When reading through NEC ampacity tables you'll notice that many make reference to an assumed "thermal resistivity of 90°C*cm/W". What's up with that?
Thermal resistivity is a characteristic of any material to dissipate heat. Higher thermal resistivities mean that the material behaves more like a thermal insulator, keeping heat from flowing out and leading to elevated temperatures. Lower thermal resistivities mean that the material behaves allows heat to flow through more readily-think of something like a heat sink or a radiator.
Soils are no exception to this characteristic. NEC Annex B provides some values for designers to reference: coastal damp soils are usually closer to 60°C*cm/W, average soils are 90°C*cm/W, and dryer, rockier soils are usually closer to 120°C*cm/W. Essentially, the code is filled with references to ampacities at 90°C*cm/W because that is an average value for soils around the United States. For a lower thermal resistivity soil (< 90°C*cm/W), conductor ampacity increases. Similarly, for a higher thermal resistivity soil (> 90°C*cm/W), conductor ampacity decreases. There's a lot more to this story than just that. Depending on the ground conditions, primarily moisture content, soils could vary from thermal resistivities below 50°C*cm/W to well over 300°C*cm/W. That leads to some big differences in ampacities.
Organic materials (grass, plants, etc.) will have much higher thermal resistivity than soil and can create air pockets that make heat transfer even worse. That's why it's important to make sure soil backfilled around cables is properly inspected.
So, given how much thermal resistivity can vary, are NEC ampacity tables even reliable?
Well, yes, they are!
Below is an example thermal resistivity dryout curve. For most moisture content values, including the natural value, soil thermal resistivities are close to or less than the NEC's value of 90°C*cm/W. Values skyrocket as moisture moves below what is known as the "critical moisture value". For the example below, the critical moisture content is approximately 4%. Values can easily approach 3x the NEC value of thermal resistivity and dramatically impact cable sizing. As long as we can stay away from that runaway dryout condition, NEC tables should remain a conservative method for sizing conductors.
But how do we keep soils from drying out?
Dryout is unlikely to occur if the external temperature of the conduit or cable that is in contact with the soil is kept low. How low is reasonable can be difficult to determine and site-specific assessment likely requires detailed geotechnical analysis. IEEE 141 makes a general recommendation of 60°C-70°C. Some conduits, like PVC, have very high thermal resistivity and will create a significant temperature differential between the surrounding soil and the conductors inside. Directly buried cables that have to pass through conduit will generally be much cooler when in direct contact with the soil than when in conduit, also lowering the potential for dryout.
Dryout is a real problem and can substantially lower conductor ampacities. In some areas with naturally low moisture contents, dryout may be unavoidable and NEC tables for ampacity may not be accurate. A detailed geotechnical analysis and considerations of conductor loading will mitigate this risk.
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