Lookup table for ambient temperature, USA. NEC Section 310.15 uprating underslab conduit.

[brycenesbitt]

I am calculating for #10 ungrounded and #12 grounded THWN-2 90C wire in existing 1" PVC conduit that's in a
ground level commercial poured concrete slab with no insulation.
That conduit then transitions to metal at the slab edges.

The desired fill works with NEC Section 310.15 if using the 104% correction factor for the "ambient temperature",
followed by de-rating for the number of conductors in the conduit.

The local soil temperature at:

Soil Temparature Maps | GreenCast | Syngenta

www.greencastonline.com

Soil Temperature Maps by Depth

Soil Temperature

This app provides soil temperature and moisture data and forecasts for horticulture, agriculture and gardening.

soiltemperature.app

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Shows a range typical 45F to low 50F. The ten year monthly peak average is 52F. But this past year
it's been warmer, and soil temperatures have risen to as much as 60F, as the area climate is warming by all measures.
I saw this old closed thread:
https://forums.mikeholt.com/threads/how-can-i-determine-ambient-temperature.76033/

Is there any update on what's considered fair game for ambient temperature for underground slab conduit, and it's short stub above the slab?
Would you consider it fair game to up the ampacity of the THWN-2 by 4% in this situation? Am I reading all the above correctly?

 

[infinity]

These ambient temperature adjustments never made much sense to me. 310.15(A) gives some guidance in the Informational notes regarding operating temperature.

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.
Informational Note No. 1: The temperature rating of a conductor [see Table 310.104(A) and Table 310.104(C)] is the maxi‐mum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation. The allowable ampacity tables, the ampacity tables of Article 310 and the ampacity tables of Informative Annex B, the ambient temperature correction factors in
310.15(B)(2), and the notes to the tables provide guidance for coordinating conductor sizes, types, allowable ampacities,
ampacities, ambient temperatures, and number of associated conductors. The principal determinants of operating temperature are as follows:
(1)
Ambient temperature — ambient temperature may vary along the conductor length as well as from time to time.
(2)
Heat generated internally in the conductor as the result of load current flow, including fundamental and harmonic currents.
(3)
The rate at which generated heat dissipates into the ambient medium. Thermal insulation that covers or surrounds conductors affects the rate of heat dissipation.
(4)
Adjacent load-carrying conductors — adjacent conductors have the dual effect of raising the ambient temperature and impeding heat dissipation.