Ampacity of Conductors in Cable Trench

[Kyle2022]

Good morning folks,

I am trying to size some 15kV cables that will be laid in cable trench. I would rather not use ventilated cable trench though some have said these can achieve "free-air" ratings.

I've gotten my hands on a study titled "Ampacity of Conductors in Ventilated Cable Trench" by Emil F. Buettner, P.E. of Oklahom Gas & Electric, 1985. This is somewhat useful, but, again, for ventilated trench.

NEC applies here, but there is nothing that applies specifically to trench. Does anyone have any thoughts on this matter? Or, maybe there is a table in NEC that has been deemed acceptable to use for cable trench installation?

Thank you for your time,

Kyle

 

[dpc]

I haven't checked the 2005 NEC, but in the past I don't believe there was a defined wiring method that really aligned with a cable trench.

I have used Trenwa trench for heavy 15 kV circuits in the past. For NEC compliance, we did Neher-McGrath calculations (we actually got Okonite to do them for us) based on the spacing we used. The cables were supported and separated using spacers and the trench was backfilled with thermally-rated sand to improve ampacity.

 

[wareagle]

I would check with Olonite. I have one of their manuals that
lists cable ampacity for cable 36" deep 90C Earth Temp RHO 90.

 

[AllGreek2me]

Kyle, we use CymeCap software to make these calculations. You'll need to consider the cable type, temp. rating, concrete encasement, soil resistivity, depth, number of cables, load, load factor, DB or duct, etc.

Check out ICEA documents P-53-426 and S-66.524

 

[apowerengr]

Why use ventilated trench when with a good backfill a direct buried application will yield higher ampacity limits for a given cable?