myectricalscience,AC Lv cable sizing for ACDB equipments electrical cable sizing,lv cable sizing calculation--cable engineering sizing calculation-myelectricalscience.

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 condition

1. 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.

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

 eg:cable manufacturer table list shown.

 

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 \ 0.647

               

            Uprated load current    =139.14 A  

With the help cable manufacturer data we have to check the Ampacity.

Ampacity, should higher than uprated load current.

xlpe,cu,unarmoured cable

 

Cross section sqmm	@flat condition Cur-1-tr-flat condition (Ampacity) flow	Cur-1@Pipe Ampsflow (ampacity)	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-3

VOLTAGE 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=√3IZ

         I=full load current

         Z=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 meter

Rac =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 0.386

                  If you have any doubt please ask in comment section I am ready to clear your doubt