Cable Sizing using Direct Buried and Ducts

[PRincez]

I'm working on the above subject and i am wondering how I can apply the two methods during cable sizing activities.

The load, 200A rated, is 350m away from the source with 2 cores per phase (VD issues). Majority of the cable runs are in an electrical cable trend; 20m approximately, runs in a road crossing PVC duct. The electrical package used for cable sizing is Amtech - there isn't an option to simulate the 2 methods, only one method cable be applied. The direct buried method yields a fairly big cable because of the distance and volts drop consideration however using the duct method gives even a far bigger cable. Since the cable is only in the duct for 15m, i am inclined to waive the 15m implication on the cable itself - the heat generated in the duct I reckon will even out and there shouldn't be trips from thermal overload of the cables.

Just want anyone's thoughts on this.

Thanks

 

[7anoter4]

It seems to me Amtech is based on BS7671.However; you have to consider the allowed minimum current on entire run. The duct run of more than 3 m has to be considered [See NEC Art.310.15(A)(2)Exception].In my opinion the permissible current in the duct run it is the minimum.
The conductor cross section area has to be calculated to withstand the short circuit current too.

 

[ScottyUK]

Amtech, for all its quirks, cryptic errors, and general awkwardness, does usually give a solution compliant with BS 7671. A 15m run in a duct is too long to ignore, so if you intend to comply with BS 7671 then install the larger cable. If volt-drop is the dominant problem then I'm surprised you're seeing thermal limitations on a cable large enough to meet the volt-drop requirements.

You can tell the software that you have three sections of cable with two intermediate junction boxes and that will allow you to input the different installation methods, but you'll have to do some manual adjustment to maintain all sections of cable at the same size. I doubt you'll see a different outcome.

 

[ppedUK]

I've been through this same scenario hundreds of times with different clients who always want to save money and reduce the cable size, particularly on large transformer LV tails.

You have to rate the cable for the worst case installation conditions. I have often heard the saying that if the duct run is less than 10M, then it can be ignored, but I have yet to see any such get-out clause in BS7671, nor any of it's previous incarnations.

I can tell you from real world experience that a cable usually gets much hotter within around 1M of it entering a duct, even if the duct is not sealed.

Also, don't forget to apply de-rating for grouping of cables in adjacent ducts, you rarely see one cable in one duct. The major cost of running cables underground is for the excavation works, and more often than not, additional ducts are installed for "future" use.