Current-carrying capacity of buried cables — soil thermal resistivity and grouping, separate from burial depth

Buried cables — burial depth and mechanical protection covers the minimum trench depth of 0.5 m for a cable buried directly in the ground, as protection against mechanical damage. This article covers a completely different question about that same buried cable: how much current may it carry? The answer to that question depends on soil properties that have nothing to do with the burial-depth requirement.

Why soil requires a different calculation than air

Reference methods — determining the installation method shows how the physical installation method is translated into a reference method (A through F) that determines which current-carrying capacity table applies. For cables buried directly in the ground (method D1) and cables in an underground duct (method D2), a different heat-dissipation model applies than for cables in air: the soil surrounding the cable must dissipate the generated heat via conduction through the ground itself, rather than via convection in air. How well the soil dissipates that heat depends on the thermal resistivity of the soil — and that property differs significantly by soil type and moisture content.

Reference values: soil resistivity and ground temperature

The current-carrying capacity tables for buried cables (method D1/D2) are drawn up for two reference values:

a thermal soil resistivity of 2.5 K·m/W (table B.52.16 of IEC 60364-5-52 Annex B gives correction factors for a different soil resistivity — dry sandy soil, for example, has a higher resistivity than moist clay soil, meaning a lower permissible current for the same cable);
a reference ground temperature of 20°C (table B.52.15 gives correction factors for a different ground temperature) — this is a different reference than the 30°C ambient air temperature used for above-ground installation methods.

Both correction factors apply only to installation methods D1 and D2 — for cables in air (methods A, B, C, E, F) the ambient air temperature tables (B.52.14) apply instead.

Grouping of multiple buried circuits

Just as with cables in air (see the guide on grouping factor Cg), a separate grouping factor applies to multiple buried circuits laid together, because each circuit heats the soil around the other circuits:

table B.52.18 gives the grouping factor for circuits buried directly in the ground (method D2) alongside each other;
table B.52.19 gives the grouping factor for circuits in separate underground ducts (method D1) alongside each other.

In both tables the grouping factor decreases as more parallel circuits are laid and as the spacing between the circuits decreases — a practical starting point for route design is to keep at least one cable diameter of spacing between parallel buried circuits, though the exact reduction factor per table value depends on the actual number of circuits and the chosen spacing.

Why this is separate from the burial-depth requirement

The 0.5 m trench depth from §522.8 is a mechanical protection requirement: it ensures a cable is not accidentally struck by a spade or excavator. This requirement says nothing about how much current the cable may carry. Conversely, the current-carrying capacity calculation involving soil resistivity and grouping factor says nothing about the required burial depth. A cable buried at exactly 0.5 m and therefore compliant with the mechanical protection requirement can still be thermally overloaded if the soil turns out to be drier, higher- resistivity ground than assumed during sizing, or if more parallel circuits are laid in the same trench than were accounted for.

Practical relevance

When designing a route with multiple buried circuits, or for a complaint about a buried cable running warmer than expected without the burial-depth requirement being in question, checking the actual soil resistivity and temperature — and the grouping factor applied for the actual configuration of parallel circuits — is the direct next step, separate from the mechanical protection of the cable itself.

Common mistakes

Assuming the burial-depth requirement (§522.8) also says something about the permissible current — these are two independent requirements with different purposes.
Applying the reference soil resistivity of 2.5 K·m/W without question for dry sandy soil, while that soil type can have a considerably higher (worse) soil resistivity, resulting in a lower permissible current.
Not applying a grouping factor for multiple parallel buried circuits in the same trench or duct bank, while table B.52.18/ B.52.19 were drawn up specifically for that purpose.
Using the ambient air temperature tables (B.52.14) for a buried cable, while for method D1/D2 the ground temperature table (B.52.15) applies — a different reference temperature with a different correction factor.