Precast Trench Ampacity 1

[chuckd83]

From my research, there is no ANSI standard to determine the ampacity of cables installed in a precast trench with solid, unventilated covers. I did find that IEC 60287-2-1 Section 4.2.6 has a method, but don't have the standard available.

What do you use? I have ETAP to calculate the ampacity using the Neher-McGrath paper, but the software requires the trench be backfilled.

 

[cuky2000]

There is no known official publication by NEC, IEEE or ICEA for cable ampacity in a trench.However, there are study performed by utilities such as Commonwealth Edison Co (Comed), Oklahoma Gas & Electic (OG&E0 and others that may be used as a goo reference.
The NEC permits under engineering supervision to perform calculation or use cable model installe in a trench
The cable model should includes stand still air velocity, geometrical parameters, working conditions, ambient temperature, heat dissipation from each cable, cables' diameters, trench width and the spacing between cables.

 

[7anoter4]

There is not much in IEC 60287-2-1 in connection with unfilled trough.

 

[chuckd83]

cuky2000, the software I have (ETAP & SKM) cannot create such a model. I believe CYME does it, but it's a $30k software. I come across this maybe once a year and cannot justify the cost. I skimmed through those studies you mentioned (Trenwa sent them to me). I didn't find anything that useful in them, but will take another look.

Thanks 7anoter4. It sounds like you just increase the ambient air temperature due to the cable heating and adjust the ampacity as needed. That may be the path I take going forward and can at least cite a standard.

 

[jghrist]

Rating of Electric Power Cables, George J. Anders, IEEE Press, 1997, addresses unfilled troughs in Section 10.6.2. It references IEC 287 (1982) and suggests using an empirical formula which gives the temperature rise of the air in the trough above ambient as W[sub]t[/sub]/(3·p) where W[sub]t[/sub] is total heat dissipation and p is the part of the trough perimeter which is effective for heat dissipation. Any portion of the perimeter exposed to sunlight is not included in p.

 

[chuckd83]

jghrist, sounds like the same as IEC 60287.

I've read BS 7671 has a correction factor. Does anyone know it or is it a table? I know it only applies to LV cables.

 

[7anoter4]

I start myself with IEC 287 in 1990 when I did the first edition of a VB45 program for underground cable duct bank ampacity calculation.
From 01/01/1997 IEC 287 was changed to 60287 and dived in 4 volumes.
The new edition of 2015 contains the same theory as 287 edition only in separate volumes and the paragraph numbers are changed.
British standard BS 7671 is changed in conformity with IEC 60364-5-52.
In my opinion you are speaking about items 54-56 from the TABLE 4A2 [see attachment]