp or LED load at low voltage, so a long secondary run wastes power and adds voltage drop. The long run is the primary series loop; the secondary is kept short and protected.
Q3Why is the transformer not mounted on the light itself?
Elevated lights must be frangible so they break away if an aircraft or vehicle strikes them, which rules out heavy equipment on the mast. A below-ground transformer is also protected from weather, jet blast and traffic.
Q4Is there one transformer for every light?
Most designs use one transformer per light or sign, rated for the connected load. Any arrangement that feeds more than one fixture from a transformer is set by the design drawings and the manufacturer’s data.
Q5How should transformer locations be recorded?
On the as-built drawings, against the circuit and fixture reference, with the transformer rating. Cans and pits stay reachable under a load-rated cover, and spare cable is left so a transformer can be lifted out for testing or replacement, with the circuit isolated, without disturbing the cable runs.
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