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Site reference conditions

The standard IEC 60287-3-1 contains reference ambient temperatures and thermal resistivities of soil in various countries. We provide you here with an overview of these values per country.

Posted 2026-01-06
Categories: User guides

Quantities related to the operating conditions of cables are liable to vary considerably from one country to another. For instance, with respect to the ambient temperature and soil thermal resistivity, the values are governed in various countries by different considerations. Superficial comparisons between the values used in the various countries may lead to erroneous conclusions if they are not based on common criteria: for example, there may be different expectations for the life of the cables, and in some countries design is based on maximum values of soil thermal resistivity, whereas in others average values are used. Particularly, in the case of soil thermal resistivity, it is well known that this quantity is very sensitive to soil moisture content and may vary significantly with time, depending on the soil type, the topographical and meteorological conditions, and the cable loading.

Numerical values should preferably be based on results of suitable measurements. Often such results are already included in national specifications as recommended values, so that the calculation may be based on these values generally used in the country in question.

The standard IEC 60287-3-1 contains reference ambient temperatures and thermal resistivities of soil in various countries.

IEC 60287-3-1

Operating conditions – Site reference conditions

In order to use the formulae given in the various parts of IEC 60287, numerical values for the physical quantities should be chosen relating to the operating conditions.

It is obviously possible to compare the results of two calculations of current rating only when the assumptions made and the numerical values of the parameters are known.

In particular, the quantities related to the operating conditions of cables are likely to vary considerably from one country to another. An enquiry into this subject has been carried out and a number of countries have replied.

Annex A of the standard summarizes the operating conditions used in various countries. Attention is drawn to the fact that the information in Annex A is intended only as a guide for cable installation designers when data provided by a user is incomplete. Care should be taken not to draw unjustified conclusions from comparisons of values for different countries. It should be remembered that the values adopted in any particular country are governed by many factors some of which might not be of equal importance in other countries.

Procedure when values are not provided in national tables

It is recommended that when there is no value laid down in the national tables for the reference ambient temperature, thermal resistivity of the soil or solar radiation as given in the following chapters should be adopted.

Ambient temperatures at sea level
Climate$\theta_{air}$ [°C]$\theta_{a}$ [°C]
 minmaxminmax
Tropical25552540
Subtropical10401530
Temperate0251020

It is essential that current ratings should be valid for the maximum temperatures given. The lower values are for winter ratings if required. The values correspond with the temperature limits of winter and summer, alternatively rainy and dry seasons.

When no information about the depth of laying is given, the standard depth is to be taken as 1 m.

Thermal resistivity of soil
Soil conditionsWeather conditions$\rho_4$ [K·m/W]
Very moistContinuously moist0.7
MoistRegular rainfall1.0
DrySeldom rains2.0
Very dryLittle or no rain3.0
Solar radiation

When no information about the intensity of solar radiation $H_{sun}$ a value of 1 000 W/m$^2$ is adopted.

Values relating to the operating conditions in various countries

AU AT CA CN EG FI FR DE IT JP MX NL NZ NO OM PL PT ES ZA SE CH GB US
Soil thermal resistivity $\rho_4$ [K·m/W]SCNO
nominalsummer1.21.01.21.21.21.01.21.01.01.01.601.752.251.751.01.21.02.51.01.21.01.21.01.01.20.9
 winter1.21.01.21.21.21.00.851.01.01.01.501.702.201.751.01.21.02.51.00.851.01.21.00.851.050.9
dry zonesummer 2.5 2.52.5 1.52.42.54.03.02.5 3.0 
 winter n.a. 2.52.5 3.94.05.53.52.5  1.05 
rangemin  0.51.0 0.81.00.50.5 
 max  3.0 2.42.54.03.0 1.5 
specific conditionwet 1.00.9 0.6 0.4 
 seabed 0.4 1.0 0.6 
 rocky soil 1.3 
 concrete backfill 0.85 
Ambient ground temperature $\theta_a$ [°C]
nominalsummer2520253035152220202530253530201515402020252515201525
 winter1820-550515202025203025515025201025255101015
rangemax 20252020 1740 15 
 min 01300 -5105 0 
 average 10 15 
dry zonecritical 50 5050505050 
Ambient air temperature $\theta_{air}$ [°C]
nominalsummer40 40 3025 3030 
 winter30  3025 20 30 
rangemax 4040404735303030 3526453630 55 40 303050
 min -20-55-10-5-2020-200 81-512-10 10 00 -25102
 average 20 25 10 2831302920 25 
 variation 20 
Solar radiation $H_{sun}$ [W/m$^2$]
Intensitynominal 1000 1000 10006507008508001000 1200 100010001000 10001000 
 max 85090011001000 1250 1200 
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