Default Material Values KOPI

Thermal Resistivity

Below are values obtained from CIGRE, Sintef and NMBU, which should be viewed as a guide only. We always recommend that a measure thermal resistivity on the masses as local conditions affect the value.

Mass	Wet [mK/W]	Dry [mK/W]	Source
Gravel (coarse-fine)	0.5	2.5	Cigre TB 714 (2017)¹²
Sand (coarse-fine)	0.5	2.0	Cigre TB 714 (2017)¹²
Sand (sorted)	0.6	3.5	Cigre TB 714 (2017)¹²
Silt	0.7	2.5	Cigre TB 714 (2017)¹²
Topsoil	0.7	2.0	NMBU measurement¹⁰
Turf	2	17	SINTEF measurement¹⁰
Clay	0.25	17	SINTEF measurement¹⁰

Mass	Thermal Resistivity [mK/W]	Source
Bentonite	0.4- 0.6	SINTEF measurement¹⁰
Tarmac	0.5-1.0	SINTEF measurement¹⁰
Concrete for duct banks	1.0	IEC 60287-2-1:2023⁹
Concrete for duct banks	0.6¹³	SINTEF measurement¹³

IEC 60287

IEC 60287 has a more general approach to masses based on moisture content presented in the table below:

Thermal resistivity [mK/W]	Mass description	Weather conditions
0.7	Very moist	Persistently moist
1.0	Moist	Regular rain
2.0	Dry	Rare rain
3.0	Very dry	Little to zero rain

Metals:

	Thermal resistivity [mK/W]	Electrical resistivity at 20°C [μΩ*m]	Temperature coefficient [1/K]	Relative permability
Steel	0.019 ¹ 4	0.138 ² ³ ¹ 4 9	0.0040 ³ 9	250 ² 5 9
Stainless steel	0.69 8	0.7 6 7 8	0.00094	1 6 7 8
Magnetic stainless steel	0.69 ⁸	0.7 ^{6 7 8}	0.00094	1000 ^{6 7 8}

Cables in Water Filled Pipes

ρ = (2 * π * U)/(1+ 0.1 * De * ((V+(Y * θ) * ln(Dd/De)))

ρ = thermal resistivity applied to the pipe filling.
θ = the average temperature of water.
Dd = inner diameter of the pipe De = diameter of the cable or an equivalent diameter of the group of the group of cable calculated according to IEC. U = 0.1 (for water)
V = 0.03 (for water)
Y = 0.001 (for water)

	Source
U = 0.1 (for water)	IEC 60287-2-1
V = 0.03 (for water)	IEC 60287-2-1
Y = 0.001 (for water)	IEC 60287-2-1

Empty water filled pipes

For empty water filled pipes and water filled pipes without a heat source, a fixed thermal resistivity of 1.6 mK/W is applied.

“The Engineering Handbook” and “The Engineering ToolBox” sets the thermal conductivity for water at 0.609 W/mK, at 26.8°C. This is recalculated to approximately 1.6 mK/W for the temperature expectet to occur in a trench.

American Society for Metals, Metals Handbook, Ninth Edition, Volume 1, Properties and Selection: Iron and Steels, Beijing, China: China Machine Press, 1988. ↩ ↩²
L. Meyerhoff, “A-C Resistance of Pipe-Cable Systems with Segmental Conductors,” AIEE. Trans., vol. 71, no. III, pp. 393-414, 1952 ↩ ↩²
Y. Sun, D. Niu and J. Sun, “Temperature and carbon content dependence of electrical resistivity of carbon steel,” in 2009 4th IEEE Conference on Industrial Electronics and Applications, Xi’an, China, 25-27 May 2009. ↩ ↩²