Direct Buried DC cable ampacity calculation for solar farm

[william879]

I am designing the cable trench for 24 direct buried cables (12 circuits) for a solar farm project. Each cable carries 168A dc current, continuously. Both "+" and "-" carry the same amount of current, which is obvious for DC.

Question 1: How am I suppose to size the cables with optimum arrangement and spacing between each cable, that guarantees each cable will carry 168ADC minimum?

Question 2: is there a code that talk about DC cable direct buried?

thank you all.

 

[undervoltage]

Calculate the applicable derating factor as per NEC or IEC which ever is applicable and make sure that your cable can carry 168A after being derated.

For the derating due to grouping, refer to the vendor's catalogue. The optimum arrangement will depend upon the number of cables per group. It is a tradeoff. You increase the number of cables per group which will result in reduced ampacity.

 

[7anoter4]

I agree with undervoltage in principle. But nor NEC neither IEC will give you directly the recommended size of the cable or the recommended distance between cables. There are national standards -as DIN-VDE 0289 part 2 for instance. According with this standard a single-core 95sqr.mm copper cable XLPE insulated and PVC jacketed -maximum conductor temperature of 90 oC-laid in the ground of 20oC and 0.7-1.2 m depth, 482 A DC is admissible. If the soil thermal resistance will be 1 K.m/w [=100 k.cm/w] but 25oC a derating factor of 0.9 will be necessary.
For 24 cables- that means 3*8- another derating factor of 0.65 is to be applied. The load factor considered by the above standard is only 70%.
Then the total derating factor will be= 0.7*.65*.9=0.4095
Now 482*.4095=197.4 A it is admissible for 24 single core cables arranged side by side. The minimum clearance between two adjacent cables has to be 7 cm [3 inches approx.]
You may calculate the permissible ampacity following "The Calculation of the Temperature Rise and Load Capability of Cable Systems", by J.H. Neher and M. H. McGrath. or IEC 60287 also.

 

[jghrist]

With this many cables in one direct buried run, you should particularly concerned with drying out of the soil. This is not particularly well covered in the original Neher McGrath paper, but is covered in IEC calculation methods and in Rating of Electric Power Cables: Ampacity Computations for Transmission, Distribution, and Industrial Applications by George J. Anders, IEEE Press, 1997.

 

[7anoter4]

Good point, jghrist !
I took it into consideration [in a modest way, may be]. If using only a clean sand backfill, well compacted and the water content of minimum 3% the thermal resistivity could be 60 K.cm/w. If the water content will decrease up to 0.1% the resistivity will be still 100.But if the water content will decrease below the critical the resistivity could be 400!
The compaction degree is an important factor also. I think a thermal resistivity test of the backfill will be required, any way.