LV Cable sizing (1)

MAIN PUMPING STATION - LV POWER CABLE SIZING CALCULATION

3

3.0 NOTES ON MAIN LVAC CABLE SIZING CALCULATION:(FROM FEEDER PILLER TO LVACDB)

3.1 Considerations made on Main LVAC Cable Sizing Calculation

a. Cable Parameters are based on M/s Oman Cable Company Catalogue (Relevant sheets are attached)

b. LV Main cables are of 1.1kV grade cables with Copper Conductor, XLPE insulated, PVC Sheathed,

c. All Voltage drops are checked with permissible limits of 2.5% as per Distribution company standard

3.0 Derating Factors for the LVAC Cable Sizing from the Transformer feeder piller to the 415V ACDB

Main Incomer cables from 1000 kVA F.P to MDB panel is proposed is as mentioned below:

2R x 1C x 630 Sq. mm Cu. XLPE insulated armoured cables for Phases and

1R x 1C x 630 Sq. mm Cu. XLPE insulated armoured cables for Neutral.

4.1 Temperature factor

i Cables (or part of route) running in enclosed trenches entering control building:

Table no. 67 of Oman Cable catalogues is referred

Similar to above philosophy, in this part of the cable laying, the overall temperature derating factor is 0.9

4.2 Group Rating factor

Group rating factors are applicable for more than one trefoils or more than 3 cable together.

Table no. 66 of Oman Cable catalogues is referred.

Hence, in this case, for entire route length of the cable group rating factor is = 1

4.3 Thermal Resitivity factor

This factor is applicable in case of cables directly buried

While in this case of cable, the same is not applicable as cables are placed in trench (air with restricted

circulation. Hence, the same is taken as = 1


4.4 Depth of laying

This factor is applicable in case of cables directly buried or placed in duct bank.


= 0.952

4.5 Overall Derating Factor

Considering the above, overall derating factor = Temperature Derating Factor x Group Derating

Factor x Termal Resistivity Derating Factor x

Derating Factor due to depth of Laying

= 0.8568

armoured type. (Ref.table #59 of Oman Cable Copmany catalogue Sheet. No.69)


4

5.0 Sample calculation

5.1 Cable sizing calculation for incomer cable of Main DB incomer

a Ampacity calculation :

Maximum demand load (Maximum con.load) = 709 kVA (Actual load 567.5 KW pf-0.8)Hence, FLC for this load = 709/(1.732*0.433)

= 945.8888 Amp

b Ampacity calculation :

Group derating factor (as explained in 4.2 above) = 1

Temp derating factor (as explained in 4.1 above) = 0.9

Thermal resistivity factor (as explained above) = 1

And, depth of laying factor = 0.952

Hence, overall derating factor is = 0.8568

Uprated current = FLC/Overall derating factor

= 945.888825477761/0.8568

= 1103.979 A

Cable size selected = 4C x 240 sqmm

Ampacity rating = 380 (from table-59 for armoured cable)

Hence, required number of runs = Upreated current / Ampacity of each cable

= 1104/380

= 2.905207

Hence,1 run of 4C x 240 sqmm XLPE/Cu Cable per phase is selected.

c Voltage drop calculation :

=

Where, I = Load current for each cable (A)

L = Total Length of cable (M) = 300 m

r = Cable Resistance at 90°C = 0.0989

R = r . L

= 0.0989*300/10^3

= 0.02967

x = Cable Reactance = 0.0710

X = x . L

= 0.071*300/10^3

= 0.0213

= 0.8

= 0.6

N = 8 Number of runs

Values of r and x are taken from cable catalogue attached with this document.

And 'L' is taken as 250 m for cable length in trench from TX-1 to Main ACDB.

Hence, = 1.732 x 945.9 x [( 0.0989 x (300/1000) x 0.8 + 0.071 x (300/1000) x 0.6)] / 2

= 7.478 Volts ……………….….Voltage drop

And hence, cumulative %ge voltage drop = 100 * 16.141 / 433

= 1.73 %

Since voltage drop is within the limits of 2.5%, the cable size of 2 run per phase of 4Cx240 sq.mm

and 1 run of 1Cx630 sq.mm for neutral selected is adequate.

Vdrop √ 3 x I x ( R cos Ø + X sin Ø) / N

Ω/km (From cable catalogue)

Ω/km (From cable catalogue)

cos Ø

sin Ø

Vdrop1

H74

Rathinam Dhinesh: For data refer catalogue

I85

Rathinam Dhinesh: Actual lenth

I86


I90


E96

Rathinam Dhinesh: Actual run


5

Reference Documents


6


7


8


9


10


11