How to calculate cable size for LT Motors

What is the difference between LT and HT Motors?

Well, as the word LT (Low Tension i.e. Low Voltage) and HT (High Tension i.e. High Voltage) or low torque and high torque respectively describe the whole story itself.

its also depends on the availability of supply voltage, i.e. in USA and EU,

LT Motors ranges = 230V-415V

while keep in mind that LT motors need more current than HT motors.

in other regions, they classified LT Motor under 1kV & HT Motor Over 1kV.

Now we have to discuss the main topic that’s how to calculate the cable size for motors?

Cable Size Calculation of 125 KW LT Motor

Motor KW = 125

Pf = 0.8, Efficiency = 94%

System Voltage, V1 = 415

Cable length  = 200 m

Load Current = P/(1.732xVxPfxEff)       —>     (P = 3 x Vx I CosΦ = for three phase circuits)

= 125000/(1.732x415x0.8×0.94)

~ 230 A

This is the full load current cable needs to cater in Ideal condition. But in practical situation, there are several de-rating factors which need to be considered.

The current rating given for the cables is defined for ambient temp of 40* C. If the ambient temp is greater than that, cable current carrying capacity de-rates.

Suppose our cable is in Air laid on cable tray,

Air Temperature in Deg. 20° 25° 30° 35° 40° 45° 50° 55°
Normal PVC 1.32 1.25 1.16 1.09 1.00 0.90 0.80 0.80
De-Rating factors HR PVC 1.22 1.17 1.12 1.06 1.00 0.94 0.87 0.80
XLPE 1.20 1.16 1.11 1.06 1.00 0.95 0.88 0.82

Rating factors related to variation in ambient air temperature

Temperature Correction Factor, K1 when cable is in the Air = 0.88 (for 50* Amb temp & XLPE cable)

Grouping of cables also de-rates cable’s current carrying capacity. If many cables are grouped together, they will all heat up. The heat won’t be able to dissipate properly hence it will warm up the cable itself and those in its contact. This will raise the temperature further. Hence we have to de-rate the current carrying capacity of the cable according to the grouping factor.

Let’s go for worst case scenario, i.e. 3 trays parallel to each other having 9 cables each touching to each other.Grouping factor for cable size for motor

No. of racks No. of cables per rack No. of cables per rack
1 2 3 6 9 1 2 3 6 9
1 1.00 0.98 0.96 0.93 0.92 1.00        0.84 0.80 0.75 0.73
2 1.00 0.95 0.93 0.90 0.89 1.00        0.80 0.76 0.71 0.69
3 1.00 0.94 0.92 0.89 0.88 1.00        0.78 0.74 0.70 0.68
6 1.00 0.93 0.90 0.87 0.86 1.00        0.76 0.72 0.65 0.66

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 select 1.1 KV, 3 core, 240 Sq.mm, Aluminum, XLPE, Armored cable for single run

Click to enlarge the table

Cable Size Calculation of 125 KW LT Motor table chart

Technical Details For 1.1 KV, 3 Core, Aluminum/Copper Conductor, XLPE Insulated, Armored Cables

Current capacity of 240 Sq.mm XLPE Armored aluminum cable in Air is 402 Amp

Total de-                                                                                               rating current of 240 Sq.mm Cable = 402×0.5984 = 240.55 Amp

Resistance = 0.162 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
1.5 43.44 37.62 46.34 40.13
2.5 29.04 25.15 30.98 26.83
4 17.78 15.40 18.98 16.44
6 11.06 9.58 11.80 10.22
10 7.40 6.41 7.88 6.82
16 4.58 3.97 4.90 4.24
25 2.89 2.50 3.08 2.67
35 2.10 1.80 2.23 1.94
50 1.55 1.30 1.65 1.44
70 1.10 0.94 1.15 1.00
95 0.79 0.68 0.83 0.70
120 0.63 0.55 0.66 0.56
150 0.52 0.46 0.55 0.48
185 0.42 0.37 0.44 0.40
240 0.34 0.30 0.35 0.30
300 0.28 0.26 0.30 0.26
400 0.24 0.22 0.24 0.22
500 0.23 0.20 0.23 0.20
630 0.20 0.18 0.21 0.18
800 0.19 0.20
1000 0.18 0.18

Voltage drop, V2 = 0.3 Volts/Km/Amp    (as per Havell’s brochure)

= 0.3x217x200/1000

= 13 V

Terminal voltage at Motor, V2 = 415-13 = 402 V

% Drop = (V2 – V1)/(V1)

= (415 – 402)x100/(415)

= 3.13%

To decide 240 Sq.mm cable, cable selection condition should be checked

  1. Cable derating Amp (240.55 Amp)is higher than full load current of load (230 Amp) = OK
  2. Cable voltage Drop (3.13%)is less than defined voltage drop (10%) = OK
  3. Cable short circuit capacity (22.56 KA) is higher than system short circuit capacity at that point ( X KA) = OK

240 Sq.mm cable satisfied all three condition, so it is advisable to use 3 Core 240 Sq.mm cable.

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