An example how to calculate voltage drop and size of electrical cable

Input information //

Electrical details:

Electrical load of 80KW, distance between source and load is 200 meters, system voltage 415V three phase, power factor is 0.8, permissible voltage drop is  5%, demand factor is 1.

Cable laying detail:

Cable is directed buried in ground in trench at the depth of 1 meter. Ground temperature is approximate 35 Deg. Number of cable per trench is 1. Number of run of cable is 1 run.

Soil details:

Thermal resistivity of soil is not known. Nature of soil is damp soil.

Ok, let’s dive into calculations…

• Consumed Load = Total Load · Demand Factor:
  Consumed Load in KW = 80 · 1 = 80 KW
• Consumed Load in KVA = KW/P.F.:
  Consumed Load in KVA = 80/0.8 = 100 KVA
• Full Load Current = (KVA · 1000) / (1.732 · Voltage):
  Full Load Current = (100 · 1000) / (1.732 · 415) = 139 Amp.

Calculating Correction Factor of Cable from following data:

Temperature Correction Factor (K1) When Cable is in the Air

Temperature Correction Factor in Air: K1
Ambient Temperature	Insulation
	PVC	XLPE/EPR
10	1.22	1.15
15	1.17	1.12
20	1.12	1.08
25	1.06	1.04
35	0.94	0.96
40	0.87	0.91
45	0.79	0.87
50	0.71	0.82
55	0.61	0.76
60	0.5	0.71
65	0	0.65
70	0	0.58
75	0	0.5
80	0	0.41

Ground Temperature Correction Factor (K2)

Ground Temperature Correction Factor: K2
Ground Temperature	Insulation
	PVC	XLPE/EPR
10	1.1	1.07
15	1.05	1.04
20	0.95	0.96
25	0.89	0.93
35	0.77	0.89
40	0.71	0.85
45	0.63	0.8
50	0.55	0.76
55	0.45	0.71
60	0	0.65
65	0	0.6
70	0	0.53
75	0	0.46
80	0	0.38

Thermal Resistance Correction Factor (K4) for Soil (When Thermal Resistance of Soil is known)

Soil Thermal Resistivity: 2.5 KM/W
Resistivity	K3
1	1.18
1.5	1.1
2	1.05
2.5	1
3	0.96

Soil Correction Factor (K4) of Soil (When Thermal Resistance of Soil is not known)

Nature of Soil	K3
Very Wet Soil	1.21
Wet Soil	1.13
Damp Soil	1.05
Dry Soil	1
Very Dry Soil	0.86

Cable Depth Correction Factor (K5)

Laying Depth (Meter)	Rating Factor
0.5	1.1
0.7	1.05
0.9	1.01
1	1
1.2	0.98
1.5	0.96

Cable Distance correction Factor (K6)

No of Circuit	Nil	Cable diameter	0.125m	0.25m	0.5m
1	1	1	1	1	1
2	0.75	0.8	0.85	0.9	0.9
3	0.65	0.7	0.75	0.8	0.85
4	0.6	0.6	0.7	0.75	0.8
5	0.55	0.55	0.65	0.7	0.8
6	0.5	0.55	0.6	0.7	0.8

Cable Grouping Factor (No of Tray Factor) (K7)

No of Cable/Tray	1	2	3	4	6	8
1	1	1	1	1	1	1
2	0.84	0.8	0.78	0.77	0.76	0.75
3	0.8	0.76	0.74	0.73	0.72	0.71
4	0.78	0.74	0.72	0.71	0.7	0.69
5	0.77	0.73	0.7	0.69	0.68	0.67
6	0.75	0.71	0.7	0.68	0.68	0.66
7	0.74	0.69	0.675	0.66	0.66	0.64
8	0.73	0.69	0.68	0.67	0.66	0.64

According to above detail correction factors:
– Ground temperature correction factor (K2) = 0.89
– Soil correction factor (K4) = 1.05
– Cable depth correction factor (K5) = 1.0
– Cable distance correction factor (K6) = 1.0
Total derating factor = k1 · k2 · k3 · K4 · K5 · K6 · K7
– Total derating factor = 0.93

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