Cable Ampacity Calcs. 1

[RIA123]

Hi Guys
I am calculating Cable Ampacity of 8x3C-750 MCM cables that run in free air for around 20 feet and then enter into a trench that is around 19 feet long. One end of the trench is open and the second end opens up in a cable pit. Cables occupy less than 12% of the trench area - so around 88% of the trench area is free for air. I am using EDSA software and it does have the capability to model a trench.
Can I assume the cables in trench to be in free air and calculate the ampacity ( considering the small length of trench with large free space for air circulation)?

Regards

RIA123

 

[jghrist]

NEC 310.15(A)(2) Exception states that "Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less."

The reasoning is that there will be longitudinal heat flow from the low ampacity section to the high ampacity section.

This exception doesn't appear to apply in your case, however, with 19' (approximately 50%) in the lower ampacity trench. Since you have the software to model the trench, why not use it?

 

[RIA123]

Thanks jghrist
My software has the capability to model cables in ductbanks,Direct buried cables, cables in air etc. but does not have the capability to model trench - so I have to make assumptions based on the actual conditions.
In this case - can the cables be assumed to be in free air as there is plenty of space for free air?

Regards

RIA123

 

[gare]

Just curious. Does "free air" imply there is air around the entire cable or set of cables and does free air imply there is some air movement? I find no definition of free air in the NEC.

 

[7anoter4]

If the cables are supported in air above the trench floor at least 2 diameters clearance between each other and the trench is well ventilated you can consider the same ampacity.
If the trench is filled with sand or soil you have to take it as buried cables.

 

[RIA123]

Thanks 7anoter4
Can I get any reference to support it?

Regards

 

[Electic]

I do not have a reference for this, but a very long time ago I read that wireway will large diameter cables randomly placed, is one location where actual ampacity might be lesser than NEC rated ampacity.

 

[7anoter4]

If I understood well: the cable is a single conductor 1*750 MCM [may be 600V rated?].There are 8*3 cable running 20 ft[6 m] in open free-air [may be supported on ladder tray?] and continue in trench for another 19 ft[6 m]. The total cross section of all cables together is 12% of the trench cross section area.
The overall diameter of this single conductor cable is 1.2 inch [30.5 mm].The cross section area of all the cables is then 27.2 sqr.inch and then the trench cross section will be 27.2/.12=226 sqr.inch.
If you'll follow the NEC art 318-10- even if it will be "ventilated" - the width of the trench has to be 30 inches minimum and then the height of the trench will be 226/30=7.5 inches.
In these conditions you have to derate the ampacity up to 70% of free-air ampacity as for a non-ventilated tray. That means 885*.7=619.5 A.
I don't think that 885 A as shown in Table 310-17 for 90oC insulation is stated for 3 cables in trefoil arrangement. It is close to VDE 0298 for flat arrangement with one diameter clearance between.
In order to preserve the same ampacity the height of the trench has to be at least 30 inches.
The cables will be laid in 3 stacked layers [ladders] with 8" vertical distance between them.
The clearance between adjacent cables is 4*diameter [trefoil arrangement].
The required width will be 30 inches, also.
These are requirements of the German Standard DIN 57298 Part 2 [VDE 0298 Part2] Tabelle 22 "Arrangements requiring no reduction ".
In these conditions you could limit the air temperature of the trench interior at not more than 5oC above the ambient then no correction nor for group neither for temperature would be required.
Ampacity of 3 single copper conductor cables in trefoil arrangement , 400 sqr.mm 06/1 kV- as per VDE 0298-in 30oC ambient air is 816.Two correction are required : one for overall dia[for 400 sqr.mm =35.5 mm] and one for conductor cross section:750 MCM copper is only 750*.5067=380 sqr.mm.
The reducing factor is then k=sqrt(30.5/35.5)*380/400=0.88
Then the actual ampacity for 750 MCM is 816*0.88=718 A.
See the attached sketch from the VDE 0298 Standard: