Isolation Transformer Selection Calculator
A working starting point for selecting an AGL isolating transformer's rated power and checking secondary current — enter the series circuit current and fixture rating to get a recommended standard transformer size.
Inputs
Recommended Selection
Standard Transformer Rating Steps
| Standard Rating | Typical Application |
|---|---|
| 45 W | Low-intensity fixtures, small signs |
| 65 W | Taxiway edge/centreline, standard signage |
| 100 W | Runway edge, taxiway centreline (higher intensity) |
| 150 W | Runway centreline, touchdown zone |
| 200 W | High-intensity runway edge, approach lighting |
| 300 W | High-intensity approach lighting, sequenced flashing systems |
Frequently Asked Questions
Quick answers about sizing an AGL isolating transformer with this calculator.
Q1What does the Isolation Transformer Selection Calculator do?
You enter the series circuit current, the fixture's rated power and secondary voltage, and a safety margin. It returns a recommended standard transformer rating, the required power including margin, the approximate secondary current, the approximate turns ratio and the headroom against the fixture rating.
Q2How is the recommended transformer size calculated?
Required power is the fixture's rated power multiplied by one plus the safety margin. The calculator then picks the smallest standard rating that is at least that value. For example, a 45 W fixture with a 15% margin needs 51.75 W, so the recommendation is 65 W.
Q3Which standard transformer ratings does the calculator use?
45 W, 65 W, 100 W, 150 W, 200 W and 300 W. Typical applications range from low-intensity fixtures and small signs at 45 W up to high-intensity approach lighting and sequenced flashing systems at 300 W.
Q4How is the secondary current worked out?
It is the fixture's rated power divided by its secondary voltage. Typical secondary voltages are 6.6 V, 12 V or 24 V depending on the lamp or LED type.
Q5What series circuit currents can I choose?
6.6 A for a standard AGL loop, 20 A for a high-current loop and 2.8 A for low-step or obstruction circuits.
Q6What safety margin should I choose?
The calculator offers 10% for a tight fit, 15% as typical and 20% as conservative. The margin is the headroom above the fixture's rated power before choosing a transformer size.
Q7What happens if the required power is above 300 W?
The calculator flags that the requirement exceeds the largest standard rating listed. Such a fixture needs a custom or higher-capacity transformer, so confirm with the manufacturer before specifying.
Q8Can I use the result as the final transformer selection?
No. It is a first-pass sizing estimate for planning. Final selection must confirm compliance with the applicable specification (ICAO Annex 14, FAA AC 150/5345-53, IEC 61822 or IS 7785), the manufacturer's rated power and insulation class, and site factors such as cable length and voltage drop.
Q9What does the headroom figure show?
It compares the selected standard rating with the fixture's rated power, as a percentage. A positive value means spare capacity, and a negative value would mean the transformer is smaller than the fixture load.
Don't Just Size the Transformer. Understand the Series Circuit.
This calculator sizes an isolation transformer for your circuit load.
Build the technical knowledge behind it — series circuit behaviour, VA loading margins, temperature rise, insulation class, and how transformer selection affects the reliability of the whole loop.
Size it correctly. Understand the circuit. Install with confidence.
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Important Notice, Disclaimer & Limitation of Liability
Please read before relying on any content on this page or any result produced by it.
- Purpose and intended users. This page is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome electrical personnel, and for people learning the subject. It is not professional engineering, legal or regulatory advice, and using it does not create any professional or client relationship.
- Safety. Work on series circuits, constant current regulators (CCRs), isolating transformers, cables and connectors can be fatal. Always apply the aerodrome’s permit-to-work and isolation/LOTO procedures, prove dead, use correctly rated instruments and PPE, and follow the manufacturer’s instructions. Nothing here authorises work on live equipment or the bypassing of any safety device or interlock.
- Aviation operations. Nothing on this page authorises installation, energisation, return to service, a change of operating category, a NOTAM or any aerodrome certification decision. These rest with the aerodrome operator, the air navigation service provider and the competent regulator.
- Standards and regulations. References to ICAO Annex 14, FAA AC 150/5345-53, IEC 61822 and IS 7785 are for orientation only. They are summarised, may be incomplete or superseded, and are not official text. Final selection must follow the current official editions, the contract specification and the manufacturer’s data. Using this calculator does not demonstrate compliance with any law, standard or regulatory approval, and official documents and the competent regulator always prevail.
- No warranty. The calculator and its content, including the table of typical applications for each rating, are generic and indicative and are provided “as is” and “as available”, without warranty of any kind, express or implied, including accuracy, completeness, currency, freedom from errors or fitness for a particular purpose. You are responsible for checking every input and every result before relying on it.
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