Cable Burial Spread: Minimum Spacing Requirements & Derating Factors for Direct Buried Cables

📅 Published: 2026-06-12 | 📁 Category: Cable Installation Guide | ⏱ 14 min read

1. Why Cable Burial Spread Affects Both Cost and Performance

When multiple armoured cables share a trench, the spacing between them — the cable burial spread — directly determines two things: the current-carrying capacity of each circuit, and the cost of excavation. Pack them too tight and mutual heating derates the cables, forcing you to oversize conductors. Spread them too far and the trench width becomes expensive to excavate and reinstate, especially in urban or highway environments.

The challenge is that neither BS 7671 nor IEC 60364-5-52 prescribes a single "correct" spacing. Instead, they provide grouping correction factors for different spacing distances, leaving the engineer to balance thermal performance against installation cost. A 2020 industry survey by the IET found that over 30% of electrical contractors routinely omit spacing derating from their cable sizing calculations — leading to cables operating above their rated temperature in service.

This guide provides a practical reference for specifying cable burial spread: the correction factors from IEC 60364-5-52 and BS 7671, depth requirements, installation methods, soil thermal resistivity impacts, and worked examples for common trench configurations. Whether you are designing a residential street lighting feeder or a utility-scale industrial power trench, understanding burial spread is essential for safe and cost-effective cable installation.

2. Burial Depth Requirements: What the Standards Say

Minimum burial depth is the starting point for any direct buried cable installation. The required depth depends on the voltage rating, the mechanical protection provided, and the risk of disturbance from surface activities.

2.1 Standard Minimum Depths

2.2 Depth Correction Factors

When cables are buried deeper than the standard reference depth, the ampacity must be derated due to higher soil thermal resistivity at depth. IEC 60364-5-52 provides depth correction factors:

Factors are approximate and vary with cable type and soil conditions. Always verify with manufacturer data or IEC 60287 calculation for critical installations.

3. Grouping Derating Factors: The Core of Burial Spread Design

When multiple circuits share a trench, mutual heating reduces each cable's ability to dissipate heat. The magnitude of this reduction depends on the number of circuits and the centre-line spacing between them. IEC 60364-5-52 Annex B and BS 7671 Table 4C2 provide the standardised grouping factors for direct buried multi-core cables.

3.1 BS 7671 Table 4C2 — Multi-Core Cables Direct Buried

Source: BS 7671 Table 4C2 (equivalent to IEC 60364-5-52 Table B.52.19). Applies to homogeneous groups of equally loaded multi-core cables, buried direct in horizontal or vertical arrangement.

3.2 Understanding the Practical Impact

A 16 mm² 4-core SWA cable rated 96 A clipped direct may have a base buried rating closer to 80 A (depending on soil conditions). If three such circuits share a trench with cables touching:

That is a 12 A difference per circuit — or 23% more capacity — simply by maintaining 250 mm spacing instead of laying cables touching. The spacing costs nothing in material but can save a full conductor size upgrade.

3.3 SORIVO vs Market Standard: Quality Assurance for Buried Cables

4. Soil Thermal Resistivity: The Hidden Variable

Grouping factors account for mutual heating between cables. But the soil thermal resistivity (measured in K·m/W) determines how effectively the combined heat dissipates into the surrounding ground. This varies dramatically with soil type, moisture content, and compaction.

4.1 Typical Soil Thermal Resistivity Values

4.2 Soil Resistivity Correction Factors

IEC 60364-5-52 Table B.52.16 provides correction factors for soils with thermal resistivity different from the reference value of 2.5 K·m/W:

4.3 Combined Derating — Grouping × Soil Resistivity × Depth

For a real installation, all three factors must be applied sequentially:

5. Trench Layout: Practical Spacing Configurations

The following section maps standard trench configurations to their equivalent grouping factors, helping designers choose the most cost-effective layout.

5.1 Trench Width vs. Derating: A Design Trade-Off

   ╔═══════════════════════════════════════════════╗
   ║  COMPACTED BACKFILL / SURFACE REINSTATEMENT    ║
   ╚═══════════════════════════════════════════════╝
   ——————————————— WARNING TAPE @ 150 mm ———————————————
   ╔═══════════════════════════════════════════════╗
   ║                BACKFILL MATERIAL                ║
   ║    ┌──────┐   ┌──────┐   ┌──────┐   ┌──────┐   ║
   ║    │SWA 1 │   │SWA 2 │   │SWA 3 │   │SWA 4 │   ║
   ║    └──────┘   └──────┘   └──────┘   └──────┘   ║
   ║    <—125—><—125—><—125—>                    ║
   ╠═══════════════════════════════════════════════╣
   ║             SAND BEDDING (100 mm)              ║
   ╚═══════════════════════════════════════════════╝
  

5.2 Three-Step Trench Design Process

1. Determine load and base cable size — calculate the required ampacity from the connected load, accounting for design margin (typically 20%).
2. Apply all derating factors — depth correction × soil resistivity × grouping factor (from Table 4C2 for your chosen spacing).
3. Check the derated ampacity ≥ design current — if not, increase cable size or increase spacing. Increasing spacing is often cheaper than upsizing conductors.

6. The 25-Year Cost of Ignoring Burial Spread

Getting burial spread wrong does not always cause an immediate failure. The damage is cumulative — accelerated thermal ageing that shortens cable life by years.

6.1 Thermal Ageing: The Hidden Cost

XLPE insulation rated for 90°C continuous operation has a design life of approximately 25 years at rated temperature. According to the Arrhenius ageing model, every 10°C increase above rated temperature reduces insulation life by approximately 50%. If grouping derating is ignored and the cable operates at 100°C conductor temperature instead of 90°C, the expected life drops from 25 years to roughly 12–13 years.

6.2 Compliance Cost

A building control or DNO inspection that finds cables operating above their rated temperature due to unaccounted grouping can result in:

• Rejection of the installation — requiring re-excavation and re-laying with correct spacing
• Re-engineering costs — redesign with upsized conductors if trench width cannot be increased
• Project delay penalties — particularly on highway or infrastructure projects with tight programme constraints

7. How to Verify Your Buried Cable's Rated Capacity

Before accepting a supplier's current rating table for direct buried cables, use these practical checks to ensure the published values apply to your installation conditions.

1. Check the reference conditions — every current rating table is published with a set of base conditions: ambient temperature (usually 20°C for ground), burial depth (usually 700 mm), and soil thermal resistivity (usually 2.5 K·m/W). If your installation differs, correction factors must be applied.
2. Confirm the cable standard — BS 5467 (PVC sheath) and BS 6724 (LSZH sheath) have identical current ratings for the same conductor size. But non-certified cables may have thinner insulation or different stranding, which can affect thermal performance.
3. Measure the actual trench width — the grouping factor in Table 4C2 assumes equally spaced cables in a homogeneous group. If your trench has asymmetric spacing or mixed cable sizes, use the worst-case factor or perform an IEC 60287 calculation.
4. Verify cable markings — every metre of compliant BS 5467 or BS 6724 cable should be marked with the standard number, conductor size, voltage rating, and manufacturer. Check that the marking matches the datasheet.
5. Request the manufacturer's derating data — reputable manufacturers publish detailed derating factors for their cables. SORIVO provides full technical datasheets including depth, grouping, and soil resistivity correction factors for all armoured cable types.
6. Consider seasonal moisture variation — soil thermal resistivity in UK summer (dry ground) can be 2–3 times higher than in winter (wet ground). For permanent installations, design for summer conditions.

For a complete walkthrough of cable verification, see our guide: How to Verify Cable Certification (methodology applies to BS standards as well).

8. Conclusion: A Systematic Approach to Burial Spread Design

Correct burial spread design follows three steps, applied in order:

1. Determine the base cable rating — select a cable size based on the load current, using the manufacturer's published direct-buried ampacity at standard reference conditions.
2. Apply all correction factors — depth × soil resistivity × grouping factor (from Table 4C2 for your chosen spacing). Never apply only one factor; the combined effect can differ significantly from any single factor.
3. Verify the derated ampacity exceeds the design current — if not, increase spacing, upsize conductors, or improve soil thermal properties (e.g., thermal backfill). Increasing spacing from touching to 250 mm is almost always the most cost-effective first step.

For the majority of low-voltage direct-buried installations, a 125–250 mm spacing between circuits with a 600 mm minimum cover depth and properly compacted sand bedding provides a reliable, code-compliant configuration that maximises ampacity while keeping trench width manageable.

At SORIVO, all armoured cables are manufactured to BS 5467 or BS 6724 with full BASEC certification, metre-marked traceability, and published derating data to support accurate trench design.

Further reading:

• Cable Laying Methods & Requirements — Direct Burial, Duct, Trench & Tray
• CU/XLPE/SWA/PVC 0.6/1kV — BS 5467 Armoured Power Cable
• CU/XLPE/LSZH/SWA/LSZH 0.6/1kV — BS 6724 Armoured Power Cable
• SWA vs AWA vs STA Armoured Cable | BS 5467/BS 6724 Comparison
• Industrial Cable Selection Guide | Conductor, Insulation & TCO

9. Frequently Asked Questions