Soil thermal resistivity — why a buried cable may carry less than the table suggests

The guide on skin effect and proximity effect at large cable cross-sections covers why a conductor's AC resistance is higher than its DC resistance. This article covers the other side of a buried cable's heat balance: not how much heat is generated in the conductor, but how well that heat can be dissipated through the soil — and why that can vary substantially by location.

Thermal resistivity: the inverse of thermal conductivity

The thermal resistivity of the soil (expressed in K·m/W — kelvin-metre per watt) is a measure of how poorly a given soil type conducts heat: the higher the value, the more temperature difference is needed to dissipate the same amount of heat per unit length of cable through the soil to the surface. IEC 60287-3-1 sets out the reference conditions (including a reference value for soil thermal resistivity) on which the standard current-rating tables for buried cables — as also used with IEC 60287-1-1 for the resistance calculation itself — are based.

Why the actual soil often deviates from the reference value

The published current-rating tables assume an average, reasonably moist reference soil. In practice, however, soil types and conditions vary substantially:

Moist clay or loam has a relatively low thermal resistivity (good heat dissipation).
Dry sand or gravel, and in particular a poorly compacted trench backfill, has a considerably higher thermal resistivity than the reference value — air trapped in the pores of a loosely compacted soil is an excellent thermal insulator.
Seasonal drying (a dry summer, soil close to a heated basement wall or a terrace) can substantially raise the thermal resistivity of the same soil type compared with a wet winter period.

When the actual soil resistivity is higher than the reference value on which a current-rating table is based, the actual, safe current- carrying capacity of the cable at that location is lower than the table value — unless a correction factor is applied.

The self-reinforcing moisture migration effect around a heavily loaded cable

For a cable that is structurally loaded close to its thermal limit, a particular mechanism can occur: the cable's own heat generation drives moisture away from the soil immediately surrounding it, which further increases the local thermal resistivity of that dried-out zone — which in turn further worsens heat dissipation and pushes the conductor temperature up even more. This moisture migration effect is a self-reinforcing mechanism that, in the worst case, can lead to a local, progressive overheating of the cable, even if the original load on its own remained within the cable's nominal current-carrying capacity according to the table value.

Note: this drying-out effect is particularly relevant for heavily and continuously loaded medium-voltage cables in drought- sensitive soils; for most lightly to moderately loaded low-voltage cables in normally moist Dutch soil the risk is limited, but not by definition absent during a prolonged dry period.

Mitigation: greater depth, spacing, or thermally stable backfill

Several commonly used measures reduce the risk from a higher-than- assumed actual soil resistivity:

Greater laying depth or greater spacing between parallel cable circuits, so that the heat from each circuit can spread over a larger soil volume before the temperatures of neighbouring circuits significantly affect each other.
Thermally stable backfill — a specially composed, compacted sand or cement-bound mixture with a guaranteed low and stable thermal resistivity regardless of moisture content — applied directly around the cable on heavily loaded medium-voltage routes.
Applying correction factors to the tabulated current rating when the actual soil resistivity has been measured beforehand or is reasonably estimated to be higher than the table's reference value.

Practical relevance

When sizing a heavily loaded buried cable route — for example a medium-voltage connection to a large-consumer connection or a charging plaza — it is not enough to consult the standard current- rating table alone without assessing the actual soil condition of the route: a dry, sandy, or loosely compacted trench backfill can noticeably lower the effective current-carrying capacity compared with the table value, and that difference does not show up in the table itself unless the correct correction factor is applied.

Common mistakes

Applying the standard current-rating table without assessing the actual soil resistivity of the route, assuming the reference value is representative everywhere.
Backfilling a trench with loose, uncompacted sand or soil without accounting for the higher thermal resistivity that air pockets in a poorly compacted backfill cause.
Ignoring the drying-out effect for a structurally heavily loaded medium-voltage cable, when precisely that loading situation can trigger the self-reinforcing mechanism.
Laying multiple cable circuits too close together without accounting for their mutual thermal interaction (and the corresponding grouping factor, similar to the principle in the guide on grouping factor Cg) in the sizing.