31.12.
03 0 Issued for approval GH HVB
DATE REV. NO. DESCRIPTION CHECKED APPROVED
REVISIONS
TOTAL NO. OF PAGES 7
LARSEN & TOUBRO LIMITED NAME SIGN DATE
ECC Division DSGN AKR 09.12.03
CHKD GH 23.12.03
APPD HVB 23.12.03
REV.
DOC. No. O 3 0 5 0 - E - C P - E E - D C - 0 0 0 6
0
RELEASED FOR PRELIMINARY TENDER INFORMATION APPROVAL CONSTRUCTION
Scale: Country: Site:
BANGLADESH CHHATAK - NOARAI
Size: Project :
A4 SURMA CEMENT PLANT
Title:
TYPICAL LV CABLE SIZING CALCULATION
Contract No: SUR500-2
This drawing and the information contained
herein are the property of LARSEN &
TOUBRO LIMITED.This shall not be
SURMA CEMENT
disclosed reproduced or used in any manner
without the prior written consent of LARSEN
& TOUBRO LIMITED. Document / Drawing No. Rev.
SUR.830.U45 0
LARSEN & TOUBRO LIMITED
ECC Division – EDRC
DOCUMENT NO. DATE
PROJECT SURMA CEMENT PLANT
SUR.830.U45 31.12.03
DESIGNED CHECKED PAGE
TITLE TYPICAL LV CABLE SIZING CALCULATION
AKR GH 2 of 7 -
TYPICAL LV CABLE SIZING CALCULATIONS
Input
Cable From : Cement Mill-1 MCC (400SU01)
Cable To : VCS of Mill fan [416FA07]
Rating of the Motor : 220 kW
Rated Voltage : 0.4 kV
Running pf (cosQ1) : 0.8
Starting pf (cosQ2) : 0.3
Efficiency of the motor : 92%
Total route length (L) : 0.125 km
System Neutral Earthing : Solidly earthed
Type of cable selected : 0.65 / 1.1kV (E), Al., Armoured, XLPE
Full load current of the feeder (If) : (220/(Sqrt(3)*0.4*0.8*0.92))
: 431.46 Amps
Sizing Criteria
Cable seclected should
- Thermally strong to withstand the full load current
- Mechanically strong to withstand shortcircuit forces
- Electrically stable to limit voltage drop within spec. requirement
(i) Sizing the cable for thermal ampacity
Conditions of laying : Laid on trays
Rating factor adopted (F) : 0.612
Size of cable selected : 3Cx120
Continuous rating of the cable (Ic) : 257 Amps (Ref: Table-14 Unistar)
Derated capacity of the cable (I) = Ic x F
= 157.28 Amps
No. of runs required = If / I
= 2.74
No. of runs selected = 3.00
LARSEN & TOUBRO LIMITED
ECC Division – EDRC
DOCUMENT NO. DATE
PROJECT SURMA CEMENT PLANT
SUR.830.U45 31.12.03
DESIGNED CHECKED PAGE
TITLE TYPICAL LV CABLE SIZING CALCULATION
AKR GH 3 of 7 -
(ii) Check for Short circuit withstand level of the cable
Actual source fault level : 50 kA
Source reactance (Xso) converted to
system voltage = kV2/MVAsc
= (0.4^2)/(sqrt(3)*0.4*50)
= 0.005 ohm
Resistance of the cable (at 20 deg) = 0.253 ohm / km (Ref: Table-1 Unistar)
Resistance of the cable (at 90 deg) = 0.3238 ohm / km
Reactance of the cable = 0.071 ohm / km (Ref: Table-2 Unistar)
Net Resistance of the cable (R) = ((0.32384*0.125)/3)
= 0.0135 ohm
Net Reactance of the cable (X) = ((0.071*0.125)/3)
0.003 ohm
Fault level at the load end of the cable = (0.4/(sqrt(3)*(R+j(Xso+X))))
= 14.9 kA
Cable short circuit withstand time = 0.5 sec
Short circuit withstand capacity of 1
run of cable = 11.3 kA for 1 sec. (Ref: Table-16 Unistar)
= 15.98 kA for 0.5 sec.
Hence for 3 runs of cable = 47.94 kA for 0.5 sec.
Hence the selected cable (3 Runs of 3Cx120 sq.mm. Al. XLPE )is suitable for withstanding a 3 phase bolted
fault at the load end.
(iii) Check for steady state voltage drop in the cable
For checking the steady state voltage drop in the cable, the presence of tap changer in the dist. trafo is
considered. Hence, during steady state condition, the tap changer shall nullify the voltage drops induced
due to source impedance and transformer regulations. Hence, the rated voltage of 400V is considered to be
applied in the sending end of the cable under steady state conditions.
The values of R & X are respectively the net resistance and reactance of the cables.
Steady state voltage drop = If [RcosQ1+ XSinQ1]
= 5.42 Volts
% Voltage Drop = 5.42 /(0.4*1000/sqrt(3))
= 2.35%
The voltage drop allowable as per specification is 5% and hence within limits. Hence the proposed cable (3
Runs of 3Cx120 sq.mm. Al. XLPE )is sufficient.
LARSEN & TOUBRO LIMITED
ECC Division – EDRC
DOCUMENT NO. DATE
PROJECT SURMA CEMENT PLANT
SUR.830.U45 31.12.03
DESIGNED CHECKED PAGE
TITLE TYPICAL LV CABLE SIZING CALCULATION
AKR GH 4 of 7 -
(iv) Check for voltage drop during starting of the motor
Since the transformer has been provided with off-circuit tap changer, it is assumed that the voltage drop
occurring due to the flow of steady state current through the source reactance and the transformer is
nullified by the operation of the tap changer.
Hence, the additional voltage drops introduced due to the flow of starting currents of large induction motor
is calculated both at the terminal of the motor and at the 400V bus to which the motor is connected
Symmetrical source fault level = 50 kA
Reactance of Source fault Xso = kV2/MVAsc
= (0.4^2)/(sqrt(3)*0.4*50)
= 0.005 ohms
Starting current Is = 3*If
= 1294.37 Amps
Max base load before starting = 1600*0.8*0.9-220 kW
= 932 kW
Rated current for base load In1 = 1681.53 Amps
Prestart Voltage V1 = 400/1.732
= 230.9468822 Volts
V1² = (Vms*cosQ2+Is*R)²+(Vms*sinQ2+Is*(X+Xso)+In1*Xso*(V1/Vbs-1))²
Vms = V1/sqrt((cosQ2+Is/V1*R)²+(sinQ2+Is/V1*(X+Xso)+In1*Xso/Vms*(V1/Vbs-1))²
Voltage at the motor terminal Vms = 216.5063976 ˜ [Vms=0.9V1 & Vbs=0.95V1]
% Voltage at the motor terminal Vms = 93.75%
% Drop at motor terminal = 6.25%
Vbs = Vms*sqrt((cosQ2+Is*R)²+(sinQ2+Is*X)²)
Voltage at the bus Vbs = 225.3125719
% Voltage at the bus Vbs = 97.6%
% Drop at the bus = 2.4%
Hence the voltage drop is within 10% at the motor terminal and 5% in switchgear in line with the
specification
Hence, the cable selected (3 Runs of 3Cx120 sq.mm. Al. XLPE) shall safely withstand the effects of short
circuit and shall carry the FLC within the permitted temperature limits and voltage drops.
LARSEN & TOUBRO LIMITED
ECC Division – EDRC
DOCUMENT NO. DATE
PROJECT SURMA CEMENT PLANT
SUR.830.U45 31.12.03
DESIGNED CHECKED PAGE
TITLE TYPICAL LV CABLE SIZING CALCULATION
AKR GH 5 of 7
INDICATIVE CABLE SIZES FOR TYPICAL LV LOADS
Running PF = 0.8 Source Fault Current = 50 kA
Efficiency = 0.9 Starting PF= 0.3
TOTAL No. OF RUNS
ROUTE LENGTH (M)
CONNTD. LOAD IN
cable from VCS to
Size of FlexibleCu
Conductor Type
VOLTAGE (V)/PH
MAX. DEMAND
CABLE SIZE
SL. No
Motor
(KVA)
From
kW
TO
1 MCC Motor VCS 231 0.37 0.51 100 1 3Cx2.5 Cu 3Cx2.5
2 MCC Motor VCS 231 1.1 1.53 100 1 3Cx2.5 Cu 3Cx2.5
3 MCC Motor VCS 231 2.2 3.06 100 1 3Cx2.5 Cu 3Cx2.5
4 MCC Motor VCS 231 3.7 5.14 100 1 3Cx2.5 Cu 3Cx2.5
5 MCC Motor VCS 231 5.5 7.64 100 1 3Cx6 Cu 3Cx6
6 MCC Motor VCS 231 7.5 10.42 100 1 3Cx6 Cu 3Cx6
7 MCC Motor VCS 231 11 15.28 100 1 3Cx10 Cu 3Cx10
8 MCC Motor VCS 231 15 20.83 100 1 3Cx10 Cu 3Cx10
9 MCC Motor VCS 231 18.5 25.69 100 1 3Cx16 Cu 3Cx16
10 MCC Motor VCS 231 22 30.56 100 1 3Cx16 Cu 3Cx16
11 MCC Motor VCS 231 30 41.67 100 1 3Cx50 Al 3Cx25
12 MCC Motor VCS 231 37 51.39 100 1 3Cx50 Al 3Cx35
13 MCC Motor VCS 231 45 62.50 100 1 3Cx70 Al 3Cx50
14 MCC Motor VCS 231 55 76.39 100 1 3Cx95 Al 3Cx70
15 MCC Motor VCS 231 75 104.17 100 1 3Cx120 Al 3Cx95
16 MCC Motor VCS 231 90 125.00 100 1 3Cx185 Al 3Cx120
17 MCC Motor VCS 231 110 152.78 100 2 3Cx95 Al 3Cx150
18 MCC Motor VCS 231 132 183.33 100 2 3Cx95 Al 2# 3Cx70
19 MCC Motor VCS 231 160 222.22 100 2 3Cx150 Al 2# 3Cx95
20 MCC Motor VCS 231 200 277.78 100 2 3Cx185 Al 2# 3Cx150
21 MCC Motor VCS 231 220 305.56 100 3 3Cx120 Al 2# 3Cx150
22 MCC Motor VCS 231 225 312.50 100 3 3Cx120 Al 2# 3Cx185
