Primary Cable VS Secondary Cable
Primary cable carries the AGL series-circuit current from the CCR to the isolation transformer's primary winding. Secondary cable connects the transformer's secondary winding to the individual light fixture. The biggest difference: the primary side is part of the CCR-controlled series circuit and can carry a relatively high circuit voltage, while the secondary side is a short, transformer-isolated connection sized to the fixture.
At a Glance
| Feature | Primary Cable | Secondary Cable |
|---|---|---|
| Connected to | CCR / AGL series circuit | Individual AGL fixture |
| Transformer side | Primary winding | Secondary winding |
| Circuit | Series circuit | Individual fixture circuit |
| Current | CCR-regulated series current | Depends on transformer/fixture |
| Voltage | Can be high, depending on AGL design | Lower, depending on transformer |
| Insulation requirement | Higher-voltage AGL insulation | Appropriate to secondary voltage |
| Typical length | Can be long | Usually short |
| Main function | Carry series-circuit power to transformer | Supply individual light |
| Connection | AGL series-circuit cable/accessories | Fixture/transformer secondary connection |
What Is Primary Cable & Secondary Cable?
Primary Cable
The cable carrying the AGL series-circuit current from the CCR to the isolation transformer's primary winding. It forms part of the CCR-controlled series circuit.
Main characteristics
- Part of the CCR-fed AGL series circuit
- Insulated for the specified series-circuit voltage
- Can run a long distance between CCR and transformer
- An open primary cable can interrupt the series circuit
Secondary Cable
The cable connecting the isolation transformer's secondary winding to the individual light fixture. It is the relatively short link on the fixture side of the transformer.
Main characteristics
- Downstream of the isolation transformer
- Rated for the transformer's secondary operating conditions
- Usually short, since the transformer sits near the fixture
- A failure here is normally localized to that one fixture
The Biggest Difference
The easiest way to remember the difference between primary cable and secondary cable:
Primary Cable
CCR → primary cable → transformer primary
Secondary Cable
Transformer secondary → secondary cable → light
Easy memory: Primary = CCR side. Secondary = light side.
Where Each Cable Sits in the Circuit
Primary Side
The primary cable is part of the AGL series circuit, carrying the CCR-regulated current to the transformer's primary winding.
Secondary Side
The secondary cable carries the transformer's secondary output the short distance to the individual light fixture.
Series-Circuit Voltage Considerations
An AGL series circuit can operate at a relatively high circuit voltage because many transformer/fixture loads are connected in series along the same CCR loop. The primary cable's insulation must therefore be suitable for the specified AGL series-circuit voltage.
This is why an ordinary LV cable should not automatically be substituted for AGL primary cable, even if its conductor current rating looks sufficient.
How Current Differs on Each Side
The primary current in a typical AGL series circuit is set and controlled by the CCR. The secondary current is determined instead by the transformer's electrical characteristics and the connected fixture load. For example, a transformer might be specified with a primary/secondary current arrangement such as 6.6 A → 6.6 A, or another approved combination such as 20 A → 6.6 A — the actual ratings must always come from the transformer specification.
| Parameter | Primary Cable | Secondary Cable |
|---|---|---|
| Current source | CCR-regulated series current | Transformer secondary output |
| Typical value | Set series-circuit current (e.g. 6.6 A / 20 A) | Depends on transformer/fixture (e.g. 6.6 A) |
| Governed by | CCR setting/brightness step | Transformer specification |
Primary Cable vs Secondary Cable Length
A major practical difference is usually the cable length. The primary cable can run a long distance as it carries the series circuit from the CCR, through duct and trench runs, to each transformer. The secondary connection is commonly much shorter because the transformer is located close to the fixture it feeds. The actual permissible lengths depend on the specific AGL design.
Construction Is Not Fixed by the Name Alone
The two cables are not necessarily different merely because one is called "primary" and the other "secondary." Their actual specifications depend on voltage rating, current rating, insulation, conductor size, environmental conditions, mechanical requirements, connector requirements, installation method, and the applicable airport/project specification.
Primary ≠ automatically larger conductor, and secondary ≠ automatically smaller cable. Each must be sized against its own project specification.
Full Signal Path
CCR to Transformer
The isolation transformer provides the electrical interface between the series circuit and the individual fixture.
Transformer to Light
One-line memory: CCR → primary cable → transformer → secondary cable → light.
Primary Cable Failure vs Secondary Cable Failure
Open Primary Cable
Because the primary is part of the series circuit, an open primary cable can interrupt the circuit path, and the transformer/fixture downstream cannot receive power from the series circuit.
Open Secondary Cable
An open secondary cable disconnects the individual fixture from its transformer. The effect is normally localized to that particular fixture/secondary connection, rather than interrupting the whole series circuit.
Full Comparison Table
| Parameter | Primary Cable | Secondary Cable |
|---|---|---|
| Position | CCR → transformer | Transformer → light |
| Transformer connection | Primary | Secondary |
| AGL circuit | Series circuit | Individual fixture connection |
| Current | CCR-controlled series current | Transformer/fixture dependent |
| Voltage | Potentially high | Transformer secondary voltage |
| Insulation | Suitable for series-circuit voltage | Suitable for secondary voltage |
| Typical distance | Longer | Shorter |
| Main concern | Medium-voltage series-circuit integrity | Reliable fixture connection |
| Failure effect | Can interrupt series circuit | Usually affects individual fixture |
Practical Difference
Think: Primary Cable
CCR → primary cable → transformer primary
Think: Secondary Cable
Transformer secondary → secondary cable → light
Myths vs Facts
Primary ≠ automatically larger conductor. Actual sizing depends on voltage rating, current rating, insulation, conductor size, environmental and mechanical requirements, and the applicable project specification.
Secondary voltage/current is set by the transformer and connected fixture design, not by simply dividing the primary series-circuit voltage.
An open primary cable is part of the series circuit and can interrupt the whole circuit path; an open secondary cable normally only disconnects the one fixture it feeds.
Choosing the Right Cable
When selecting primary and secondary cable for an AGL project, establish:
- Whether the run sits on the CCR-fed series side or the transformer-to-fixture secondary side
- The specified series-circuit voltage for the primary run, and the transformer's secondary rating for the secondary run
- Required insulation system and mechanical/environmental rating for each side
- Compatibility with the specified connectors, transformers and accessories
- Applicable ICAO/national AGL requirements
- Airport-specific technical specifications and approved cable types
Which Cable Applies Where?
Primary cable is the CCR-side link in the AGL series circuit, carrying a regulated series current to the isolation transformer's primary winding at an insulation level suited to the series-circuit voltage. Secondary cable is the light-side link, carrying the transformer's secondary output the short distance to the fixture, rated to that fixture's own operating conditions.
The final selection for an actual airport project should always be based on the applicable current ICAO/national AGL standard, the approved circuit design and project cable specifications — not on a generic comparison alone.
Engineering & Expert Notes
Treat Primary Circuits as Hazardous
SafetyThey carry series current. Follow isolation procedures.
Check Secondary Cable Condition at Each Visit
MaintenanceCorrosion and flexing cause faults.
Label Cables Clearly
IdentificationAvoid confusion between primary and secondary.
Record Types and Routes
RecordsKeep as-builts current.
Frequently Asked Questions
Primary cable carries the CCR-regulated series-circuit current from the CCR to the isolation transformer's primary winding. Secondary cable connects the transformer's secondary winding to the individual light fixture.
Usually, yes — the primary cable runs the series circuit from the CCR to each transformer, while the secondary cable is typically a short connection because the transformer sits close to its fixture. The actual permissible lengths still depend on the AGL design.
Because it forms part of the CCR-controlled series circuit, which can reach a relatively high circuit voltage depending on how many transformer/fixture loads are connected in series. The secondary cable only needs to suit its own transformer's secondary conditions.
An open primary cable is part of the series circuit and can interrupt the circuit path, cutting power to everything downstream of that break. An open secondary cable normally only disconnects the one fixture it feeds.
No. Conductor size depends on voltage rating, current rating, insulation, environmental and mechanical requirements, and the project specification — not on whether it's labelled primary or secondary.
The primary current is set by the CCR's regulated series current. The secondary current is determined by the transformer's specification (for example a 6.6 A/6.6 A or 20 A/6.6 A primary/secondary arrangement) and the connected fixture.
Governing Documents
For an actual airport project, verify the current applicable editions of the relevant standards and the specific cable/transformer manufacturer's datasheets before using this comparison for project design.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — series circuit requirements, CCR characteristics and isolation transformer interfacing for AGL.
FAA AC 150/5345-26
Specification for L-823 plug and receptacle, cable connectors — including the isolating-transformer-lead connectors used at the primary/secondary interface.
ICAO Aerodrome Design Manual
Applicable material covering series circuit design, primary/secondary cable insulation requirements and electrical infrastructure 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 primary/secondary cable voltage rating, insulation system and connector compatibility requirements can vary with the governing standard, transformer manufacturer and approved airport design. For final design, use the current applicable standard and manufacturer datasheets rather than a generic comparison page.
Primary cable is the CCR-fed series-circuit link into the isolation transformer; secondary cable is the transformer-to-fixture link on the light side — the two serve different parts of the circuit and are specified independently.
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