What are transformer wattage ratings?
The power rating of the small isolation transformer at every light. It is a ceiling on the load the unit can carry, so the aim is a rating just above the fixture, not far above it.
Every light on a series circuit sits behind its own isolation transformer, and each one carries a wattage rating on its nameplate. It is easy to read it as the load. It is the ceiling: the most power the unit can supply to the fixture at rated current.
1Made in standard steps
Ratings come in set sizes, for example 30/45, 65, 100, 150, 200, 300 and 500 W (sizes from FAA AC 150/5345-47, which also lists small 10/15 and 20/25 W units). The 30/45 W unit is dual-rated, so a 45 W fixture on it sits at its upper rating, not over it. You do not order a transformer for exactly 45 W; you take the nearest standard size at or above the fixture's real draw.
2Too small runs hot
If the fixture draws more than the rating, the transformer is overloaded. It runs hot, its output sags, the lamp is dimmer than it should be, and the insulation and life suffer. This is the one to avoid first.
3Too large wastes power
Bigger is not safer. A lightly loaded transformer still draws magnetising current, so efficiency and power factor fall, and the loss is paid at every light on the loop. The target is a unit loaded close to its rating without exceeding it.
4It all adds up on the CCR
The regulator supplies the sum of every transformer's load plus losses. Rating choices repeated across hundreds of lights decide whether the circuit sits comfortably within the CCR rating, so a sensible size at each light protects the whole loop.
| Rating | Loading of a 50 W fixture | Result | Effect on the circuit |
|---|---|---|---|
| 30/45 W | About 111% of the 45 W upper rating | Overloaded | Runs hot, lamp dims, early failure |
| 65 W | About 77% | Well matched | Efficient, room to spare |
| 100 W | 50% | Lightly loaded | Lower efficiency, extra load on the CCR |
| 200 W | About 25% | Much too large | Magnetising loss at every light |
Frequently Asked Questions
Q1What is the wattage rating of an isolation transformer?
It is the maximum power the transformer is built to supply to the fixture on its secondary, at the rated series current. It is printed on the nameplate, and the fixture's load should sit within it.
Q2What are the standard wattage ratings?
Series isolation transformers are made in set sizes, for example 30/45, 65, 100, 150, 200, 300 and 500 W, and the 30/45 W unit is dual-rated; smaller 10/15 and 20/25 W units also exist. The sizes that apply depend on the specification and the manufacturer, so check the data sheet and the design.
Q3Is a bigger transformer safer?
No. A transformer carrying much less than its rating still draws magnetising current, so its efficiency and power factor fall, and that cost is added at every light on the circuit. An overloaded transformer is worse, because it runs hot and the lamp is dimmer than it should be.
Q4How do I pick the right rating?
Find the fixture's real draw on the secondary, plus anything else connected there, then take the nearest standard rating at or above it. Check that the unit is not heavily oversized, and allow for a likely lamp change such as halogen to LED.
Q5Does the wattage rating change the series current?
No. The series current is set by the CCR, and the transformer ratio sets the secondary current, for example 6.6 A : 6.6 A. The wattage rating says how much power the transformer can deliver at that current, not what the current is.
Q6How do the ratings affect the CCR?
Each transformer's load, plus its losses, adds to the series circuit, so the regulator must supply the sum. Oversizing transformers across a circuit adds avoidable load on the CCR.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.