Ampacity calculations for deeply buried objects

When simulating objects that are buried deeper than a couple of metres one needs to consider the thermal inertia of the surrounding soil. Cableizer offers an alternative approach to calculate the ampacity with an equivalent depth for deeply buried objects.

Posted 2021-04-18
Categories: Theory

For cable systems buried at a depth of more than 5 m Cableizer indicates the following information: 'IEC standard gives no guidelines for objects at large depths'. In this blog post, we want to give you some background about deeply buried objects and what to consider when simulating them in Cableizer.

Basically, you can use the IEC method for any burial depth but your ampacity will become more and more conservative. The reason is that it will take years for the temperature to reach and heat up the surrounding soil. Even at the expected lifetime of your cable system (which for example is in the range of 40 years for XLPE cables), depending on your environmental conditions, a steady-state may not have appeared even with continuous load. In case of a cyclic load, it will take even much longer to reach the final temperatures.

The IEC 60287-3-1 standard actually gives no guidelines for cables installed at shallow (<0.5 m) or larger depths (>3 m). There was a lot of discussion about different burial depths in CIGRE Guide 640 (2015). Many state a limit at 5 m beyond which they reconsidere the conditions. Some DSO/TSO use a significantly lower ambient temperature of around 10 °C when the burial depths exceed 5 m. The discussion is complex and impossible to fully describe in a short information text. It was our decision to set the threshold value for the information at 5 m instead of 3 or 10 m.

An alternative approach that we have implemented in order to consider deeply installed objects like tunnels is our Deep burial thermal inertia advanced option. The calculation of the equivalent depth of a deep tunnel is based on the IEEE transactions on power delivery paper 'Ampacity Calculation for Deeply Installed Cables' by E. Dorison, G.J. Anders, and F. Lesur, dated 2010. You can read more about the deep burial thermal inertia method and its implementation in Cableizer by clicking the preceding link.


The following two graphs show values of the equivalent depth $L_{deep}$ as a function of the laying depth $L_{cm}$ (top) and the transient load period $\tau_L$ (bottom). In both examples, a heat source or tunnel with an outer diameter$D_o$ of 1 m and a soil thermal diffusivity $\delta_{soil}$ of 5e-07 m2/s was assumed. In Cableizer, these values can all be freely adjusted to the specific conditions of your individual projects.

Equivalent depth [m] as a function of actual depth (10 - 80 m) with $\tau_L$ = 40 years.

Equivalent depth [m] as a function of transient load period (1 - 80 years) with $L_{cm}$ = 40 m.

The above graphs show that the equivalent depth is always lower than the actual burial depth. Under some conditions such as deep burial or low expected life time ($\tau_L$), the difference can be quite considerable. As a consequence, the ampacity of your project will be larger (and less conservative) with the Deep burial thermal inertia advanced option activated.