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Thermal resistance for 480VAC feeders at varying depths?

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Dale001289

Senior Member
Location
Georgia
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'?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
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

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
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

Senior Member
Location
Georgia
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

Senior Member
Location
Georgia
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

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
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.
 

Dale001289

Senior Member
Location
Georgia
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.

Excellent
Thanks again


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