Low Voltage Direct Buried Cables

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timm333

Senior Member
Location
Minneapolis, MN
Occupation
Electrical Design Engineer
We have to lay direct buried cables on 480V secondary side of transformer. The cables are multiconductor, with five conductors per phase in parallel. NEC 300.5 states that the minimum cover for direct-buried shall be 24”, but it does not tell about spacing between direct-buried cables.

When cables are run in underground PVC conduits: the PVC conduits can be laid in screened-sand in two vertical layers, and the minimum required spacing between adjacent PVC conduits (both horizontal and vertical) is 7.5”.

When we lay cables as direct-buried, can we still lay cables in two vertical layers? Also when we lay cables as direct buried, what would be the minimum required spacing between adjacent cables? Thanks
 
timm333 said:
We have to lay direct buried cables on 480V secondary side of transformer. The cables are multiconductor, with five conductors per phase in parallel. NEC 300.5 states that the minimum cover for direct-buried shall be 24”, but it does not tell about spacing between direct-buried cables.

When cables are run in underground PVC conduits: the PVC conduits can be laid in screened-sand in two vertical layers, and the minimum required spacing between adjacent PVC conduits (both horizontal and vertical) is 7.5”.

When we lay cables as direct-buried, can we still lay cables in two vertical layers? Also when we lay cables as direct buried, what would be the minimum required spacing between adjacent cables? Thanks
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Look into Duct and or Bore spacers. You need to be concerned about proper backfill of material if you are going to have a space between cables, also.

If you don't space them, you'll have to look at derating:
(3) Adjustment Factors.
More Than Three Current-Carrying Conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a)^. Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
 
There is no requirement in NEC that the backfill soil (for direct buried cables) needs to be compacted. Should backfill soil still be compacted?
 
timm333 said:
Should backfill soil still be compacted?
Click to expand...
That more up to how it will impact other things than it has to do with the electrical installation itself. If you don't compact it at very least there likely will be settling sometime down the road.
 
The horizontal distance between cables influences only on ampacity indeed. You may run the cables even triplexes [in contact one with other] .However, in no situation 3 conductors’ cable or three triplexes cables’ ampacity will decrease up to the value recommended in NEC Table 310.16
Let's take, for instance , 500 MCM copper conductor 75oC. In 310.16 it is 380 A. In Annex B [informative] 3 cables in triplex arrangement [Detail 7] it is 536 A.
In Annex B [Informative] B.2 paragraph is indicated:
For additional information concerning the application of these ampacities, see IEEE STD 835, Standard Power Cable Ampacity Tables.
In IEEE 835/1994, one triplex circuit it is 543 A [even the soil temperature is 25oC instead of 20 oC as in NEC] and for double circuit triplex it is 479 A.
 
The considered arrangements and cable types in IEEE STD 835 it is as following:IEEE 835 Explanations 01.jpg
 
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