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How do I select the ambient temp for burried cables in a raceway

How do I select the ambient temp for burried cables in a raceway

How do I select the ambient temp for burried cables in a raceway

(OP)
I have to size some THWN-2 conductors for use in underground conduit between two pieces of outdoor equipment. I need to figure out the conductor size and I am getting hung up on how to figure out the derating due to the ambient temperature in the area.

My first question is how do I select the ambient for an outdoor area with seasonal variability? Do I have to take the highest temp that can ever be seen in a region or is there some accepted ambient based on the temperature range of that location?

Secondly if only a few feet (less than 5' out of a run of around 80') is actually above ground can I use a soil ambient instead of an air ambient? If so, how do I figure that number out? Is there a rule of thumb based on air temp?

Thanks in advance for any help.

RE: How do I select the ambient temp for burried cables in a raceway

(OP)
Just to clarify I am trying to figure out what ambient range I need to use in the table 310.15(B)2

RE: How do I select the ambient temp for burried cables in a raceway

Based on use of THWN-2, I'm assuming this circuit is less than 2000 V. The soil temperature is not going to exceed 30 deg C unless this is in the middle of Death Valley, so no temperature derating should be required.

In general, you need to use the lowest ampacity that would apply to the circuit if there is more than one applicable ampacity. See 310.15(2).

RE: How do I select the ambient temp for burried cables in a raceway

"My first question is how do I select the ambient for an outdoor area with seasonal variability? Do I have to take the highest temp that can ever be seen in a region or is there some accepted ambient based on the temperature range of that location?"

See 310.15(A)(3)Temperature Limitation of Conductors: No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. In no case shall conductors be associated together in such a way, with respect to type of circuit the wiring method employed, or the number of conductors, that the limiting temperature of any conductor is exceeded.

This means you take the highest ambient temperature that will be seen - not an average temp.

"Secondly if only a few feet (less than 5' out of a run of around 80') is actually above ground can I use a soil ambient instead of an air ambient? If so, how do I figure that number out? Is there a rule of thumb based on air temp?"

See code reference and my opinion listed above.

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