Duct Bank Depth

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billyzee

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Does anyone have a good story on why Annex B requires you to derate ampacity by 6% per foot increase in burial depth? I don't get this. Isn't the thermal conductivity of dirt, rocks and just about everything better than the thermal conductivity of air. And not only that isn't the ambient temperature of lower when you go deeper.

Anyway, it's not a big deal. I can live with requirement. I just don't get it.

Thank you,
 
Maybe it has to do with the thermal mass. A ground-source heat pump will raise the temperature of the earth around the pipes over the cooling season, and lower it during the heating season. Dirt is actually a pretty good insulator, as evidenced by the fact that temperatures are constant year-round just a few feet underground.
 
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