How is a series cable sized?
Choosing the conductor for the primary loop: strong enough to install, insulated for the voltage, and large enough that it wastes little power and leaves the regulator headroom.
The current in a series circuit is only a few amperes, so a thin conductor can carry it. Yet cable size still matters, because the same current flows through kilometres of cable. Cable sizing balances loss, voltage and cost over that whole loop.
1Start with the loop length
Resistance depends on the total length of the series loop, out and back, and on the conductor size. For copper, about 3.08 Ω per km at 6 mm² falls to about 1.15 Ω per km at 16 mm². That resistance sets the I²R loss and the voltage drop, and both feed straight into the load calculation.
| Conductor | Resistance per km (copper, 20°C) | Loss at 6.6 A, 5 km loop | Voltage drop |
|---|---|---|---|
| 6 mm² | 3.08 Ω/km | 671 W | 102 V |
| 10 mm² | 1.83 Ω/km | 399 W | 60 V |
| 16 mm² | 1.15 Ω/km | 250 W | 38 V |
2Check it against the regulator
Cable loss and drop add to the voltage the CCR must supply. The total must stay under the regulator’s rated output voltage at the top step, with margin. A larger cable can bring a marginal circuit back inside the limit, and sometimes lets a smaller regulator do the job.
3Insulation, strength and installation
Primary series cable is rated for 5 kV class insulation, because the circuit voltage and fault voltage are high. The conductor must also be strong enough to pull into duct or lay direct, so very small sizes are ruled out whatever the current. Ground temperature and the method of installation raise resistance in service, so allow for a warm cable, not the 20°C figure.
- Find the loop length. Add the outgoing and returning runs of the whole series circuit.
- Pick a trial size. Start from the minimum size the standard and the installer allow.
- Calculate. Resistance, I²R loss at the top-step current, and voltage drop.
- Check the regulator. Total circuit voltage, with the extra loss, under the CCR rating.
- Check strength and installation. Pulling tension, duct or burial, temperature.
- Step up if it fails. Take the next standard size and repeat, then stop at the smallest that passes.
| Term | Meaning |
|---|---|
| Conductor size | Cross-section of the copper, in mm² or AWG |
| Loop length | Total cable length in the series circuit, out and back |
| I²R loss | Power wasted as heat in the cable |
| Voltage drop | Voltage used up in the cable, current × resistance |
| Insulation rating | Voltage class the cable insulation is built for, 5 kV for series circuits |
| Operating temperature | Cable temperature in service, which raises resistance |
Frequently Asked Questions
Q1What is cable sizing in airfield lighting?
Choosing the conductor size of the primary series cable so that it carries the circuit current safely, wastes little power, keeps the regulator inside its voltage limit, and is strong enough to install and last.
Q2Why is the current rarely the limit?
Series circuits run at a few amperes, so even a small conductor can carry the current without overheating. Loss, voltage drop and mechanical strength usually decide the size, not the current rating.
Q3How does cable size affect the load?
A larger conductor has lower resistance, so less I squared R power is wasted and less regulator voltage is used up. On a long circuit that can reduce the load and the regulator size needed.
Q4Why must the cable have a high insulation rating?
At full brightness a series circuit can run anywhere from several hundred volts to a few kilovolts, depending on its length and load, and a fault or surge can push the voltage on the cable well above that. Primary cable is therefore rated for 5 kV class insulation, to suit the circuit and the standard in use.
Q5Is a bigger cable always better?
No. Larger conductors cost more, are harder to pull and terminate, and past a point save little. The aim is the smallest standard size that meets loss, voltage and strength limits, with some room for growth.
Q6Does temperature matter?
Yes. Copper resistance rises as the cable warms, so loss and voltage drop are higher in service than the tabulated 20 degree figure. Duct or direct burial and the surrounding temperature feed into the calculation.
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