Sizing duirect buried cables

[Colinjohnson]

Gentlemen,

How would you size this particular cable ?

Service - 400V AC 50HZ TPN
Load 1666A
Power factor .91
Cable:- Single core XLPE AWA armour to BS5467 laid up in trefoil.
Installation - Direct buried in soil at 500mm below surface groups spaced 600mm apart

I have referred this to Pirelli who advise three parallel 400mm sq cables will suffice. However running the calculation on two software packages gives the answer as three 630mm sq cables or three 1000mm sq cables per phase !!

I am sure three 400mm sq would do the job but can I prove it ?

Your comments much appreciated.

CJ

 

[alehman]

In the U.S., the National Electrical Code provides tables for sizing conductors. If the the load is non-continuous and the insulation is rated 75C, you would need 3 at 1250kcmil (630 sq mm). For continuous load, the NEC requires 125% ampacity however, which would be 4 at 900kcmil (450 sq mm).

The NEC also permits sizing by engineering anaylsis as you are attempting. These methods depend on the soil conditions, thermal resistivity, spacing of the cables, type of insulation, etc. As you see, there is no "one" answer.