Cable Burial Spacing & Derating: IEC 60364-5-52

01What spacing actually controls

When several circuits share one trench, each cable is heated not only by its own losses but by the losses of its neighbours. The soil around the trench has a finite ability to carry that combined heat away, so the more cables you put into one thermal field, the less current each one may carry. That is the whole mechanism, and the standards express it as a grouping factor — a multiplier below 1 applied to the single-circuit rating.

Spacing is the only derating variable that costs nothing in material. Conductor size costs copper. Soil replacement costs excavation and backfill. Grouping is bought with trench width, and trench width is usually the cheapest metre in the project. It is also the correction most often left off the sizing sheet, because a cable sized at its catalogue value looks compliant right up to the moment the second circuit is added to the same trench.

Three things are worth stating before the tables, because they decide whether the arithmetic means anything:

• The factors assume a homogeneous group. Same cable type, same size, same loading, laid in one horizontal row at a consistent clearance. A trench that mixes a 95 mm² feeder with a 6 mm² lighting circuit is not covered by a single row of the table.
• Ducted and direct-buried are different columns. A duct traps an air gap and an extra wall between the cable and the soil. IEC 60364-5-52 treats ducts (reference method D1) and direct burial (D2) as separate cases, and the soil correction is materially different between them — see section 04.
• The published rating already contains a set of reference conditions. Ground temperature is taken as 20 °C, not the 30 °C used for cables in air. Soil thermal resistivity is taken as 2.5 K·m/W. If your site differs on either count, the correction applies on top of the grouping factor.

02Burial depth: what is required, and what is only practice

Burial depth attracts more confident numbers than the standards support. In the UK, BS 7671 does not prescribe a single minimum burial depth for a cable: it requires the depth to be sufficient to protect the cable against the foreseeable disturbance of the ground, which is a judgement about the site rather than a figure taken off a page. Ploughing depth on agricultural land, vehicle loading on a car park, and a domestic garden are three different exposures and attract three different answers.

What exists in practice is a body of utility, highway-authority and project conventions that engineers quote as if it were a standard. The table below separates the two: it gives the figures we see specified on UK low-voltage buried routes, and states what each figure is actually based on.

Cover is measured from the finished ground level to the top of the cable. These are practice figures, not a single standardised minimum — the governing document on any project is the specification, the network operator's requirement, or the highway authority's condition. Verify against that, not against this table.

Depth and thermal rating are two different questions

A common conflation is to treat burial depth as a rating input. It is, but weakly, and not through a universal coefficient. Deeper burial puts the cable into soil that moves less heat and, in some ground, is drier and more thermally resistive; shallower burial exposes the route to seasonal surface temperature swing. Both effects are real, and neither is captured by a single depth multiplier that can be applied to any cable.

IEC 60364-5-52 does provide a correction for ambient ground temperature — Table B.52.15, referenced to 20 °C, where a 25 °C ground costs a 90 °C XLPE cable about 4 % of its rating and a 30 °C ground about 7 %. That is a temperature correction, not a depth correction. Where depth matters thermally, the honest route is a calculation per the IEC 60287 series or the manufacturer's published depth data for that specific construction — not a generic percentage.

03The grouping factors: IEC 60364-5-52 Table B.52.18

This is the table the whole article turns on. It gives the reduction factor for more than one circuit of single-core or multi-core cable laid directly in the ground — reference method D2 — as a function of the number of circuits and the clearance between them.

Values as published in IEC 60364-5-52, Table B.52.18, for multi-core and single-core cables laid directly in the ground. Clearance is measured cable surface to cable surface, centre-line spacing is not the input. The table assumes equally loaded cables of the same construction in a single horizontal layer.

How to read it without misreading it

Two features of the table explain most of the argument that happens on site. The first is that the penalty for the first few circuits is steep and then flattens: going from one circuit to four touching costs 40 % of the rating, going from four to eight touching costs another 28 %, but going from eight to twelve touching costs only 16 %. Mutual heating is a crowded-field effect, and the field is already crowded.

The second is that the spacing columns pay off differently at different circuit counts. At two circuits, moving from touching to 0.25 m buys 0.15 of factor (0.75 to 0.90). At twelve circuits the same move buys 0.23 (0.36 to 0.59), because a widely spaced group of many circuits is a fundamentally easier thermal problem than a tightly stacked one. On a heavily loaded corridor, the case for a wider trench is strongest not when there are two feeders, but when there are eight.

04Soil thermal resistivity: Table B.52.16 cuts two ways

Grouping accounts for heat arriving from the neighbours. Soil thermal resistivity, expressed in K·m/W, decides how easily the combined heat leaves. The published buried ratings are referenced to a resistivity of 2.5 K·m/W; a site that is wetter and colder than that will carry more current, and a dry site will carry less.

This is where a detail in the standard is regularly missed, and it is the single correction most often applied to the wrong column. IEC 60364-5-52 Table B.52.16 publishes two sets of factors: one for cables drawn into buried ducts, one for cables laid direct in the ground. The direct-buried column is markedly more generous on wet soils, because a cable in intimate contact with the soil sheds heat far better than the same cable inside a duct.

Both columns are from the same table of IEC 60364-5-52 and both are referenced to 2.5 K·m/W. The factors are averaged over the conductor sizes and installation types covered by the source tables, with a stated accuracy of about ±5 %. They assume uniform soil; where partial drying-out around the cable is foreseeable, the standard directs you to the IEC 60287 series instead.

Reading a soil you have not tested

Most LV projects do not commission a thermal resistivity test, and for those the standard offers a second, empirical table that maps soil description to a correction factor. It is coarser, but it is stated in terms a site engineer can actually answer.

• Very wet or saturated soil — factor 1.21
• Wet soil — factor 1.13
• Damp soil — factor 1.05
• Dry soil — factor 1.00
• Very dry or sunbaked soil — factor 0.86

Note which way the sign points. A dry soil does not add a small penalty on top of a good design; it removes 14 % of the rating before any grouping factor is applied, and on a multi-circuit trench it multiplies against the grouping loss rather than adding to it. A six-circuit trench in sunbaked ground is at 0.50 × 0.86 = 0.43 of the single-circuit rating before the ground temperature correction is even considered.

05Trench layout: spacing, width and the trade-off

The table gives factors by clearance. The drawing has to give millimetres. The two are linked by cable diameter, which is why a 4 × 16 mm² armoured cable roughly 22 mm across and a 4 × 95 mm² at 41 mm generate very different trench widths for the same nominal clearance.

The table below converts the spacing columns into practical route arrangements for a typical four-circuit LV trench, using a nominal 20–22 mm cable diameter as the worked basis.

Widths are indicative, based on a nominal 20–22 mm overall diameter — the published figure for a 4 × 16 mm² copper armoured cable at 0.6/1 kV. Add working space, bedding and the requirement to keep the sand envelope intact; the finished excavation is wider than the cable rows. Factors are read from Table 2 above.

The row worth pausing on is the second one. At four circuits the table gives 0.60 for touching and 0.60 for one cable diameter. Widening a four-circuit trench from touching to a one-diameter clearance buys exactly nothing, yet it costs excavation and reinstatement. The gain starts at the 0.125 m column. This is the sort of detail that a general rule of thumb — "always leave a diameter between cables" — gets wrong, and it is why the factor should be read from the table rather than from habit.

Design sequence

1. Start from the load and a first-pass conductor size using the single-circuit published rating for the installation method.
2. Multiply the three corrections — grouping from Table B.52.18, soil from Table B.52.16, ambient ground temperature from Table B.52.15.
3. Compare the derated rating with the design current. If it fails, change the cheapest variable first: trench width, then soil treatment, then conductor size.
4. Record the assumptions — circuit count, clearance, soil description, ground temperature — on the cable schedule. An unrecorded assumption is an unverifiable design.

06Two trenches, one cable: a worked comparison

The clearest way to see what spacing is worth is to hold the cable constant and vary only the trench. Take a 4 × 25 mm² copper XLPE-insulated steel-wire-armoured cable to BS 5467, published by the manufacturer at 107 A direct buried at 20 °C ground. That published figure already assumes the reference conditions: 20 °C ground and 2.5 K·m/W soil.

Step 1 — spacing only, reference soil

Four circuits, soil at the 2.5 K·m/W reference, so the soil correction is 1.00 and the ambient correction is 1.00. The only variable is clearance.

Arithmetic: 107 A × grouping factor, rounded to the nearest amp. Ambient ground temperature 20 °C and soil thermal resistivity 2.5 K·m/W, so both ambient and soil corrections equal 1.00 and do not appear.

Step 2 — add the soil, hold the spacing

Now hold the layout at 0.125 m centres — grouping 0.70 — and let the soil vary across the range a UK site actually presents. The direct-buried column of Table B.52.16 applies.

Arithmetic: 107 A × 0.70 (grouping, Table B.52.18) × soil correction (Table B.52.16, direct-buried column), rounded to the nearest amp.

The pair of tables makes the design argument concrete. The same cable, in the same four-circuit trench at the same clearance, is worth 67 A in very dry ground and 96 A in moist ground — and the spread from trench layout alone, at reference soil, is 64 A to 86 A. Grouping and soil are not a refinement to a cable selection; together they decide it. A designer who takes the catalogue number of 107 A and stops has overstated the route by up to 60 %.

07The 25-year cost of a tight trench

An under-spaced buried route rarely fails on the day it is energised. It runs hot, and hot runs short. XLPE insulation has a maximum continuous conductor temperature of 90 °C, and the ageing of the polymer accelerates with temperature above its design point. The exact relationship depends on the compound, the activation energy and the loading profile, so no single number of degrees-to-years can be quoted honestly — but the direction is not in doubt, and the failure mode is expensive because the cable is underground.

The commercial case for adequate spacing rests on three costs, only the first of which appears in the tender:

• Excavation and reinstatement. The visible cost. On an urban or highway route, reinstatement typically dominates the trench cost, and the marginal cost of widening an already-open trench by 300 mm is a fraction of the cost of opening it a second time.
• Replacement. A cable that has aged prematurely has to be replaced along its full length — and by the time it is replaced, the road, the surface, the planting and possibly the buildings either side are back in service.
• Compliance and programme risk. Where a designer or an inspecting body finds that grouping was not applied, the remedy is re-excavation or a redesign with larger conductors. On a programme-constrained infrastructure job, the delay usually costs more than the copper.

08How to verify a supplier's buried rating

A buried rating is only usable if you know the conditions it was measured or calculated under. When a supplier's datasheet gives a single number for current in the ground, the questions below establish whether it applies to your route.

Five checks before you accept the number

• What is the reference ground temperature? A figure quoted at 20 °C ground is not valid on a 30 °C site without the Table B.52.15 correction.
• What is the reference soil resistivity? If the datasheet does not say, it is most likely 2.5 K·m/W — but ask, because a figure quoted on a favourable soil assumption flatters the cable.
• Is it one cable or a group? Single-circuit ratings and grouped ratings are different quantities and are not interchangeable.
• Direct buried or in duct? The two carry different factors on both the grouping and the soil side.
• Is the construction what the standard requires? Conductor stranding and class, insulation compound and thickness, bedding, armour wire diameter and sheath material are all specified by the product standard.

On our own armoured range, the published tables give both reference conditions side by side — for example 4 × 70 mm² copper XLPE SWA at 246 A in air at 30 °C and 188 A direct buried at 20 °C. The 58 A between those two columns is not a manufacturing tolerance; it is the difference between two sets of reference conditions, and it is the reason a single "rated current" on a quotation should always be questioned. The same applies to the halogen-free version of the cable, which carries the same current ratings as the PVC version at the same cross-section: the sheath choice is a fire-performance decision, not an electrical one.

Related reading on this point: cable ampacity derating for temperature, altitude and grouping, and the soil thermal resistivity guide for the underlying physics.

09What we publish, and what we will not pretend to publish

This article cites two tables from IEC 60364-5-52 and works them in full, because those tables are published, stable and verifiable. It also states plainly what it does not do.

It does not give a single "correct" burial spacing. No standard does, because the answer depends on how many circuits share the trench and what the ground is like. Anyone quoting one number for all projects is quoting a habit.

It does not give statutory burial depths. The depth figures in Table 1 are UK practice, labelled as such, and the governing document on your project is the specification or the network operator's requirement. Where the standard does state a principle — that the depth must suit the foreseeable disturbance of the ground — we have written the principle rather than inventing a clause number for it.

Finally, it does not quote a generic depth correction factor, because we have not found one that is valid across cables and installations. Where depth matters thermally, the route is an IEC 60287 calculation or the manufacturer's data for the specific construction.