AGL-CS-029 — Transformer Loading and VA Calculation
The AGL Knowledge Hub | Engineering Case Study Series. A loading-assessment investigation where incorrectly selected transformers were operating above rated capacity — from load measurement through percentage-loading calculation to correct VA selection.
Important: This is an engineering simulation created for training. It is not presented as an actual airport incident. All readings, values, dates and outcomes are illustrative, and no airport, organisation, person or manufacturer is identified or implied.
1Case Study Overview
During a runway LED lighting upgrade project, maintenance engineers observed that several newly installed isolating transformers were operating at noticeably higher temperatures than adjacent units.
The Constant Current Regulator (CCR) was operating normally, and all runway edge lights were functional.
A review of the installation records revealed that the transformers had been selected based on physical availability rather than load calculation.
The investigation focused on calculating the actual transformer loading and determining whether the installed VA ratings were appropriate for the connected LED fixtures.
This case demonstrates that correct VA calculation is essential for transformer temperature rise, voltage regulation, insulation life, and long-term AGL system reliability.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand transformer VA ratings.
- Calculate transformer loading correctly.
- Determine the required transformer VA for an AGL fixture.
- Calculate percentage loading.
- Evaluate transformer spare capacity.
- Understand the effect of overloading and underloading.
- Select appropriate transformer ratings for LED and conventional AGL fixtures.
3System Background
An AGL isolating transformer transfers power from the 6.6 A primary series circuit to the secondary lighting fixture.
The transformer must be capable of supplying the required apparent power (VA) demanded by the connected fixture.
Transformer loading affects:
- Temperature rise.
- Voltage regulation.
- Efficiency.
- Insulation aging.
- Service life.
4Problem Statement
The reported condition was:
- Several transformers overheating.
- LED fixtures operating normally.
- CCR healthy.
- No cable or connector faults.
- Different transformer VA ratings installed.
Are the transformers operating within acceptable loading limits, and what VA rating should have been selected?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| LED fixture rating | 52 VA |
| Installed transformer | 45 VA |
| Adjacent transformer | 65 VA |
| Transformer temperature | Elevated |
The installed transformer rating appeared smaller than the fixture requirement.
6Initial Engineering Hypotheses
The maintenance team considered:
Incorrect transformer VA selection.
High transformer loading.
Poor voltage regulation.
Thermal overloading.
Calculation error during installation.
7Transformer Loading Principle
Transformer loading is calculated as:
Example:
- Transformer rating: 65 VA
- Connected load: 52 VA
This is generally acceptable.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Clamp meter.
- Thermal imaging camera.
- Transformer nameplate verification.
- Approved AGL circuit test equipment.
- Manufacturer data sheets.
- Circuit drawings.
- Airport-approved PPE.
9Investigation
Step 1: Verify Fixture Load
The connected LED fixture measured:
| Secondary voltage | 18.5 V |
| Secondary current | 2.8 A |
The actual fixture load was approximately 52 VA.
Step 2: Calculate Transformer Loading
Installed transformer: 45 VA
The transformer was overloaded.
Correct transformer: 65 VA
This loading was acceptable.
Step 3: Temperature Correlation
Measured temperatures:
| 45 VA transformer | 79°C |
| 65 VA transformer | 53°C |
The overloaded transformer exhibited significantly higher temperature.
Step 4: Voltage Regulation Assessment
The overloaded transformer showed:
- Increased voltage drop.
- Reduced regulation.
- Higher winding temperature.
- Increased copper losses.
These effects explained the elevated operating temperature.
10Engineering Calculation Examples
Example 1: Determine Required VA Rating
Given:
- Fixture load: 48 VA
Allow 20% spare capacity.
Nearest standard rating: 65 VA
Example 2: Calculate Loading
Transformer: 100 VA
Fixture load: 68 VA
This transformer is comfortably loaded.
Example 3: Compare Two Transformer Ratings
| 45 VA | 40 VA load → 89% loading |
| 65 VA | 40 VA load → 62% loading |
The 65 VA transformer provides greater thermal margin.
11Root Cause
The installed 45 VA transformer was overloaded by a 52 VA LED fixture.
The transformer operated above its intended loading range, causing:
- Excessive temperature rise.
- Increased copper losses.
- Poor voltage regulation.
- Accelerated insulation aging.
The problem was caused by incorrect VA calculation during transformer selection.
12Corrective Action
The maintenance team:
- Calculated the actual fixture load.
- Verified transformer nameplate ratings.
- Replaced overloaded 45 VA transformers.
- Installed 65 VA approved transformers.
- Verified loading calculations.
- Measured operating temperature.
- Confirmed voltage regulation.
- Updated transformer selection procedures.
- Revised installation documentation.
- Trained maintenance personnel.
13Post-Repair Verification
The overheating was eliminated after correct transformer selection.
14Root-Cause Analysis
Transformer overheating
Fixture load determined
52 VA confirmed
45 VA identified
116% overload confirmed
Incorrect VA calculation
Install correctly rated transformer
15Why This Case Was Important
Incorrect transformer loading affects:
- Temperature rise.
- Insulation life.
- Reliability.
- Energy losses.
- Maintenance frequency.
Proper VA calculation is a design, procurement, and maintenance responsibility.
16Engineering Lessons
Always calculate actual fixture VA before selecting a transformer.
Always calculate actual fixture VA before selecting a transformer.
Transformer loading should normally remain below the rated VA.
Transformer loading should normally remain below the rated VA.
Provide spare capacity for reliable operation.
Provide spare capacity for reliable operation.
Temperature rise is a useful loading indicator.
Temperature rise is a useful loading indicator.
Nameplate verification should be mandatory during installation.
Nameplate verification should be mandatory during installation.
17Engineer's Decision Point
You have: LED fixture load: 54 VA. Available transformers: 45 VA and 65 VA. Which transformer should you install?
18Knowledge Check
19Interview Questions
20Frequently Asked Questions
Quick answers about transformer loading and VA calculation, based on this case study.
Q1What is AGL-CS-029?
AGL-CS-029 is an engineering training case study on transformer loading and VA calculation, following overheating of several newly installed transformers during a runway LED upgrade. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why were the transformers running hot?
They had been selected based on physical availability rather than load calculation. A 45 VA transformer was serving a 52 VA LED fixture, so it was overloaded, while adjacent transformers were rated 65 VA.
Q3What does an overloaded transformer cause?
The root cause was incorrect VA calculation during selection. The overload caused excessive temperature rise, increased copper losses, poor voltage regulation and accelerated insulation ageing.
Q4How do you choose the right transformer for a 54 VA LED fixture?
Install the 65 VA transformer. A 45 VA unit would be overloaded at 120% (54/45), while the 65 VA unit is loaded at about 83% (54/65) with spare capacity.
Q5Which tools were used to check the loading?
The team used a true-RMS multimeter, clamp meter, thermal imaging camera, transformer nameplate verification, approved AGL circuit test equipment, manufacturer data sheets and circuit drawings.
Q6How was the problem corrected?
The team calculated the actual fixture load, verified nameplate ratings, replaced the overloaded 45 VA transformers and installed 65 VA approved units. They verified loading and voltage regulation, measured operating temperature, updated selection procedures and installation documentation, and trained maintenance personnel.
Q7How was the repair verified?
Transformer loading was 80%, temperature was 53°C, voltage regulation was normal and light output was stable. The overheating was eliminated after correct transformer selection.
Q8What is the key lesson?
Select transformers by calculated load and verified nameplate rating, not by what is physically available. Checking VA against the fixture load before installation avoids overheating and premature insulation ageing.
Engineering & Expert Notes
Add Up the Real Load, Including Losses
CalculationUse the fixture's actual VA at each setting and include any extra load on the secondary. Do not rely on a nameplate from a different model.
Leave Headroom on the Transformer Rating
MarginRunning near the limit leaves no tolerance for ageing or variation. Choose a rating with sensible spare capacity.
Use Temperature as a Quick Check
DiagnosisA hot transformer under normal conditions suggests overloading. Compare with correctly loaded units.
Record the Basis of Selection
RecordsWriting down the load and rating used means future changes can be checked against it.
21Case Study Conclusion
Correct transformer loading and VA calculation are essential engineering requirements that directly affect temperature rise, voltage regulation, insulation life, and long-term reliability of AGL isolating transformers.
Systematic load measurement, VA calculation, percentage-loading assessment, thermal verification, and appropriate spare-capacity selection can prevent transformer overheating and significantly improve the reliability of the airfield lighting system.
In AGL engineering, a transformer should be selected by electrical calculation, not by physical availability or visual similarity.
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