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Technical information on British high-voltage power lines



This page covers a few of the basic technical details on British “pylon wires”, specifically 132 to 400 kV power lines carried on lattice towers (commonly referred to as “electricity pylons”).

This page is not exhaustive and is not in any way authoritative. The information on this page is provided solely to offer a little bit of background knowledge to anyone curious about pylons. The information presented here is collated from a variety of sources and is not guaranteed to all be correct or accurate. Quite possibly the entire page is nonsense. Caveat lector!

Circuit count

Most power is transmitted and distributed in three phases. Three-phase power transmission does not require a return conductor: the current from each wire is returned via the other two. This is why power lines are found with multiples of three cables. Each three-phase circuit requires three cables. The 11 kV feeds to farms and villages on wood poles are generally single circuit, with three cables to a pole, on pin insulators sat on a metal crossbeam. The higher voltage 33 kV feeds, also on wood poles, are more commonly double-circuit with either two parallel sets of three-wire poles, or double poles with six wires.

Steel lattice transmission towers can be either single circuit or double circuit, although double circuit is by far the most common. Each of the three phases per circuit can be a single cable or a bundle of two, three or four cables separated by spacers. Although lower voltage towers may continue the practice of conductor cables only, higher voltage lines (in particular 132 kV and above) typically have a fourth or seventh cable at the top for earth protection. (In some locations there are two such cables at the top to make eight in total.)

Typical double-circuit towers have six arms, three per side (forming three crossarms), with one phase per arm, although other arrangements exist. Typical single-circuit towers have three arms, two on one side and one on the other, but there are also single-circuit lines using double-circuit towers with one side of each tower unused.


The diagram above suggests that phase 1 of each circuit is on the top crossarm, phase 2 of each circuit is on the middle crossarm, and phase 3 on bottom crossarm. This can be true, and is known as untransposed phasing, but is often not the way that the circuits are set up. For most supergrid lines, one circuit has the phases arranged top-to-bottom and the other circuit arranged bottom-to-top. This transposed phasing arrangement helps cancel out the electromagnetic fields emanating from the line. According to National Grid’s Phasing page, “only 50% of double-circuit overhead lines currently have optimum phasing” as transposed phasing was not part of the original design for the National Grid; the figure rises to 90% for 275 and 400 kV as the supergrid was designed with transposed phasing in mind.

Voltage and conductors

“Pylon wires” in the UK operate at a number of voltages. Supergrid lines operate at 275 and 400 kV and are classed as “transmission” lines: power lines that transmit power around the country. Below this are the 132 kV lines, classed as “distribution” in England and transmission in Scotland. 132 kV was the National Grid voltage from the 1930s until the creation of the Supergrid at 275 and later 400 kV. Some 132 kV lines run on wood or metal poles instead (for example the “Trident” wood pole type), while some 33 kV and 66 kV distribution lines run on lattice towers. It would appear that—in England at least—132 kV lines are demoted out of the National Grid; in [Development Near Lines] Appendix II, National Grid note: “Regional electricity companies generally own and operate lines with a voltage of 132kV and below.”

Higher voltages require larger insulators and larger clearances. With traditional insulators, at lower voltages, the number of insulators in the string is a good guide to the voltage: a single insulator (or two) implies 11 kV and a string of three insulators implies 33 kV. For 132 kV, you are more likely to see nine or ten insulators in a suspension string. For 132 kV, you’ll see around 10–11 insulators in a suspension insulator string (on a suspension tower), and around 8–9 insulators in a tension insulator string (on an angle or terminal tower). For 400 kV (a little over 3 × 132 kV), you would expect three times as many, but this is not the case. The Balfour Beatty L6 towers in Sundon for example have 21 insulators per string on both suspension and tension towers. L2 towers vary in the region of around 14–20, with 400 kV towers appearing to have around 18–20.

National Grid’s Terminology – an introduction page gives an idea of the amount of power that each voltage of line supports. These figures are indicative only, as the power rating varies by conductor type.

Voltage Current per phase Power per circuit Factor difference
Factor Cumulative
400 V (final distribution) 200 A 150 kW 1
11 kV (distribution) 150 A 3 MW 20
132 kV (distribution) 300 A 70 MW ~23 467
400 kV (transmission) 1000 A 700 MW ~10 4667

In [Development Near Lines] Appendix II, National Grid give the following figures:

Voltage Factor difference
Factor Cumulative
132 kV 1
275 kV 6
400 kV 3 18

The increase in voltage is only one reason for the huge increase in capacity of a 400 kV line over a 132 kV line. 400 kV lines use not only higher cross-section conductors (cables) but also multiple conductors per phase, known as “bundles”. These heavier conductors and higher conductor counts require stronger towers to support the additional weight. The higher voltages require taller towers to provide greater clearance between the conductors and everything that they pass over (buildings, trees and the ground), and greater clearance between the phases. The end result is that a 400 kV tower is up to twice the height of a 132 kV tower. 132 kV towers come in at around 26 to 27 metres in height. 275 kV L3 towers are around 37 to 38 metres tall. 275/400 kV L2 towers are over 41 m tall and the L8 replacement type is 46 m tall, while L6 400 kV towers are around 50 m tall. L12, which superseded L6, is shorter at 46 m.

Conductor bundles

132 kV circuits normally have a single cable per phase. For increased load carrying capacity, each phase can be formed of a bundle of two, three or more cables. This is uncommon for 132 kV, but standard practice for 275 and 400 kV. Most 132 kV tower types only support single cables, but L7 is rated for twin cables at what appears to be the standard diameter of 0.175″. L7 can also take twin heavy duty 0.4″ supergrid cables, although under these conditions the maximum distance between towers is reduced from 1000 feet to 785 feet due to the increased cable weight.

Supergrid lines are generally twin, triple or quad conductor: bundles of two, three and four cables per phase. The ubiquitous L2 series accommodates twin 0.4″ cables at 275 or 400 kV, while the smaller L3 only accommodates twin 0.175″ and has clearances only for 275 kV. (Note that modern cables are metric, but the Imperial system of measurements was in use when these tower types were introduced.) L2 was followed by L6; 20% taller in height and 47% wider at the base, L6 has the required strength to take quad 0.4″ cables per phase, although many can be seen strung as triple. Later tower series appear to have gone in for thicker cable, using all aluminium construction to keep the weight down, and thus only twin bundles, although this needs to be verified from source material. The new T-pylons however are quad bundle again. The implication thus is that any line using conventional lattice towers and triple or quad conductor bundles will be L6. The converse is not true: twin conductor does not rule out L6, as any tower type can be downrated to a lower voltage and lower conductor count, as is seen in various places around the country. For twin conductor, the tower type must be identified visually.

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L6 tower with quad conductor bundles separated by spacers
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L2 tower with twin conductor bundles, including twin jumpers and twin insulators, with spacers on the jumpers and just after the insulators

Generally, the conductors in a bundle are held apart by spacers positioned at regular intervals along each span:

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L2 twin conductor spacers
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L6 quad conductor spacers

A close-up photo taken at Blaw Knox L6 D60 tower 4KG116 on Flickr depicts a triple-conductor spacer; note the distinctly triskelion design.

The usage of spacers in bundles is typically but not universally the case. The 132 kV L7 Harker-Penrith line is strung twin conductor without spacers from Harker Substation to somewhere around Newby West outside Carlisle (by Newby Cross the line has changed to single conductor). The outer conductor for each phase is suspended lower than the inner conductor from the suspension towers, while at the tension towers they are level in height and held apart by the usual spacer next to the insulators. This can been on L7 D suspension tower DS 21 at 54.905° W 2.992° W near Grinsdale and L7 D30 tension tower DS 10 at 54.934° N 2.991° W between Rockcliffe and Cargo. Flickr photo 132kV Pylon, Double-Cabled depicts one of the twin conductor L7 D towers on this line.

The number of sub-conductors is not always constant along a power line. A Flickr photo of Blaw Knox L6 D60 tower 4KG116 depicts the changeover point from twin-conductor to triple-conductor bundles, or vice versa depending on perspective.

Conductor types

Unexpected tree and animal types appear in various places. These terms refer to the cable types used for the conductors and earthwires. The different cable types include:

All-Aluminium Alloy Conductor; each type is named after a type of tree
Aluminium Conductor Steel Reinforced; each type is named after a type of mammal
All-Aluminium Alloy Conductor Steel Reinforced
Aluminium Conductor Alloy Reinforced
Possibly steel-cored aluminium; seems to be an older term for ACSR; SCA was the specified conductor type for L2 and L6

All such cable types are described as “conductors” even though they are also used for earthwires and the earthwire is not regarded as a conductor.

Copper and aluminium are used for their good electrical conductivity. Steel was traditionally used for the inner strands for its mechanical strength, but for lightness aluminium alloy typically substitutes for steel now.

Some applicable conductor types are listed below. The diameters given are nominal only according to British Standards and the weights can vary slightly by manufacturer.

Imperial conductor types
Area Example usage
0.175 PL16 (single), L7 (twin), L3 (twin)
0.4 L132 (single), L2 (twin), L6 (quad), L7 (twin, reduced span)
Metric conductor types
Designation Type Nominal area Weight Example usage
Assignment Series/bundle
Horse ACSR 70 mm² 546 kg/km Earthwire L4
Lynx 175 mm² 865 kg/km 132 kV PL1, PL16, L4; L7 (twin)
275 kV L3 (twin)
Earthwire L132, L2, L3, L4, L7, L8
Zebra 400 mm² 1683 kg/km 132 kV L132, L4; L7 (single/twin); L2 (twin)
275 kV L2 (twin)
400 kV L2 (twin); L6 (twin/quad)
Earthwire L4, L4M
Finch 560 mm² 132 kV L7(c)
Upas AAAC 300 mm² 997 kg/km 132 kV L4M; L7 (twin)
275 kV L3 (twin)
Totara 425 mm² 1372 kg/km 132 kV L7
275 kV L2 (twin)
400 kV
Rubus 500 mm² 1622 kg/km 132 kV L7(c)
275 kV L2 (twin)
400 kV L2, L6 (both twin)
Sorbus 560 mm² 1822 kg/km 275 kV L2 (twin)
400 kV
Araucaria 700 mm² 2266 kg/km 275 kV L3
400 kV L6 (twin, triple), L12 (twin)
Redwood 850 mm² 2753 kg/km 400 kV L6, L12 (both twin)
AACSR 60 mm² Earthwire L4(m)
Keziah 160 mm² Earthwire L2, L3, L4M, L6, L7(c)
Collybia ACAR 500 mm² 275 kV L2 (twin)
400 kV

See NPS/001/007 – Technical Specification for Overhead Line Conductors from Northern Powergrid for additional details. More detailed and more complete lists of the various conductor types can be found under Common UK conductor specifications (external material) and the ACSR (British Standard) and AAAC (British Standard) pages from Witthinrich GmbH. The named conductor assignments are taken from [SWS Forum 08/06/2017] and [Uprating of Overhead Lines] Appendix D Table D1 (PDF page 205), both external material. [NSP/004/030] gives some of the conductor and earthwire types used with the various tower series albeit by cross section only, and this is ambiguous. The definition of Collybia is known only from Invitation to Tender no. Europeaid/114479/D/S/SL.

Imperial/metric equivalence

There is a very curious equivalence between imperial and metric ACSR conductor figures. 1 square inch is equivalent to 645.16 square millimetres, yet the astute will notice that the conversion factor for conductors is not 645.16 but 1000! 0.4 square inch ASCR is equivalent to 400 mm² ASCR (Zebra) and 0.175 square inch ACSR is equivalent to 175 mm² ACSR (Lynx): the metric figure is simply 1000 times the imperial figure. How does this make sense? Apparently this is simply a matter of pure coincidence.

The imperial figures are said to refer to the copper equivalent area: that is, the cross-sectional area of copper needed to achieve the same conductive capacity as the aluminium being used. The metric figures however refer to the actual aluminium area. By chance, the latter method results in a number approximately 1000 times greater.

To convert from imperial to metric, first multiply the imperial figure by the ratio of the resistivities of the elements. The resistivity of aluminium at 20 °C is 26.5 nΩ⋅m; for copper the figure is 16.78 nΩ⋅m. For 0.4 in² copper-equivalent area, the aluminium area would be 0.4 in² × (26.5 nΩ⋅m ÷ 16.78 nΩ⋅m) = 0.632 in². This figure can now be directly converted to square millimetres, giving 407.6 mm². This gives us a nominal aluminium area of 400 mm². For Lynx, 0.175 in² × (26.5 ÷ 16.78) = 0.276 in², or 178.3 mm², for a nominal aluminium area of 175 mm². The steel core strands in the ACSR conductor appear not to be counted; their resistivity will be something like 10–20% at most that of the aluminium (depending on the exact type of steel used), and because of the skin effect will be carrying proportionally less of the current.

Limits and uprating

Transmission and distribution line voltages are limited by the line dimensions: insulator string length, crossarm spacing and tower height. The limit on the current is a little different. Metal expands as it heats up, and this applies to pylon wires just as it does railway track or any other metal object. Various techniques exist to accommodate expansion due to heat: gridiron and invar pendulums for clocks, expansion joints and pre-stressed track for railway lines, and automatic tensioning equipment in railway overhead lines. As the sun warms the overhead wires on an electrified railway, heavy weights or powerful springs take up the slack to keep the wires taut; if this is not done, there is a danger of dewirement (overhead line damage), something that can occur in extreme weather conditions. Railway track is pre-stressed to rail neutral temperature for installation, but this too has its limits: if the weather is too hot or too cold, the steel rails will attempt to expand or contract beyond the design limitations.

Overhead power cables are not fitted with any such compensation mechanisms: they simply sag lower as they warm up, and pull tighter as they cool down. Sag is a key factor in the design of overhead lines: the lines must remain at a safe height above the ground even in extreme heat. The sag limit also sets the maximum current on a line: the higher the current, the more the wires heat from resistance, and therefore the more they sag.

As global temperature rises and demands on the power infrastructure by an increasingly electrified society grow, the thermal limit of power lines can be reached. Solutions to this include additional lines, line replacement with higher-capacity towers (larger size for higher voltage and stronger to carry more and larger conductors), voltage increase through adapting existing towers for greater clearances and higher insulation and more tolerant conductors that better withstand heat (e.g. the introduction of invar alloy) or that have better conductivity for their weight. Improvements in conductor technology allows for lines to be uprated by exchanging older conductors with new ones.

Thermal limits and uprating are discussed extensively in [Uprating of Overhead Lines] and not covered in detail here. In particular, the following passage summarises uprating through reconductoring:

Re-conductoring is the most common and effective method of current uprating that requires minimal modifications of existing structure. Although this method is comparatively expensive than any other current uprating method, it is cheaper than building a new line [2.24, 2.26]. Replacement by a conductor with a slightly high cross sectional area (same conductor weight) or by High Temperature Low Sag (HTLS) conductors can provide significant current uprating without any structural modification. For example, the replacement of Aluminium Conductor Steel Reinforced (ACSR) by All Aluminium Alloy Conductor (AAAC) of same cross-sectional area can improve the thermal rating up to 40% [2.35]. In the UK, AAAC is extensively used to replace ACSR which allows an increase in maximum operating temperature from 50 °C to 75 °C, with a corresponding 25% increase in thermal rating [2.32, 2.36].


Power lines sometimes need to cross each other. There is more than one way to achieve this. One method is to simply keep one set of wires higher up than the other, which can be seen in example below from Redbourn in Hertfordshire. The two L2 towers are placed very close to each other to minimise sag and maintain clearance. The 132 kV line running between the PL16 towers—which are only around 60% of the height of a standard height L2 tower—simply passes underneath the 400 kV line. The L2 towers are extended a little in height and the sag of the 132 kV lines keeps the two lines separated.

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Simple (duck-under) crossing
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Same crossing, showing the difference in wire height from the side

A more conventional approach is a diamond crossing. Here, one of the lines is split off onto two low-height gantries placed either side of a tower from the other line. The gantries carry the conductors around the outside of the other line’s tower, where they join together at the next tower. This can be seen in Flickr photograph L3 pylons | Near Atworth, Wiltshire: here, the dual-voltage 132 kV/33 kV Bath-Norrington AG line on L3 towers crosses the 132 kV Y line on PL16 towers at 51.385° N 2.196° W. The 132 kV line also passes over two 33 kV lines in the vicinity.

Circuit termination

Sealing end compound

A sealing end compound is a fenced-off enclosure where the route changes from overhead lines to underground cables. The overhead lines terminate on a special tower, with downleads connecting the overhead cables to the underground cables. See Sealing-end compounds on EMFS.info for more details. Illustrated below is the St Albans sealing end compound near the entrance to Nightingale Lane off Highfield Park Drive (51.737° N 0.303° W). This spur connects Cell Barnes substation in St Albans to the Elstree–Rye House 132 kV line at Coursers Farm Anaerobic Digestion Plant between Tyttenhanger and Colney Heath, using JL Eve L16 towers. The line reaches St Albans on towers, and changes to underground cables until the substation. The photographs were taken on 1st May 2021.

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Sealing end compound; note the downleads between the tower and the ground-level equipment
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Tower detail
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Cable sealing ends (the three special structures) terminate the downleads
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Cable sealing end detail

Sealing end platform

These are the “baskets”, the enclosed raised platforms placed alongside towers where the downleads from the tower connect to underground cables. This approach is possibly intended to reduce the land usage requirement compared to a sealing end compound, but some sealing end platforms are nonetheless enclosed within a compound. Typically there is a separate pillar for each phase, and two separate assemblies, one per circuit (each one carries three phases).

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Pepperstock L4 DT decommissioned sealing end platform; M1 motorway in the background
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Slip End sealing end platform under construction around an existing L7 D30 tower

Both photos below show non-operational sealing end platforms, as there are no known operational ones in the area. The SEP at Pepperstock (just outside of Luton) has had its downleads removed, de-energising the line; it has been in this state at least as far back as November 2008. A new sealing end compound is being constructed around the L7 DT in Redbourn, so this line may be returned to service.

The nearby Slip End SEP is brand new. An L7 D30 through tower has been fitted with a top crossarm extension (a non-standard design as this is a D30 instead of a DT), and the SEP has been added to the tower sometime between May and August 2022. There are again no downleads in the photograph; by now these may have beens fitted.


Transposition exchanges the vertical positions of the phases on the line. For example, moving the top phase to the middle crossarms, middle phase to the bottom crossarms and bottom phase to the top crossarms. Such towers were designated DX (double circuit) and SX (single circuit)

For details on transposition please refer to another reference source such as the transposition and transposition tower pages on Wikipedia. Transposition in this manner is no longer used in the UK, but some historic transposition towers remain in service albeit as regular through towers now. Flash Bristow quoted an explanation from Ian McAulay on her Electricity Pylon Frequently Asked Questions page as follows:

Early transmission practice in the UK did use transposition towers, but they were found to be unnecessary and the transpositions were generally eliminated when reconductoring lines that included them. Where they remain, transposition towers are just used as ordinary tension towers, but the extended top and bottom crossarms are characteristic.

Double-circuit lines require special transposition towers. This is not necessary with single-circuit lines although the L132 specification from 1940 included single-circuit transposition towers.

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Single-circuit transposition on PL1 towers in Scotland; photo courtesy Ian McAulay (CC-BY-NC)
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PL1a transposition tower in Bexley, London, circa 1933; image © Bexley Archives