The low voltage or low tension motor is called LT motor. the LT motors mostly operated between the voltage ranges from 240v to 415v .
LT motors mostly having high current and low voltage. moreover we can say up to 1KV motors are LT motors.
The high voltage or high tension motor is called HT motor. the HT motors mostly operated between the voltages range from 3.3KV to 11KV. HT motors mostly having low current and high voltages. moreover we can say above 1KV motors are HT motors.
How to calculate cable size of LT motor:
Now we are going to calculate cable size of LT motor with examples, here we will consider 100kw LT motors and motor having efficiency 94%, power factor 0.8 , system terminal voltage 415v and cable length is 200m. so we will find cable size for this LT motor.
Cable Size Calculation of 100 KW LT Motor
generally we know power formula P= √3 x V x I CosΦ
let I = P / √3 V x CosΦ
we know √3= 1.732, CosΦ = power factor = 0.8
and voltage v=415v so apply these values into above equation
~ 185 A
This is full load current for LT motor in normal condition. but in real condition we need to consider some other factors to size the cable.
let us consider normal ambient temperature is 40* C. in case the ambient temperature is increase then current carrying capacity of the cable is de rated. that could be understand by below de rating table
Suppose we are run the cable in Air laid on cable tray,
|Air Temperature in Deg.||20°||25°||30°||35°||40°||45°||50°||55°|
|De-Rating factors||HR PVC||1.22||1.17||1.12||1.06||1.00||0.94||0.87||0.80|
Rating factors related to variation in ambient air temperature
Normally cable de rating factor formula is
K1= Temperature Correction Factor
K2= Cable Grouping Factor
Temperature Correction Factor, K1 when cable is in the Air = 0.88 (for 50* Amb temp & XLPE cable)
Grouping factor of the cable also de rate the current carrying capacity of the cable. if more than 3 cables run together in a single tray. then it will be produce heat it self, and this will increase the temperature of the cable. then current carrying capacity of the cable is de rated.
lets consider some worst cases , i.e. 3 trays parallel to each other having 9 cables each touching to each other.
|No. of racks||No. of||cables per rack||No. of cables per rack|
Cable Grouping Factor (No of Tray Factor), K2 = 0.68 (for 3 trays having 9 cable each)
Total de-rating factor = K1 x K2
= 0.88×0.68 = 0.5984
Let’s consider 3 core, 185 Sq.mm, Aluminum, XLPE, Armored cable for single run
Click to enlarge the table
let see that details for 3 Core, Aluminum/Copper Conductor, XLPE Insulated, Armored Cables
Current capacity of 185 Sq.mm XLPE Armored aluminum cable in Air is 339 Amp
Total de-rating current of 185 Sq.mm Cable = 339×0.5984 = 202.85 Amp
Resistance = 0.211 Ω/Km and
Reactance = 0.072 Ω /Km
|Estimated Voltage Drops in PVC/XLPE Aluminum Cables For A.C. System|
|(Voltage drop – Volts/Km/Amps)|
|Nominal area of conductor (sq. mm)||P.V.C. Cable||XLPE Cable|
|Single Phase||Three Phase||SinglePhase||Three System|
from above table we consider Voltage drop, for 3 phase 185sqmm XLPE cable is 0.40
voltage drop V2 = 0.4 Volts/Km/Amp
= 17.3 V
Terminal voltage at Motor, V2 = 415-13 = 402 V
% Drop = (V2 – V1)/(V1)
= (415 – 402)x100/(415)
To decide 185 Sq.mm cable, cable selection should be satisfied following condition
- Cable derating Amp (202.85 Amp)is higher than full load current of load (185 Amp) = OK
- Cable voltage Drop (4.18%)is less than defined voltage drop (10%) = OK
- Cable short circuit capacity (17.39 KA) is higher than system short circuit capacity at that point ( X KA) = OK
So 100kw LT motor for cable 200m length required 185sqmm XLPE cable.