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Including Soil Drying Time in Cable Ampacity Calculations

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  • Last Updated July 27, 2020

Including Soil Drying Time in Cable Ampacity Calculations

In soil drying time in cable ampacity calculations, previous papers proposed a method to find the maximum possible dried area of soil expected due to heating by underground cables. A method was also proposed to include this dried soil area in cable ampacity calculations. This assumed cables would be loaded 100% of the time. To dry soil to the maximum possible extent may take very long time periods, and in many installations the cables actually carry current for a limited time. In these cases the soil may never dry to its maximum extent.

This paper proposes a method to approximately determine the radius of dried soil that will occur due to cable heating for limited periods of time. It similarly proposes a method to approximate the time it will take for the dried soil to be replenished to its original moisture content after heating by the cables has ceased.

Buried cables heat the surrounding soil, decreasing its moisture content, increasing the soil’s thermal resistance. This gradual drying may eventually cause cable overheating. Previous papers have proposed a method for determining the maximum diameter of the dried soil area and described how this dried area may be included in Neher-McGrath cable ampacity calculations. This method was based upon the worst-case assumption that the buried cables would provide a constant heat rate 100% of the time.

Many installations have a load factor that provides for cable heating far less than this worst-case condition. Many photovoltaic installations deliver power only six to eight hours a day, with very little output the remaining 16-18 hours. This type of heating pattern will allow the soil to dry for a few hours followed by a longer period of time where surrounding soil moisture can be replenished. Using the maximum possible diameter of dried soil is overly conservative, causing cables to be unnecessarily oversized, resulting in increased cost.

Soil will be subject to drying only during the time the cable is carrying current. A method is proposed to calculate the radius of soil that will dry during the time period the cable is heating the soil. This paper proposes an approximate method to determine the radius of dry soil that will occur during any arbitrary time period and heat rate. Once this dried soil radius is known, it can be used in cable ampacity calculations in lieu of the maximum possible dried radius. Since the radius of soil around a cable that will dry during a limited time will be less than the maximum possible radius, the result will be an increase in allowable cable ampacity compared to the worst possible case.

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Including Soil Drying Time in Cable Ampacity Calculations