![]() ![]() Thermal conductivity of soil is dependent on two factors: the soil’s density and water content. This causes an increase of temperature to the surrounding soil, further de-rating the ampacity of the cable. As the thermal conductivity of the soil decreases, the temperature gradient increases. How effectively the heat distributes through the soil and away from the cables is determined by the thermal conductivity of the soil. What Fourier’s Law states is that a temperature change in the surrounding material will only occur if there is a gradient in the heat flow. K = Thermal conductivity of the material (W/m-K) The relationship between heat flow and the resulting temperature gradient can be represented by Fourier’s Law of thermal conduction: Convection takes part in the air space between the cables and the conduits in the duct bank, but the heating effects due to this method of heat transfer is minimal in comparison to the heat transferred through conduction. For underground duct banks, the primary means of heat transfer is through conduction. ![]() If there is a temperature difference between two systems, heat will transfer from the hotter system to the colder system until the system reaches a point of thermal equilibrium. Conduction of heat from the cables to the surrounding materials occurs due to temperature differences between materials. As the cables generate heat, it is dissipated to the surrounding soil and concrete encasement materials. The thermal properties of the soil in which the duct bank is installed are the most critical aspect to the design. Heating effects caused by the surrounding soil In reality, any heat-producing object within close proximity of the duct bank will transfer heat in the duct bank system and further de-rate the ampacity of the cabling system. Mutual heating effects can also occur between two duct banks installed close to each other. The effects of mutual heating between the cables are dependent upon the spacing between each of the cables and the symmetry in which the cables are arranged. Each cable in the duct bank will generate its own source of heat and conduct this heat to the other cables. Most electrical duct bank installations will contain more than just one cable. However, these must be taken into account in high-voltage applications. In low-voltage applications, the temperature rise due to dielectric losses is relatively small compared to the overall temperature rise of the cable. Heat created by dielectric losses is due to the energy needed for the electrons to overcome the opposition of the electromagnetic field’s alternating polarities. The temperature rise of the cable is generated from the current producing (I 2R) losses through the cable’s conductor, sheath, and conduit as well as the temperature rise produced from the cable’s dielectric losses. Common practice for large underground duct bank cables is to utilize the 90☌ rating. All cables have a temperature threshold that must not be exceeded for extended periods of times, or physical deterioration of the cable will begin - increasing the possibility of a fault occurring. Engineering solutions Heating effects of the cableĪs the cables in the duct bank transmit current, heat is generated due to losses in the cable.Heating effects caused by the surrounding soil.In order to properly size the duct bank, the following factors must be considered and evaluated: Electrical engineers designing electrical duct banks must understand the soil’s thermal properties and the effects it will have on the overall installation. Improper heating calculations can lead to an undersized duct bank, which can be a costly mistake to fix - if it’s even possible. Performing electrical cable heating calculations can be a cumbersome task, but they’re essential to accurately determining the allowable ampacity of an underground electrical duct bank installation.
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