§01 — What the network demands
Environmental and electrical conditions on 6–35 kV distribution feeders
A distribution feeder is specified against the network study, not the catalogue. Four inputs settle most of the design: system voltage, earth-fault arrangement, fault level with clearing time, and the thermal environment. Ring mains, substation-to-ring-main-unit circuits and transformer spurs place different demands on the same product family, and a cable that passes every type test can still run hot in the ground if those inputs were assumed.
| Design input | Condition encountered on site | What it fixes in the cable |
|---|---|---|
| System voltage | 11 kV or 33 kV primary distribution | Insulation designation — 6.35/11 kV, 12.7/22 kV or 19/33 kV — and XLPE wall thickness |
| Earthing arrangement | Solidly, resistance- or impedance-earthed | U0 selection: insulation must withstand the earth-fault voltage for the real clearing time |
| Fault level | Taken from the utility’s network study | Conductor and metallic screen sizes against adiabatic temperature rise |
| Способ прокладки | Direct burial, duct bank, trenchless crossing | Armour type, oversheath, bedding and backfill specification |
| Thermal surroundings | Soil resistivity, ambient temperature, grouping | Ampacity derating; dry or rocky backfill cuts the usable rating |
Soil thermal resistivity is the value most often assumed rather than measured. Where no survey exists, specify conservative backfill and spacing — cheaper than replacing a feeder sized on optimistic data.
§02 — Recommended cable families
Families that match utility duties, and why
11 kV XLPE for ring mains and spurs
The default choice for urban ring mains and transformer spurs: conductors to IEC 60228, XLPE insulation, and a copper screen sized for the network’s earth-fault current. Constructions and test levels are on the 11 kV XLPE cable page.
33 kV XLPE for primary feeders
Chosen where distance or load growth justifies a higher voltage, cutting losses and voltage drop on substation-to-substation runs. See 33 kV XLPE cable within the wider medium and high voltage range.
Steel-tape armoured for direct burial
Where the trench is the only protection, armouring absorbs the mechanical load of backfill and later excavation. Configurations are shown on the YJV22 armoured power cable page.
LV XLPE for substation secondaries
Transformer secondaries, substation auxiliaries and street lighting run at 0.6/1 kV under IEC 60502-1 rather than Part 2: see the LV XLPE power cable page.
§03 — Selection checklist
Decisions that cause failures when they are skipped
| Decision | What has to be settled | Failure mode if ignored |
|---|---|---|
| U0/U designation | Insulation level matched to earthing and fault-clearing time | Insulation too thin for a slow-clearing system, or wasted spend on a solidly earthed one |
| Screen bonding | Single-point, cross-bonded or both-ends bonding against route length | Circulating sheath current, lost ampacity, unsafe sheath voltages |
| Armour selection | Steel tape for burial, steel wire under tensile load; non-magnetic on single-core | Armour damage, or heavy induced losses on single-core runs |
| Water treeing | Water-blocked or tree-retardant build in permanently wet ground | Insulation breakdown well before design life |
| Accessories | Joints and terminations specified with the cable, matched to size and screen | Failures concentrated at accessories instead of along the route |
Accessories fail more often than cable. Specify terminations, joints and installer competence at the same time as the cable.
§04 — Questions from network engineers
Frequently asked questions
Should an 11 kV feeder be direct-buried or installed in ducts?
Direct burial is cheaper and suits rural routes with few future interventions. Ducting costs more but allows replacement and extra circuits without reopening the trench, and is preferred in cities and at road and rail crossings. Ducted cable still has to be rated for the thermal environment inside the duct.
How is the U0/U designation chosen?
U is the rated voltage between conductors and U0 between conductor and earth or metallic screen. The ratio follows the system’s earth-fault behaviour: a solidly earthed network with fast clearing stresses insulation less than one that tolerates a fault for longer. Give the earthing arrangement and clearing time with your enquiry.
Which standard covers these cables?
IEC 60502-2 governs extruded-insulated power cables from 6 kV up to 30 kV — construction, insulation thickness, routine, sample and type tests. LV cables fall under IEC 60502-1. Cables for 33 kV systems are commonly specified to IEC 60840 or to the utility’s own national specification.
How should the metallic screen be earthed on a long feeder?
Short routes are usually earthed at both ends. Beyond a length where sheath circulating current becomes significant, single-point bonding with an earth continuity conductor, or cross-bonding at transposition points, is used. Fix the arrangement before drum lengths and sheath voltage limits, because it changes both rating and accessories.
§05 — Next step
From network data to a quotation
An enquiry that returns an accurate price carries the system voltage, earthing arrangement, fault level and clearing time, route length, installation method and applicable standard. A typical 11 kV distribution project brief shows the detail that makes quotations comparable, and our testing and certification page lists the documents supplied per shipment.
Send the network data and route profile and we will return a construction proposal with ratings and quantities: Request a quotation →