Cable Calculation

Dear Friend,

Your general and wide question will have a general and wide answer. Please explain the matter specifically. What is the voltage level? What is load parameters? What is the cable laying type, ambient temperature, grouping factor? Which standard and code is applicable for the project? What is the cable insulation, EPR, PVC or XLPE? and ....

ok

take the data

3.3 kv votage level

40 KA for .16 sec short crcuit current level

500 kw,860 kw motor ,derating factor take 0.69

cable shuld be AL XLPE

LENGHT IS 300 METER

POWER FACTOR .85 EFFI. .85

CABLE SHULD BE OF THREE CORE

PLS SHOW ME THE FULL PROCEDURE OF CALCULATION SO I CAN DO FOR OTHER MOTORS

The cable sizing has the following sequence.

This calculation is only for reference, you should study related national codes and standards and also refer to cable manufacturer technical catalogues.

1-Calculation of Load current:

In=P/(√3*VL*PF*Eff.)=860/(1.732*3.3*0.85*0.85)=208A (for 860kW motor)

2- Short circuit withstand current of cable (IEC Standard):

Min. cross sectional area for Aluminium conductor, XPLE insulated cable

Smin=Icc*(√t)/k= 40000*√(0.16)/94=170.2 mm²

You should also check the shield and armour current capacity for earth fault currents if applicable

3- Permissible voltage Drop value (Operation: <5% and Starting <15%)

ΔV%=√3*In*[(R*Cos(φ) +X*Sin(φ)]*100/VL=1.732*208[R*0.85+X*0.53]*100/3300 R and X shall be extracted from MFR's technical catalogue

3- Current Carrying capacity of cable by considering all de rating factors such as ambient and grouping factors shall be > 208A

THANKS UPTO THAT. BUT

HOW WILL U SELECT R AND X VALUE .(WHAT BASIS FOR SLECTING R AND X)

You don't select R and X. R is the active resistance and X is the inductive Resistance of selected cable. The value of R and X is to be extracted from cable MFR's technical catalogue or related standards.

For example: Manufacturer: EUPEN, 3-core, Al/XLPE, 240mm²

the active resistance(R) at 90ºC is 0.166 Ω/km and inductive resistance(X) is 0.105 Ω/km and the short circuit withstand current for 0.2 sec. is 50kA.

The biggest concern in MV cable sizingis always short circuit. Hence you must have sufficient knowledge of the protection device above the cable. If it is a Breaker, you need to check how many cycles are required for tripping on a fault. Then leave sufficient margin and then calculate the cable size. If it is a Fuse, check the characteristics; hopefully the fuse will operate in less than half a cycle, which will drastically lower the cable size. Next comes ampacity. Voltage drop is never an issue in MV\HV where currents are low and cable sizes are high. Hence, strictly, the order for MV cable sizing is :

Short circuit...........AMpacity...........voltage drop

ok how we use derating current in the cable selection