Thermal resistance for 480VAC feeders at varying depths?

[Dale001289]

Is there a simple guideline (without using Etap or Neher-McGrath) to determine basic conductor sizes in concrete at a depth of say 4' versus a duct bank at 26'?

 

[ptonsparky]

As an EC, not an EE, with limited large project experience I would use the standard ampacity charts for WAGs.

 

[don_resqcapt19]

Look at Informative Annex B, section B.3(b). You may be able to use this for a simple calculation, but if these are critical, heavily loaded circuits, you need to run the full calculation. The 6% per foot of additional depth would be a huge change in ampacity for your circuit.

 

[Julius Right]

In my opinion 4'=48" but 26' =26*12=312". I think it has to be 2.5'=30".
Usually, the depth of a duct bank-see NEC art.310 or IEEE 835/ 3.3.1 Duct banks (30 inches cover over top of duct bank).
The heat evacuation from a cable through different layers of different thermal resistance is like the current run through different impedances and the temperature drop it is instead of voltage drop. Approximately 10-15% of the temperature drop through cable insulation, jacket, through conduit and concrete does not depend on earth thermal resistance and depth but the rest of 85-90% run through earth.
So, I agree with don_resqcapt19.
NEC Informative Annex B Application Information for Ampacity Calculation
8.3 Criteria Modifications.
Where the burial depth of direct burial or electrical duct bank circuits are modified from the values shown in a figure or table, ampacities can be modified as shown in (a) and (b) as follows:
(b) Where burial depths are deeper than shown in a specific underground ampacity table or figure, an ampacity derating factor of 6 percent per increased 300 mm (foot) of depth for all values of Rho can be utilized. No rating change is needed where the burial depth is decreased.

 

[Dale001289]

In my opinion 4'=48" but 26' =26*12=312". I think it has to be 2.5'=30".
Usually, the depth of a duct bank-see NEC art.310 or IEEE 835/ 3.3.1 Duct banks (30 inches cover over top of duct bank).
The heat evacuation from a cable through different layers of different thermal resistance is like the current run through different impedances and the temperature drop it is instead of voltage drop. Approximately 10-15% of the temperature drop through cable insulation, jacket, through conduit and concrete does not depend on earth thermal resistance and depth but the rest of 85-90% run through earth.
So, I agree with don_resqcapt19.
NEC Informative Annex B Application Information for Ampacity Calculation
8.3 Criteria Modifications.
Where the burial depth of direct burial or electrical duct bank circuits are modified from the values shown in a figure or table, ampacities can be modified as shown in (a) and (b) as follows:
(b) Where burial depths are deeper than shown in a specific underground ampacity table or figure, an ampacity derating factor of 6 percent per increased 300 mm (foot) of depth for all values of Rho can be utilized. No rating change is needed where the burial depth is decreased.

Very nice feedback - thanks to all


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[Dale001289]

In my opinion 4'=48" but 26' =26*12=312". I think it has to be 2.5'=30".
Usually, the depth of a duct bank-see NEC art.310 or IEEE 835/ 3.3.1 Duct banks (30 inches cover over top of duct bank).
The heat evacuation from a cable through different layers of different thermal resistance is like the current run through different impedances and the temperature drop it is instead of voltage drop. Approximately 10-15% of the temperature drop through cable insulation, jacket, through conduit and concrete does not depend on earth thermal resistance and depth but the rest of 85-90% run through earth.
So, I agree with don_resqcapt19.
NEC Informative Annex B Application Information for Ampacity Calculation
8.3 Criteria Modifications.
Where the burial depth of direct burial or electrical duct bank circuits are modified from the values shown in a figure or table, ampacities can be modified as shown in (a) and (b) as follows:
(b) Where burial depths are deeper than shown in a specific underground ampacity table or figure, an ampacity derating factor of 6 percent per increased 300 mm (foot) of depth for all values of Rho can be utilized. No rating change is needed where the burial depth is decreased.

Julius
I just sent you a message thru the forum.


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[Julius Right]

Sorry, I used a wrong program. Now I checked it vis-a-vis IEEE 835 [the basis of NEC].
Revising some details, according to IEEE 835, for 312" depth the ampacity per each one of 17*3=51 cables [17 parallel per one phase] is 292 A only, and for 4'=48" the ampacity is 347 A.
So, for 26'=312" depth the total ampacity per one phase will be 17*292= 4964 A [500 mcm copper,90oC,1 kV] and for 4'=48" the total ampacity is 17*347=5899 A.
LF=50% Earth thermal resistivity=55 duct bank dimensions 24" height,48" width.