Hello,friends I am going to explain a simple steps on calculating cable size.
Cable sizing is very important for substation designing, Correct sizing of cable protect the cable from burning, short-circuit current, voltage drop and other abnormal condition.Cable sizing is measured on sq mm.Cable should statify two condition1. Ampacity(continuous current carrying capacity of conductor)2.voltage drop must be within 5%(Volt difference b\t sending end and receiving end)
Select the type of cable or cable data,
1.Conductor(cu orAl)
2.Insulator(pvc,xlpe,epr etc.)
3.No of core(SINGLE OR MULTI-CORE).
NOTE: Normally xlpe insulation is taken for modern substation. so i am selecting copper, xlpe and multi-core.Now we going to do the cable sizing calculation side, believe me this going to be easy for you.STEP-1,COLLECT LOAD DATA.EXAMPLE-Consider Heat ventilation air condition (hvac unit-1) on ACDB requires operating power of 15000w connected on 1-PHASE station service transformer(8.4kv\240v) Now through station service trafo data we came to know voltage is 240v. power factor is 0.85(active power).we all know the formula P=V*I*COS(fi)Find VA rating for load; p/cos(fi)Find full load current(I)=va/(v*qty of load)Here Qty of HVAC load is 1 so voltage is multiplied with 1 if load qty is 2 then multiply by 2.
Now find the tripping current and breaker rating ie,multiply full load current into 120%
VA rating ie, 15000/0.85
= 17647.5 VA
Full load current ie, 17647/(240*1)
= 73.5291A.
Tripping current ie,73.5291*1.2
=88.235A
Selected breaker rating is 90A.STEP-2,FIND THE TOTAL CORRECTION FACTOR OR DERATING FACTOR.Selecting "derating factor" is based on where the cable is going to install(eg. outside building or inside building,cable trench etc,conduit buried).This may be choose with help of "single line diagram" by seeing location of equipment.Now you select where the cable is going to install and check your selection on "installation method" with help of cable manufacturer data.EXAMPLE TABLE(Real manufacturer has lot of installation method)derating factor manufacturer table.eg:cable manufacturer table list shown.
case no install-method
no of run
max sq mm
no of cores
tot,derating factor 1 trench,outside building 1 upto 400 2 0.721 2 conduid buried 4 25to150 4 0.647 Total correction factor depends on soil temperature,air temperature,soil resistivity,and air resistivity. that will provide by manufacturer.Example:If I am going to select "Conduid buried" method on flat condition then cable size is upto"16sqmm" and total derating factor is 0.647.Uprated load current should be found by the formula,Uprated load current =breaker tripping current \ total correction factor= 90 \ 0.647Uprated load current =139.14 AWith the help cable manufacturer data we have to check the Ampacity.Ampacity, should higher than uprated load current.
Cur-1-tr-flat condition (Ampacity) flow Rac -90deg celc Xac-90deg celc Rdc outer dia 2C*25 118 90 0.927 0.1241 0.727 22.4 2C*35 142 107 0.669 0.1184 0.524 24.6 2C*50 175 129 0.494 0.1115 0.387 -
According to manufacturer data I've selected "2C*35mm^2"Ampacity of cable is "142 A"STEP-3VOLTAGE DROP must within 5% for total connected load.
now here only single load so below 1% is required.
For single phase Vd=2IZ
For 3Phase Vd=√3IZI=full load currentZ=Impedence (Rx*cos(fi)+Xac*sin(fi))2-phase and neutral.(1 phase)
Note cos(fi) sin(fi) 0.85 0.53
Vd=2I*L((Racc*cos(fi)+Xac*sin(fi))/(1000*no of runs)
L = length in meterRac =Resistance of selected cable.Xac =Reactance of selected cable
Vd =73.529*20(0.669*0.85+0.1184*0.53)/(1000*1)
Vd =0.928v
To find voltage drop in percentage %=(Vd / operating voltage*100)
% of voltage drop is 0.386If you have any doubt please ask in comment section I am ready to clear your doubt
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