6.6/6.6 A VS 20/6.6 A
A 6.6/6.6 A isolation transformer has the same rated current on both sides — essentially a 1:1 current-ratio transformer whose main job is electrical isolation. A 20/6.6 A transformer is rated for a 20 A primary and a 6.6 A secondary, so it provides isolation and current transformation together. The notation describes rated currents, not voltage — always confirm actual ratings from the nameplate/datasheet.
At a Glance
| Feature | 6.6/6.6 A Transformer | 20/6.6 A Transformer |
|---|---|---|
| Primary rated current | 6.6 A | 20 A |
| Secondary rated current | 6.6 A | 6.6 A |
| Current ratio | 1:1 | 3.03:1 |
| Main function | Isolation | Isolation + current transformation |
| 6.6 A AGL circuit | Directly suited | Not normally the primary-side arrangement |
| Secondary AGL circuit | 6.6 A | 6.6 A |
| Primary current capability | Lower | Higher |
| Typical application | Standard AGL series circuit | Higher-current/special circuit arrangement |
| Main difference | Same rated current on both sides | 20 A primary → 6.6 A secondary |
Same Current on Both Sides
The 6.6/6.6 A transformer is rated at 6.6 A on the primary and 6.6 A on the secondary. Because the rated current is the same on both sides, it is essentially a 1:1 current-ratio isolation transformer. Its primary purpose is electrical isolation between the series circuit and the individual light fixture — not current reduction.
6.6/6.6 A Flow
The rated current entering the transformer equals the rated current leaving it — a straightforward 1:1 isolation stage.
Key Points
- Primary: 6.6 A · Secondary: 6.6 A
- Current ratio ≈ 1:1
- Suited to the standard 6.6 A AGL series lighting circuit
- Function is isolation, not current transformation
Higher-Current Primary, 6.6 A Secondary
The 20/6.6 A transformer is rated at 20 A on the primary and 6.6 A on the secondary. It is designed to interface a higher-current primary circuit with a 6.6 A secondary circuit, so it provides both galvanic isolation and current transformation in the same unit.
20/6.6 A Flow
The 6.6 A secondary side then interfaces with an AGL lighting load designed for that current.
Key Points
- Primary: 20 A · Secondary: 6.6 A
- Current ratio ≈ 3.03:1
- Provides isolation and current transformation
- Interfaces a higher-current arrangement with a 6.6 A load side
Current Ratio Side by Side
The easiest way to remember the difference is by the current ratio each transformer carries between primary and secondary.
The Underlying Relationship
For an ideal transformer, approximately:
V₁ × I₁ ≈ V₂ × I₂ — so when the secondary current is lower than the primary current, the corresponding voltage relationship differs accordingly. For the rated current ratio, I₁ / I₂ = 20 / 6.6 ≈ 3.03, giving a current ratio of approximately 3.03 : 1. The actual voltage and power ratings must always be checked from the transformer nameplate/datasheet.
A Simplified Series Circuit
In a simplified AGL series circuit, the Constant Current Regulator (CCR) drives 6.6 A through the series loop into a 6.6/6.6 A transformer, which isolates the individual light from the series circuit and passes the same 6.6 A through to the fixture.
CCR → 6.6/6.6 A → Fixture
The transformer isolates the individual light from the series circuit without changing the rated current.
Interfacing a Higher-Current Circuit
Where the primary-side circuit runs at 20 A, a 20/6.6 A transformer isolates and steps the current down so the 6.6 A secondary side can interface with an AGL lighting load designed for that current.
20 A Circuit → 20/6.6 A → Fixture
The 6.6 A secondary side can then interface with an AGL lighting load designed for that current.
Current Rating Is Not Voltage Rating
Do not read 6.6/6.6 A as 6.6 kV / 6.6 kV. Likewise, 20/6.6 A does not by itself specify the transformer's voltage ratio. This notation describes current ratings only. The nameplate/datasheet must always be checked for primary voltage, secondary voltage, rated current, VA/kVA, frequency, insulation level, temperature rise and the applicable standard.
Parameter-by-Parameter View
| Parameter | 6.6/6.6 A | 20/6.6 A |
|---|---|---|
| Primary current | 6.6 A | 20 A |
| Secondary current | 6.6 A | 6.6 A |
| Current ratio | 1:1 | 3.03:1 |
| Isolation | Yes | Yes |
| Current conversion | No significant current conversion | Yes |
| Standard 6.6 A series circuit | Typical fit | Depends on application |
| Primary-side circuit | 6.6 A | 20 A |
| Secondary-side circuit | 6.6 A | 6.6 A |
Common Misreadings
The two transformers have different primary current ratings and current ratios. A connected circuit must match the transformer's rated current, voltage, VA rating and intended application — not just one shared number.
These figures describe rated current, not voltage. Reading 6.6/6.6 A as 6.6 kV/6.6 kV, or assuming 20/6.6 A sets the voltage ratio, is incorrect — voltage must be confirmed separately from the nameplate/datasheet.
It performs a different function. The 6.6/6.6 A unit provides isolation at a 1:1 current ratio, while the 20/6.6 A unit provides isolation together with current transformation across a ≈3.03:1 ratio.
Choosing the Right Rating
When identifying or specifying a 6.6/6.6 A or 20/6.6 A transformer, consider:
- The rated current on the primary-side circuit the transformer will actually be connected to
- Whether the secondary-side fixture requires the standard 6.6 A AGL series current
- Whether isolation alone is sufficient, or current transformation is also required
- Confirming primary voltage, secondary voltage, VA/kVA, frequency, insulation level and temperature rise from the nameplate/datasheet
- Applicable ICAO/national AGL requirements and airport-specific technical specifications
- Never assuming interchangeability based on a shared "6.6 A" figure alone
Which Transformer Applies Where?
A 6.6/6.6 A transformer carries the same rated current on both sides, making it a 1:1 current-ratio isolation transformer suited to the standard 6.6 A AGL series lighting circuit. A 20/6.6 A transformer is rated for a 20 A primary and a 6.6 A secondary, so it provides isolation together with current transformation for a higher-current or special circuit arrangement.
The notation describes rated current only — it does not specify voltage. Final selection for an actual project should always be based on the transformer nameplate/datasheet, the approved circuit design and the applicable ICAO/national AGL standard.
Engineering & Expert Notes
Never Mix Current Ratings on a Circuit by Assumption
CompatibilityThe primary current rating must match the series circuit. A unit that fits physically may not be suitable electrically.
Check the Nameplate at Every Replacement
VerificationRead the rating before fitting. Mislabelled or mixed spares are a common cause of wrong replacements.
Know Which Circuit Each Location Belongs To
RecordsMark the circuit current on drawings and registers. It prevents errors when working on mixed airfields.
Treat Primary Connections With Care
SafetyThe primary side carries the series current. Follow isolation and permit procedures before any disconnection.
Frequently Asked Questions
It states the primary and secondary rated currents of the transformer — for example, 20/6.6 A means a 20 A primary and a 6.6 A secondary. It does not describe voltage.
Not significantly. Since the rated current is the same on both sides, it functions essentially as a 1:1 current-ratio isolation transformer.
Approximately 3.03:1 (20 A ÷ 6.6 A ≈ 3.03), calculated from the rated primary and secondary currents.
Not automatically. The connected circuit's rated current, voltage, VA rating and intended application must match the transformer — the two types should not be treated as interchangeable simply because both have a 6.6 A side.
No. The figures describe rated current, not voltage. Voltage ratings must be confirmed separately from the transformer nameplate/datasheet.
To interface a higher-current primary circuit arrangement with a secondary circuit designed for the standard 6.6 A AGL fixture current, combining isolation with current transformation in one unit.
Yes. Both the 6.6/6.6 A and 20/6.6 A units provide electrical isolation; the difference is whether current transformation also occurs across the unit.
Governing Documents
For an actual airport project, verify the current applicable editions of the relevant standards and the specific transformer nameplate/datasheet before using this comparison for project design.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — series circuit requirements and isolation transformer current characteristics for AGL.
ICAO Aerodrome Design Manual
Applicable material covering series circuit design and isolation transformer rating guidance.
National CAA / DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI requirements, airport-specific technical specifications, project drawings and contract specifications.
Final Engineering Note
Exact voltage, VA rating and insulation requirements can vary with the governing standard, equipment manufacturer and approved airport design. For final design, use the current applicable standard and manufacturer datasheet rather than a generic comparison page.
6.6/6.6 A describes a 1:1 current-ratio isolation transformer for the standard AGL series circuit; 20/6.6 A describes an isolation transformer that also transforms current from a 20 A primary to a 6.6 A secondary. The notation is current only — voltage, VA and insulation must always be confirmed from the nameplate/datasheet.
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