Ambient Temperature for Underground Cable Ampacity Calculations 1

[Tony2802]

Hi,

This question has already been posted in two other threads, but the answers provided were not really helpful.

In Neher-Mcgrath and IEC 60287, it is stated that the ambient temperature is the soil temperature at cable laying depth. As I understand it, those methods consider an isothermal earth surface at ambient temperature. Wouldn't it make more sense to consider ambient temperature at earth surface?

In summer, in canada, the ambient earth surface is going to be hotter than the ambient earth at the cable depth. If you consider the temperature at cable depth, it's going to be too optimistic.

In winter, in canada, the ambient earth surface is going to be colder than the ambient earth at the cable depth. If you consider the temperature at cable depth, it's going to be too pessimistic.

What are your thoughts? If user Power0020 sees this, did you ever find an answer to this question?

Thank you

 

[7anoter4]

NEC 2017 Annex B Art. B2 it is written:
These ampacities were calculated as detailed in
the basic ampacity paper, AIEE Paper 57-660, The Calculation of
the Temperature Rise and Load Capability of Cable Systems, by
J. H. Neher and M. H. McGrath. For additional information
concerning the application of these ampacities, see IEEE
STD 835-1994, Standard Power Cable Ampacity Tables.

NEC Earth temperature 20oC ;IEEE 835/1994 25oC
RHO 90[oC cm/W] NEC; 60,90,120 IEEE 835
IEC 60287-2-1 does not indicate nor Ta neither RHO. However, IEC 60502-2 [for Medium Voltage cables] Ta=20oC RHO 150 and IEC 60364-5-52 for low voltage cables 20oC earth RHO 250.
Any way the concrete RHO is also different. IEC 100oC.cm/W= 1K.m/W. IEEE 835 55 oC.cm/W

 

[7anoter4]

In the Neher and McGrath publication it is an explanation why the Earth temperature [average] is constant [as all agreed with 20oC -if the depth it is more than 30 inches]:
The combined heat flow from earth core and solar sources results in an earth temperature which decreases with depth in summer; increases with depth in winter; remains about constant at any given depth on the average over a year; approximates constancy at all depths at midseason, and in turn results in flow of heat from cable sources to earth's surface, directly to surface in midseason and winter and indirectly to surface in summer.

 

[LittleInch]

Soil temperature at about 1m depth typically follows the average air temperature with a time shift of a couple of months.

given that this is where the cable sits, why would you want to take a ground temp reading which has much bigger fluctuations?

Heat travels in all directions so why wouldn't you balance out the temp at 2m depth with that at ground level?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dpc]

I don't think anyone will be able to give you a definitive answer. It's an engineering judgement and a fundamental limitation of Neher McGrath method. But obviously, you must err on the pessimistic side. There are so many variables in this calculation, that the accuracy is always questionable. If my cable sizing hinged on my interpretation of how to select the soil temperature, I'd probably feel I was cutting it too close.

 

[FacEngrPE]

If the Neher and McGrath method results in uneconomical wire sizes the other approach is to enhance cooling.
[ul]
[li]Circulating liquid. Link[/li]
[li]Air cooling (forced or gravity) Link[/li]
[/ul]

 

[Power0020]

The assumption of soil temperature as the ambient for underground cables is based on infinite/too high heat capacity of soil that the cable heat generation will be absorbed totally by earth and not going to the ambient air. As this is an iterative process, the starting temperature of soil is used to initially estimate the thermal resistance (note that soil temperature change around the cable with cable heating). It is assumed that the temperature variation around the cable is not affecting the whole soil though, which seems to be correct given that heat flux dissipates around a bigger area outside the cable.

The only good thing to think that this assumption is correct is the very good correlation of the calculated and measured ampacities, which is acceptable.

 

[Tony2802]

Thank you very much everyone. I now understand better why the IEC 60287 and Neher-Mcgrath methods use the soil temperature at cable depth and not the ambient temperature.