HOW TO SELECT AN ELECTRIC CABLE
VOLTAGE AND MEDIUM VOLTAGE
A reference document to be used whenever you are required to select a Low
Voltage (less than 1000V) or a medium voltage (6,6kV- 33kV) cable. The article
discusses load carrying capacity, volt drop and fault current calculations.
R. HARDIE Pr. Eng
DIVISIONAL MANAGER - TECHNICAL
ABERDARE POWER CABLES - SELECTION OF ELECTRIC CABLE
The following variables need to be known:
• Power - This can be in kVA, kW or in Amps.
• Voltage
• Permissible voltage drop - (Usually 5%)
• Distance to load
• Fault current:
o Short circuit (Symmetrical Fault Current)
o Earth fault (Asymmetrical Fault Current)
• Mechanical Conditions:
o Temperature, depth of burial, soil thermal resistivity, presence of
other cables, or other heat sources.
o Armouring requirements.
o Sheath requirements.
A: CALCULATION OF CURRENT
Whether we are given kW, kVA or Horsepower, we need to convert this to the
required full load current. (I)
For motor starting current use: 3 x I for Star-Delta (soft start).
or 6 x I for Direct on line starting.
Select a cable based on the required maximum current.
Now check to see whether the volt drop is acceptable.
B: PERMISSIBLE VOLT DROP
Look up the mV/A/m figure - The relevant table has been reproduced later from
our Facts and Figures booklet.
Volt drop = mV/A/m x Current (Amps) x Distance (metres).
= mVolts (Convert to volts by dividing by 1000).
Usually 5% volt drop is allowed. For a 400 Volt 3 phase system, 5% is equal to
20 Volts.
EXAMPLE: Selection of low voltage cable.
100kW motor at 400 volts, cable to be installed under standard conditions in air,
over 50 meters, using a star delta starter.
Refer to the following table for cable selection:
Select a 50mm² cable and note mV/A/m figure of 0.817.
Starting current = 3 x 160 = 480 Amps.
Volt drop = 0.817 x 480 x 50 = 19 Volts (Acceptable)
CABLE SELECTION TABLE
CURRENT RATING AND VOLTAGE DROP OF 3 AND 4 CORE PVC INSULATED CABLES
WITH STRANDED COPPER CONDUCTORS
AREA mm² Z ohm/km SUSTAINED CURRENT RATING (A) VOLT DROP
mV/A/m
GROUND DUCT AIR
1,5 14,48 23 18 18 25,080
2,5 8,87 30 24 24 15,363
4 5,52 38 31 32 9,561
6 3,69 48 39 40 6,391
10 2,19 64 52 54 3,793
16 1,38 82 67 72 2,390
25 0,8749 126 101 113 1,515
35 0,6335 147 120 136 1,097
50 0,4718 176 144 167 0,817
70 0,3325 215 175 207 0,576
95 0,2460 257 210 253 0,427
120 0,2012 292 239 293 0,348
150 0,1698 328 269 336 0,294
185 0,1445 369 303 384 0,250
240 0,1220 422 348 447 0,211
300 0,1090 472 397 509 0,189
CURRENT RATING AND VOLTAGE DROP OF 3 AND 4 CORE PVC INSULATED CABLES
WITH SOLID ALUMINIUM CONDUCTORS
AREA mm² Z ohm/km SUSTAINED CURRENT RATING (A) VOLT DROP
mV/A/m
GROUND DUCT AIR
25 1,4446 98 79 88 2,502
35 1,0465 119 95 108 1,813
50 0,7749 142 113 132 1,342
70 0,5388 171 138 164 0,933
95 0,3934 204 165 201 0,681
120 0,3148 235 189 234 0,545
150 0,2607 263 213 270 0,452
185 0,2133 295 240 308 0,369
240 0,1708 340 278 362 0,296
If the distance was 200 meters, then volt drop would have been:
= 0.817 x 480 x 200 = 78 Volts. (19.5% Unacceptable)
Cable size in this case would be calculated as follows:
Cable Size/Volt Drop
70mm² Copper: 0.576 x 480A x 200m = 55 296mV = 13.8% (Too high)
95mm² Copper: 0.427 x 480A x 200m = 40 992mV = 10.2% (Too high)
120mm² Copper: 0.348 x 480A x 200m = 33 408mV = 8.4% (Too high)
150mm² Copper: 0.294 x 480A x 200m = 28 224mV = 7.0% (Too high)
185mm² Copper: 0.250 x 480A x 200m = 24 000mV = 6.0% (Too high)
240mm² Copper: 0.211 x 480A x 200m = 20 256mV = 5.0% (Just right)
At last a cable that meets the volt drop requirement.
REVERSE CALCULATION
In practice it is sometimes easier to first calculate the volt drop requirements, and
then check whether the cable is large enough.
(% Volt drop x Voltage x 1000mV)/Current x Distance = mV/A/m
Select a cable from the previous table with a volt drop mV/A/m figure just smaller
than the above calculated value. Check that the current rating of this cable is
adequate for the full load current required.
Using the previous example:
(5% x 400V x 1 000mV)/ (480A x 200m) = 0.21mV/A/m
Corresponding cable = 240mm² Copper cable
USE OF A HIGHER VOLTAGE
Problems sometimes occur when the distance is great, and the cable becomes
excessively large (and expensive) in order to comply with volt drop requirements.
Then we must consider using a higher voltage.
The advantage of a higher system voltage, for example Interdac 3 at 1.9/3.3kV is
that:
1. Current is lower for same power (smaller conductor).
2. Higher voltage drop is allowed (5% of 3.3kV = 165 Volts).
CURRENT RATING (AND DERATING) OF CABLES
The allowable current, or the current rating of an electric cable is that current
which will result in the conductor temperature rising to the maximum operating
temperature.
For XLPE this is 90°C
For PILC this is 65°C
For PVC this is 70°C
When a cable is installed in a hot environment, for example against a boiler, it
stands to reason that the cable will have a lower current rating than the same
cable installed in cooler conditions.
Brochure quoted current ratings are based on the following standard conditions:
Ground Temperature: 25°C
Ambient Temperature: 30°C
Ground Thermal resistivity (g): 1.2K.m/W
Depth of laying to top surface of cable or duct 0.8m
CURRENT RATINGS ARE BASED ON THE FOLLOWING:
PAPER INSULATED CABLES
Maximum sustained conductor temperature 70 deg C
Ground Temperature 25 deg C
Ambient air temperature (free air-shaded) 30 deg C
Ground Thermal Resistivity 1,2K.m/W
Depth of laying to top of cable or duct 800 mm
DERATING FACTORS FOR NON-STANDARD CONDITIONS
Depth of laying - multicore PILC cables (up to 300mm²)
Depth of laying (mm) Direct in ground In single way ducts
800 1,00 1,00
1100 0,98 0,99
1250 0,96 0,97
1500 0,95 0,96
2000 0,92 0,94
Ground Thermal Resistivity (Multi-core PILC cables)
Thermal Resistivity (Km/W) Direct in ground In single way ducts
1,0 1,07 1,03
1,5 0,92 0,95
2,0 0,84 0,89
2,5 0,76 0,84
Grouping of PILC cables in Horizontal Formation at standard depths of laying and in standard soil
conditions (multicore cables).
No of Direct in ground In single way ducts
cables in
group
Axial spacing (mm) Axial spacing (mm)
Touching 150 300 450 600 Touching 300 450 600
2 0,80 0,85 0,89 0,90 0,92 0,88 0,91 0,93 0,94
3 0,69 0,75 0,80 0,84 0,86 0,80 0,84 0,87 0,89
4 0,63 0,70 0,77 0,80 0,84 0,75 0,81 0,84 0,87
5 0,57 0,66 0,73 0,78 0,81 0,71 0,77 0,82 0,85
6 0,55 0,63 0,71 0,76 0,80 0,69 0,75 0,80 0,84
Ground Temperature derating factors
Maximum Conductor Ground Temperatures (deg C)
Temperature (70 deg C)
25 30 35 40 45
1,00 0,95 0,90 0,85 0,80
Air Temperature derating factors
Maximum Conductor Air Temperatures (deg C)
Temperature (70 deg C)
25 30 35 40 45
1,10 1,00 0,94 0,87 0,79
XLPE INSULATED CABLES
Maximum sustained conductor temperature 90 deg C
Ground Temperature 25 deg C
Ambient air temperature (free air-shaded) 30 deg C
Ground Thermal Resistivity 1,2K.m/W
Depth of laying to top of cable or duct 800 mm
DERATING FACTORS FOR NON-STANDARD CONDITIONS
Derating factors of laying - multicore XLPE cables (up to 300mm²)
Depth of laying (mm) Direct in ground In single way ducts
500 - 800 1,00 1,00
850 - 1000 0,97 0,96
1050 - 1200 0,95 0,95
1250 - 1400 0,93 0,95
1450 - 1600 0,92 0,94
Ground thermal resistivity - multicore XLPE cables (up to 300mm²)
Thermal Resistivity (Km/W) Direct in ground In single way ducts
0,7 1,23 1,28
1,0 1,08 1,12
1,2 1,00 1,00
1,5 0,90 0,93
2,0 0,80 0,85
2,5 0,72 0,80
3,0 0,66 0,74
Grouping of XLPE cables in horizontal formation at standard depths of laying and in standard soil
conditions (multicore cables).
No of Direct in ground In single way ducts
cables in
group
Axial spacing (mm) Axial spacing (mm)
Touching 250 700 Touching 250 700
2 0,79 0,85 0,87 0,87 0,91 0,93
3 0,69 0,75 0,79 0,80 0,86 0,91
4 0,63 0,68 0,75 0,75 0,80 0,87
5 0,58 0,64 0,72 0,72 0,78 0,86
6 0,55 0,60 0,69 0,69 0,74 0,83
Ground temperature derating factors
Maximum Conductor Ground Temperatures (deg C)
Temperature (90 deg C)
25 30 35 40 45
1,0 0,96 0,92 0,88 0,84
Air Temperature derating factors
Maximum Conductor Air Temperatures (deg C)
Temperature (90 deg C)
30 35 40 45 50
1,0 0,95 0,89 0,84 0,78
PVC INSULATED CABLES
Maximum sustained conductor temperature 70 deg C
Ground Temperature 25 deg C
Ambient air temperature (free air-shaded) 30 deg C
Ground Thermal Resistivity 1,2K.m/W
Depth of laying to top of cable or duct 500 mm
SUSTAINED CURRENT RATING FACTORS FOR NON-STANDARD
CONDITIONS
Derating factors for depth of laying - multicore cables (up to 300mm²)
Depth of laying (mm) Direct in ground In single way ducts
500 1,00 1,00
800 0,97 0,97
1000 0,95 0,96
1250 0,94 0,95
1500 0,93 0,94
2000 0,92 0,93
Derating factors for ground thermal resistivity (multicore cables)
Thermal Resistivity (Km/W) Direct in ground In single way ducts
1,00 1,08 1,04
1,5 0,93 0,96
2,0 0,83 0,88
2,5 0,78 0,87
Derating factors for grouping of cables in horizontal formation, at standard depths of laying and in
standard soil conditions. Multicore cables (up to 300mm²)
No of Direct in ground In single way ducts
cables in
group
Axial spacing (mm) Axial spacing (mm)
Touching 150 300 450 600 Touching 300 450 600
2 0,81 0,87 0,91 0,93 0,94 0,90 0,93 0,95 0,96
3 0,70 0,78 0,84 0,87 0,90 0,82 0,87 0,90 0,93
4 0,63 0,74 0,81 0,86 0,89 0,78 0,85 0,89 0,91
5 0,59 0,70 0,78 0,83 0,87 0,75 0,82 0,87 0,90
6 0,55 0,67 0,76 0,82 0,86 0,72 0,81 0,86 0,90
Ground Temperature derating factors - multicore (up to 300mm²)
Maximum Conductor Ground Temperatures (deg C)
Temperature (70 deg C)
25 30 35 40
0,90 0,85 0,80 0,74
Air Temperature derating factors
Maximum Conductor Air Temperatures (deg C)
Temperature (70 deg C)
30 35 40 45
1,00 0,94 0,87 0,79
Derating factors for grouping of multicore cables installed horizontally in air
No of cables 1 2 3 6 9
Condition DERATING FACTOR
Cables touching 1 0,9 0,85 0,8 0,75
Clearance D* between Cables 1 0,95 0,9 0,88 0,85
*D is the overall diameter of one cable
FAULT CURRENT
This topic is mainly concerned with higher voltages than LV.
Two types of fault current need to be considered:
1. Short circuit current (sometimes called symmetrical fault current).
2. Earth fault current (sometimes called asymmetrical fault current).
SHORT CIRCUIT CURRENT
When we experience a fault on an electrical system, the protection equipment
will detect the fault and trip out the faulty circuit. It cannot however trip
instantaneously, and a delay, (of up to a few seconds) will occur.
The formula for the short circuit current withstand capability of an electric cable
is:
Where:
Isc = Short circuit rating of cable (kA)
A = Area of conductor (mm²)
t = Time to trip (in seconds)
K = 96 for PVC, Copper conductor
= 62 for PVC, Aluminium conductor
= 116 for PAPER, Copper conductor
= 78 for PAPER, Aluminium conductor
= 143 for XLPE, Copper conductor
= 98 XLPE, Aluminium conductor
Example: For a 70mm², PVC insulated Copper conductor, the 1 second short
circuit rating is:
If the protection is fast, (say 0.2 seconds) then the conductor will have a 0.2
second short circuit rating of:
EARTH FAULT CURRENT
If the phase conductor comes into contact with the earth conductor (lead sheath,
copper tape or armour) then we must ensure that this earth conductor can
survive the resulting fault current.
Where:
K Depends on the earth path material. (Steel wire armour, lead sheath, or copper
tape)
A = Cross-sectional area of earth path.
t = Fault duration in seconds.
Values of earth fault ratings are available in our brochures.
EXAMPLE OF CABLE SELECTION FOR MEDIUM VOLTAGE (11kV)
We wish to supply a 2MVA, 11kV transformer from a Eskom supply point which
is 3km away. We are to use an underground Paper Insulated, Copper conductor
cable.
The depth of burial of the cable is 1.25m. Ground thermal resistance is 2 K.m/W.
The ground temperature is 25°C and there are no other cables in the trench.
Short circuit level may be assumed to be 250MVA, and the earth fault level
100MVA, and it may be assumed that a fault will be cleared in ½ second.
Using the previous formulae:
Derating factor for Depth of Burial at 1.25m is 0.96.
Derating factor for Soil Thermal Resistivity at 2 K.m/W is 0.84.
Derating factor for Ground Temperature of 25°C is 1.00.
Total derating factor = 0.96 x 0.84 x 1.00 = 0.806
Either derate the standard cable current rating as given in our catalogues by
multiplying this value by the total derating factor as calculated above, or increase
the required load current by dividing the load current by the total derating factor.
I required = 105/0.806 = 130 Amps
Our Paper Insulated Cable catalogue shows that:
A 35mm² Copper conductor cable would be capable of carrying this load (130A).
Cable size required is thus 35mm², Copper conductors, 3 Core general purpose
belted cable.
Checking for volt drop:
Volt drop is seldom a problem at Medium Voltage, even for long runs of small
conductor size as shown above.
Checking for fault current:
Prospective symmetrical (Short circuit) current.
The selected cable short circuit current withstand is:
This cable will not survive the prospective short circuit current of 13.137kA.
Smallest standard size is a 95mm² Copper conductor. This has a current rating of
235 Amps under standard conditions.
Prospective asymmetrical (earth) fault current:
Lead area for a 95mm² x 3 Core PILC cable: = 198.2mm²
This cable will survive the prospective earth fault current of 5.254kA.
In order to survive short circuit conditions, in many cases the cable conductor
size is required to be larger than dictated by the full load current. Less commonly
the cable size is chosen in order to survive the prospective earth fault current.
The use of large conductors can sometimes be avoided by improving the speed
of protection, and in the case of earth fault current, by the use of sensitive earth
fault protection equipment.
Contents and images ©1997, 1998
Aberdare Cables (Pty) Limited. All rights reserved.
Terms and conditions for use of this document.
Designed and created by Intekom (Pty) Limited
