Secondary Cable
The low‑voltage, flexible cable that connects each isolating transformer to its associated light fixture, sign, or beacon in the AGL series circuit. Like the primary loop it's fed from, the secondary circuit is current-driven rather than voltage-driven — the transformer holds its secondary current fixed at its rated value (typically 6.6A or 20A) and voltage floats with the connected load. Secondary cable provides reliable power transmission in the harsh airfield environment with excellent flexibility for installation in confined spaces.
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What Is It?
The Secondary Cable (also known as the feeder cable, transformer‑to‑fixture cable, or lead‑in cable) is the low‑voltage power distribution cable used in airfield ground lighting systems to connect each isolating transformer to its corresponding light fixture, sign, or beacon. Like the primary cable, the secondary circuit is a constant-current loop, not a constant-voltage one: the isolating transformer holds its secondary current fixed at its rated value (typically 6.6A, sometimes 20A, matching the transformer's L‑830/L‑831 rating), while the voltage across the loop floats to whatever the connected fixture's impedance requires. Depending on the transformer's wattage rating (30W up to 500W in the standard FAA range), that secondary voltage can run from around 4V AC on a small 30W transformer to well over 50V AC on a high-wattage unit.
Secondary cable is typically:
- Two-conductor, in practice: Because the secondary is an isolated, floating AC loop with no earth or chassis return, it always needs two conductors to complete the circuit — either a genuine two-core cable or, more commonly on AGL projects, two single-core L‑824-type runs pulled together from the transformer to the fixture.
- Flexible construction: Designed with a fine‑stranded conductor to provide excellent flexibility for installation in confined spaces, around bends, and inside light fixture bases and junction boxes.
- Low‑voltage insulation: Insulated for the relatively low secondary voltage (typically 600V rating, but with robust insulation to withstand moisture and mechanical stress).
- Colour‑coded: Typically white, grey, or black, with coloured stripes or markers to identify phase and polarity.
The cable is installed from the isolating transformer secondary terminals to the light fixture or sign, often passing through conduits, junction boxes, or directly buried. Proper cable selection, installation, and termination are critical for ensuring reliable fixture operation and minimising voltage drop.
Purpose & Function
The secondary cable's job is to carry the transformer's fixed secondary current to the fixture with as little resistive loss as possible, since the fixture's light output and colour temperature are directly tied to the voltage actually developed across its lamp or LED driver by that current. Even a short run of undersized or degraded secondary cable adds resistance to the loop, which — carrying the same fixed current — shows up as extra voltage dropped in the cable rather than reaching the fixture, dimming it below its intended intensity step. That matters on precision approach and CAT II/III runways where intensity settings are calibrated against a defined photometric standard. Its fine-stranded, flexible construction also serves a practical installation purpose, letting the cable be routed through the tight confines of a fixture base or junction box without stressing the termination.
Where Is It Used?
The secondary cable is used throughout the airfield for all transformer‑to‑fixture connections, including:
- Runway Lighting Fixtures: Connecting runway edge lights, centreline lights, threshold lights, end lights, and touchdown zone lights to their respective isolating transformers.
- Taxiway Lighting Fixtures: Connecting taxiway edge lights, centreline lights, stop bars, clearance bars, and guidance signs.
- Approach Lighting Systems: Connecting ALSF‑1, ALSF‑2, MALSR, and other approach lighting fixtures to their transformers.
- Obstruction Lighting: Connecting obstruction lights on structures to their power sources.
- Airport Beacons: Connecting rotating beacons and heliport beacons to their transformers.
- Signs and Markers: Connecting illuminated signs, guidance signs, and other visual aids to their power supplies.
The cable is available in various conductor sizes (typically 1.5mm², 2.5mm², or 4mm²) selected mainly for the length of run and acceptable voltage drop rather than for widely varying current, since the secondary current itself is fixed by the transformer's rating (typically 6.6A or 20A) and comfortably within any of these conductor sizes' ampacity. It is designed for use in temperatures ranging from -40°C to +85°C and is resistant to oils, fuels, and de‑icing chemicals commonly found on airfields.
Installation & Location
The cable is run from the isolating transformer's secondary terminals to the fixture, sign, or beacon it serves, generally the shortest practical route to keep voltage drop to a minimum, since the transformer-to-fixture run is where undersizing or excess length has the most direct effect on the light's actual output. Routing typically passes through the same duct, conduit, or direct-buried trench as the primary cable serving that fixture, though kept as a separate, clearly identifiable conductor given its different voltage and current characteristics. Termination at the fixture end is made inside the fixture's base or a dedicated junction chamber, with enough slack cable coiled in to allow the fixture to be lifted or serviced without straining the connection, and at the transformer end, terminated directly to the secondary winding terminals inside the transformer's own enclosure. Where the run passes through a handhole or junction box en route, connections are kept to a minimum, since each additional joint is a further point where resistance and voltage drop can creep in.
How Does It Work?
The secondary cable functions as the final power delivery link between the isolating transformer and the light fixture. Its design ensures efficient, reliable, and safe power transmission at low voltage:
- Conductor: Typically made of high‑conductivity annealed copper, stranded to provide flexibility and reduce fatigue from vibration and thermal cycling. The fine stranding allows for easy routing in confined spaces and around tight bends.
- Insulation: A layer of PVC, EPR, or XLPE provides electrical insulation and mechanical protection. The insulation is rated for the secondary voltage (typically 600V) and is resistant to moisture, oils, and chemicals.
- Sheath: An outer PVC or TPU sheath provides additional mechanical protection against abrasion, impacts, and chemical attack. For direct‑burial applications, a robust sheath with enhanced moisture resistance is used.
- Voltage Drop: Since the secondary current is fixed by the transformer's rating, any resistance in the cable shows up directly as I²R loss and a voltage drop along the run. The cable is sized so this drop does not exceed acceptable limits (typically 5‑10%) at the transformer's rated secondary current, ensuring the fixture receives sufficient voltage for proper illumination.
- Flexibility: The fine stranding and soft insulation allow the cable to be bent and routed through conduits, fixture bases, and junction boxes without damage, simplifying installation and maintenance.
In operation, the isolating transformer holds the current on its secondary at a fixed value (typically 6.6A or 20A, matching its L‑830/L‑831 rating) and the secondary voltage floats to whatever the fixture's impedance calls for — anywhere from a few volts on a small 30W transformer up to 50V or more on a high-wattage unit. The secondary cable carries this fixed-current, load-dependent-voltage power to the lamp or LED driver. The cable is designed to have low resistance per metre, minimising the voltage lost in the cable itself and ensuring consistent illumination across all fixtures.
System Architecture & Control Logic
Cable is sized so voltage drop over the full run stays within 5–10% of the fixture's rated voltage.
Physical Construction
Construction mirrors the primary cable's layered approach but is scaled down for the much lower secondary voltage. The conductor is finely stranded annealed copper, using more and thinner strands than a primary cable of similar cross‑section, since the cable's main routing challenge is threading through cramped transformer and fixture bases and tight bends rather than long straight runs. A single layer of insulation, rated for the modest secondary voltage in most circuits (typically a few volts up to several tens of volts, depending on the transformer's wattage) but still built to resist moisture and mechanical wear, surrounds each conductor. Because the secondary is a floating, isolated loop with no earth return, a complete connection always needs two conductors — most commonly run as a pair of single-core L‑824-type cables from transformer to fixture, though a genuine two-core jacketed cable is also used where a return path or separate live/neutral feed is needed for an LED driver. Colour‑coded insulation or printed stripes distinguish phase and polarity at a glance during termination. The cable terminates directly at the transformer's secondary leads at one end and at the fixture's internal wiring at the other, typically by twist‑on connectors, crimp splices, or solder joints sealed with heat‑shrink, rather than a heavy waterproof connector, reflecting the lower voltage and the fact that these joints usually sit inside an already‑sealed fixture base or transformer case.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Conductor Material | Annealed copper (EC grade), finely stranded |
| Conductor Size (Cross‑section) | 1.5mm², 2.5mm², 4mm², 6mm² (AWG 16 to 10) |
| Conductor Stranding | Fine stranded (Class 5 or 6) for maximum flexibility |
| Insulation Material | PVC, EPR, or XLPE (cross‑linked polyethylene) |
| Sheath Material | PVC, TPU, or flexible rubber (for extreme flexibility) |
| Voltage Rating | 0.6/1kV (nominal), 600V continuous |
| Current Rating (continuous, cable ampacity) | 1.5mm²: 15A; 2.5mm²: 22A; 4mm²: 30A — well above the fixed 6.6A/20A secondary current these cables actually carry |
| Operating Temperature | -40°C to +90°C (XLPE); -40°C to +70°C (PVC) |
| Maximum Conductor Temperature (short circuit) | 250°C (XLPE), 160°C (PVC) for 5 seconds |
| Resistance (20°C, per km) | 12.1 Ω/km (1.5mm²), 7.41 Ω/km (2.5mm²), 4.61 Ω/km (4mm²), 3.08 Ω/km (6mm²) |
| Capacitance (per km) | Approx. 0.1‑0.2 µF/km (depending on insulation type) |
| Protection Class (installed) | IP68 (buried or in ducts) |
| Bending Radius (minimum) | 6 × cable diameter (for flexible cables) |
| Standards | IEC 60228, BS 7211, UL 62, FAA AC 150/5345‑47 (secondary lead cable, via the isolation transformer spec) |
Electrical Requirements
| Requirement | Typical Value |
|---|---|
| Circuit Position | Secondary (low-voltage) loop, isolating transformer to fixture |
| Voltage Rating (cable insulation) | 0.6/1kV nominal, 600V continuous |
| Actual Operating Voltage | Floats with load, roughly 4V–50V+ AC depending on transformer wattage — not a fixed supply voltage |
| Current (fixed by transformer rating) | 6.6A or 20A, matching the L‑830/L‑831 transformer's secondary rating — comfortably within 1.5mm²–4mm² cable ampacity |
| Insulation Resistance | ≥ 50 MΩ (megger test, cable-to-ground) |
| Isolation | Electrically isolated from primary series loop by the transformer |
| Earthing/Grounding | Not intentionally earthed; fixture body bonded separately to earth grid |
| Fault Protection | Secondary faults typically load the transformer rather than the series loop |
Applicable Standards
IEC 60228
Conductors of insulated cables — stranding and conductor sizes
BS 7211
Electric cables — thermosetting insulated, non‑armoured cables for voltages up to 450/750V
UL 62
Standard for flexible cords and cables (North American standard)
FAA AC 150/5345‑47
Specification for Series-to-Series Isolation Transformers for Airport Lighting Systems — covers the transformer's leads, including the secondary lead cable (AC 150/5345‑7 / L‑824 is scoped to underground primary circuit cable and explicitly excludes transformer leads, so it is not cited here)
Installation Requirements
Shortest Practical Routing
ElectricalRun the cable from the isolating transformer's secondary terminals to the fixture by the shortest practical route, since undersizing or excess length on this run has the most direct effect on the light's actual output.
Separation from Primary Cable
ElectricalKeep the secondary cable as a separate, clearly identifiable conductor from the primary cable sharing the same duct, conduit, or trench, given the different voltage and current characteristics.
Fixture-End Termination & Slack
ElectricalTerminate inside the fixture's base or a dedicated junction chamber with enough slack coiled in to allow the fixture to be lifted or serviced without straining the connection.
Transformer-End Termination
ElectricalTerminate directly to the secondary winding terminals inside the transformer's own enclosure, confirming polarity matches the colour-coded insulation before making off.
Minimise Intermediate Joints
ElectricalWhere the run passes through a handhole or junction box, keep connections to a minimum, since each additional joint is a further point where resistance and voltage drop can creep in.
Heat-Shrink Seal on Joints
ElectricalSeal twist-on connectors, crimp splices, or solder joints with heat-shrink before the fixture base or transformer case is closed, since these joints are not accessed again without opening a sealed enclosure.
Pre-Energisation Sign-off
DocumentationRecord cable route, termination points, and polarity before the fixture or transformer case is closed, since these are not inspectable once assembly is complete.
Testing & Commissioning
Insulation Resistance (Megger) Test
Pre-EnergisationMegger-test the secondary cable before backfill, confirming insulation resistance clears the acceptable minimum along the full run.
Continuity & Loop Resistance Test
Pre-EnergisationVerify conductor continuity end to end and record loop resistance, ruling out nicks or partial breaks introduced during pulling.
Secondary Voltage Verification
EnergisationWith the CCR and isolating transformer energised, confirm secondary voltage at the fixture end matches the transformer's rated ratio.
Route, Depth & Separation Verification
Pre-EnergisationConfirm burial depth, routing, and separation from the primary circuit match the design drawing before the trench is closed.
Documentation & As-Built Sign-off
Final AcceptanceRecord insulation, continuity, and voltage readings alongside as-built route and depth in the commissioning dossier for handover.
Inspection & Maintenance
Insulation Resistance Test
AnnualMegger-test each secondary run to confirm insulation resistance stays above the minimum, catching moisture ingress or sheath damage before it causes a fault.
Voltage Drop Check
AnnualMeasure voltage at the fixture end of long runs against the source, confirming drop stays within tolerance for the conductor size and run length in use.
Termination Seal Inspection
AnnualCheck heat-shrink tubing, self-amalgamating tape, or waterproof connectors at each cable end for degradation that could let moisture travel along the strands.
Sheath & Insulation Inspection
AnnualInspect exposed sheath for abrasion, nicks, or cracking from installation or thermal cycling, especially at conduit entries and fixture bases.
Termination Resistance Check
AnnualMeasure resistance across terminations and splices; since the secondary current is fixed by the transformer and well within the cable's ampacity, a high-resistance joint — not conductor overload — is the realistic source of localised heating.
Bend Radius Compliance Check
Semi-annualVerify routed cable maintains at least 6× its diameter in bend radius, since tighter bends stress fine-stranded conductors and insulation over time.
Continuity Test
AnnualTest end-to-end conductor continuity to catch a fine-stranded conductor damaged during installation before it fully opens under vibration or thermal cycling.
Duct / Trench Condition Check
AnnualWhere buried, check duct and trench markers and, on excavation, verify the cable sheath is intact with no water pooling around joints.
Common Field Issues
Incorrect Cable Size Selection
Using a secondary cable that is undersized for the run length can result in excessive voltage drop, dim lights, or reduced LED driver performance, since the fixed secondary current (6.6A or 20A) drops proportionally more voltage across a smaller conductor's resistance. Always calculate the voltage drop based on the cable length, resistance per metre, and the transformer's rated secondary current, and select the appropriate cable size.
Mechanical Damage During Installation
The fine‑stranded conductors of flexible secondary cables are more susceptible to nicks and damage during stripping and routing. Use proper stripping tools, avoid sharp bends, and protect the cable from abrasion in conduits and fixture bases.
Moisture Ingress at Cable Ends
If cable ends are not properly sealed or terminated, moisture can enter the conductor strands and travel along the cable, causing corrosion and insulation breakdown. Use heat‑shrink tubing, self‑amalgamating tape, or waterproof connectors to seal terminations.
Excessive Voltage Drop on Long Runs
Long secondary cable runs (especially for fixtures at the end of a taxiway or runway) can result in excessive voltage drop, reducing the voltage available at the fixture. Use larger conductor sizes or reduce the run length by repositioning the transformer closer to the fixture.
Insulation Voltage Mismatch on High-Wattage Runs
Because the secondary current is fixed by the transformer's rating (6.6A or 20A), standard secondary cable sizes (1.5mm²–4mm²) are rarely current-overloaded in normal service. The real risk on a high-wattage transformer (150W and above) is that its secondary voltage is correspondingly higher, and a cable or termination selected without accounting for that voltage can see insulation stress it wasn't rated for. Always confirm cable and connector voltage ratings against the specific transformer's wattage, not just its current rating.
Chemical Attack on Sheath
Fuel, oil, de‑icing fluids, and other chemicals can degrade PVC or thermoplastic sheaths over time. Use cables with chemically resistant sheaths (e.g., TPU or EPR) in areas where chemical exposure is likely.
Incorrect Termination at Transformer or Fixture
Using the wrong connector or poor termination technique can result in high resistance, overheating, and eventual failure. Always use approved connectors (such as L‑823 secondary connectors) and follow manufacturer's termination instructions.
Troubleshooting Guide
Fixture Dim or Voltage Low at the Load
Cable or Termination Heat Damage
Insulation Resistance Failure / Moisture at Cable Ends
Intermittent or Open Circuit
Sheath Cracking or Softening Over Time
High-Resistance Termination at Transformer or Fixture
Failure Modes & Root Causes
Undersized Conductor for the Run
High-Resistance Termination Heating
Moisture Ingress at Unsealed Ends
Fine-Strand Conductor Mechanical Damage
Chemical Attack on the Sheath
Safety & Operational Notes
Isolate the Primary Loop First
Electrical SafetyAlthough the secondary cable itself carries only a low, transformer-isolated voltage, its supplying isolating transformer sits on a constant-current series loop that can carry several kV; de-energise and lock out the associated CCR before disconnecting or splicing a secondary cable.
Moisture Ingress at Splices
Electrical SafetySecondary connections run in duct or direct burial and are exposed to groundwater; use only submersible-rated splice kits, fully encapsulate every joint, and never leave a temporary or taped connection buried or left in a base can.
Excavation & Cable Strike Awareness
OperationalBefore digging or trenching near a secondary run, confirm cable routing against as-built drawings or a cable locator, since a strike on a live loop can injure personnel and take out lighting on the associated circuit.
Airside Work Coordination
OperationalAny work on or near an active movement area requires ATC/airside coordination and a NOTAM where the associated fixture will be dark or out of tolerance, with vehicle and personnel movement kept clear per the aerodrome's safety management procedures.
FOD Control at the Base Can
OperationalKeep tools, fasteners, and removed gaskets accounted for at all times when a base can is open for cable work, and secure or remove any loose material before reinstating the can, to prevent foreign object debris on the movement area.
Insulation Test Before Re-Energisation
Electrical SafetyAfter any secondary cable or connector work, megger test the run and the affected primary series loop before restoring power, since a residual fault at re-energisation can trip the CCR or damage healthy fixtures on the same circuit.
PPE & Grounding
Personnel SafetyUse insulated tools and appropriate PPE when working on secondary connections, and confirm the transformer case and base can are bonded/grounded per the electrical design before reconnecting the fixture.
Restore to Compliant State
ComplianceDo not leave a fixture dim, flickering, or intermittent after cable work; verify continuity and photometric output before closing out the work and cancelling the associated NOTAM.
Engineering & Expert Notes
Secondary Cable Faults Often Present as a Primary-Side Symptom
ElectricalA fixture behaving as if the CCR is misbehaving, such as flickering or dropping out at certain steps, can actually originate from insulation breakdown in the low-voltage secondary run rather than the primary loop; check secondary cable condition before assuming a CCR or primary-circuit fault.
Submersible-Rated Splices Are Only as Good as Full Encapsulation
DurabilityA submersible splice kit that isn't fully encapsulated during assembly, leaving even a small void or air gap, doesn't provide its rated protection despite using the correct kit; verify complete encapsulation by feel and visual check before burying or closing a base can.
Trend Secondary Insulation Resistance Alongside the Primary Loop
TroubleshootingBecause a secondary cable's insulation resistance can degrade independently of the primary series loop it's fed from, testing only the primary loop after cable work can miss a developing secondary-side fault; test both circuits at every visit, not just the one thought to be affected.
Record As-Built Secondary Routing, Not Just the Transformer Location
OperationalSecondary runs between an isolating transformer and its fixture are often shorter and less rigorously as-built than primary loops; recording actual routing at installation saves excavation guesswork during a future fault-find.
A Temporary Splice Left in Place Becomes a Permanent Liability
WorkmanshipA taped or unencapsulated temporary connection made to restore service quickly is easy to forget once the fixture is working again; track temporary repairs explicitly and schedule a permanent splice, since an unrecorded temporary joint eventually fails without warning.
Voltage Drop Across a Long Secondary Run Affects Intensity, Not Just Faults
DesignA secondary cable run longer than the design assumed can introduce enough voltage drop to dim a fixture measurably even with no fault present; check secondary cable length against design calculations when a fixture reads consistently low but otherwise tests healthy.
Frequently Asked Questions
Short answers about AGL secondary cable: what it connects, sizing, voltage drop, standards, maintenance and common faults.
Q1What is a secondary cable?
The secondary cable, also called the feeder, transformer-to-fixture or lead-in cable, is the low-voltage cable connecting each isolating transformer to its light fixture, sign or beacon.
Q2Is the secondary circuit constant voltage?
No. Like the primary, it is a constant-current loop. The transformer holds its secondary current fixed at its rated value, typically 6.6 A or sometimes 20 A, and the voltage floats to whatever the fixture's impedance requires, roughly 4 V to 50 V or more depending on transformer wattage.
Q3What is the purpose of the secondary cable?
To carry the transformer's fixed secondary current to the fixture with as little resistive loss as possible. Extra resistance from undersized or degraded cable shows up as extra voltage drop, which reduces the voltage developed across the lamp or LED driver and so the light output.
Q4What sizes and construction does it have?
Finely stranded (Class 5 or 6) annealed copper in 1.5, 2.5, 4 or 6 mm² (AWG 16 to 10), with PVC, EPR or XLPE insulation and a PVC, TPU or rubber sheath. Voltage rating is 0.6/1 kV, and temperature range is −40 °C to +90 °C for XLPE or +70 °C for PVC.
Q5What is the resistance of secondary cable?
At 20 °C, about 12.1 Ω/km for 1.5 mm², 7.41 Ω/km for 2.5 mm², 4.61 Ω/km for 4 mm² and 3.08 Ω/km for 6 mm². These figures are used to calculate voltage drop for a given run.
Q6Where is the secondary cable used?
For all transformer-to-fixture connections: runway and taxiway lights, stop bars, clearance bars and signs, approach lighting (ALSF-1, ALSF-2, MALSR), and obstruction lights.
Q7How is the secondary cable routed?
From the isolating transformer's secondary terminals to the fixture by the shortest practical route, generally in the same duct, conduit or trench as the primary cable, to keep voltage drop to a minimum.
Q8Which standards apply to secondary cable?
The main references are IEC 60228 (conductors of insulated cables), BS 7211, UL 62 and FAA AC 150/5345-47 (series-to-series isolation transformers, covering the transformer's leads). Always verify the current edition and your approved requirements.
Q9How often should secondary cable be tested?
Typically annually: insulation resistance (megger), voltage drop at the fixture end of long runs, termination seal inspection, sheath and insulation inspection and termination resistance checks. Your approved maintenance programme takes precedence.
Q10Why is a fixture dim or the voltage low at the load?
Excessive voltage drop from an undersized cable or a long run. Calculate the expected drop from cable size, length and the transformer's rated current, measure the actual voltage at the fixture, then replace the run with correctly sized cable or move the transformer closer to the fixture.
Q11Why is there heat damage at a cable or termination?
Usually a high-resistance joint rather than conductor overload, since the secondary current is fixed and well within standard cable ampacity. A poor crimp, splice or connector is the likelier heat source, so measure resistance across terminations and re-make any that are high.
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