Derating 15 kv cable in a concrete trench

[diamonddean]

I am having no luck on finding guidance on how to derate 15KV cable installed in concrete trench with a lid. Trench is 4' x 4' and over 2000' long.
Any help would be appreciated

 

[Mayimbe]

I use the IEEE 835 - 1994 standard.

 

[diamonddean]

There is no trench configuration in IEEE 835 ?

 

[Mayimbe]

There is no trench configuration in IEEE 835 ?

Are you asking me or telling me?

question:

Concrete trench isnt the same that concrete buried duct?

 

[mkgrady]

Are you asking me or telling me?

question:

Concrete trench isnt the same that concrete buried duct?

I would say no. There is no duct. Is this trench crowded with other cables? How about checking the manufacturers literature on the cable or calling the manufacturers rep.

I've installed miles and miles of 15KV cable in UG ducts, in trenches and through vaults. I didn't design any of it but the subject of derating never came up. Maybe it was because it was usually sixed several times larger than seemed necessary.

 

[rbalex]

Assuming they were neatly installed, I?d be inclined to treat them as a covered cable tray application. [392.13(A)(1) or 392.13(B)(1)] Otherwise, it?s simply a NEC non-compliant installation [328.10] and derating is moot.

 

[bob]

I am having no luck on finding guidance on how to derate 15KV cable installed in concrete trench with a lid. Trench is 4' x 4' and over 2000' long.
Any help would be appreciated

How many cable are you laying in the trench? Are the bundled or just random lay?

 

[diamonddean]

There are six 15 kv cables with a max. load of 360A each. Actually the cables will be placed on an open cable tray (6" x 30") inside the trench. SInce the trench is covered (although not sealed) there will be an increase in ambient temperature inside the trench and therefore require a derating of the cable. Cables can be triplexed or laid in a single row.

 

[rbalex]

There are six 15 kv cables with a max. load of 360A each. Actually the cables will be placed on an open cable tray (6" x 30") inside the trench. SInce the trench is covered (although not sealed) there will be an increase in ambient temperature inside the trench and therefore require a derating of the cable. Cables can be triplexed or laid in a single row.

That narrows it down to 392.13(B)(1) then; "sealed" or not, the air-flow is highly restricted. Sections 392.13(B)(2)&(3) require a basically "free air-flow" installation. The ampacity is 70 percent of either Table 310.69 or 310.70, whichever applies.

 

[weressl]

Assuming they were neatly installed, I?d be inclined to treat them as a covered cable tray application. [392.13(A)(1) or 392.13(B)(1)] Otherwise, it?s simply a NEC non-compliant installation [328.10] and derating is moot.

Hmmm.....even though you and I did go to different schools, seem like we are in agreement a lot. I replied the same, although not as detailed as you, to his other post.

Maybe we are going through the same school of (professional) life? :grin: