Voltage Drop Calculations

The voltage drop of any insulated cable is dependent upon the route length under consideration (in meters), the required current rating (in amperes) and the relevant total impedance per unit length of the cable. The maximum impedance and voltage drop applicable to each cable at maximum conductor temperature and under a.c. conditions is given in the tables. For cables operating under dc conditions, the appropriate voltage drops may be calculated using the formula.

2 x route length x current x resistance x 10¯³.

The values detailed in the tables are given in m/V/Am, (volts/100 per ampere per metre), and the nominal
maximum acceptable volt drop specified by the IEE Regulations is 2.5% of the system voltage, i.e. 0.025 x 415
= 10.5 volts for 3 phase working or 0.025 x 240 = 6.0 volts for single phase working.

Consider a 3 phase system
The requirement may be for a load of 1000A to be transmitted over a route length of 150m, the cable to be
clipped to the wall and close protection provided. The rating tables in the IEE Regulations indicate that a
35mm copper conductor PVC SWA PVC cable would be suitable for the loading required, but the voltage drop
must be checked.

Volt drop = Y x current x length
= 1.1 x 100 x 150 millivolts
= 1.1 x 100 x 150 volts/1000
= 16.5 volts
where Y = value from tables in mV/A/m Unless a particular value of voltage drop, acceptable to the user, is
specified, the IEE Regulations figure of 10.5 volts must be adhered to.

Thus: total volt drop = 10.5 volts
10.5 = Y x 100 x 150
Therefore Y = 10.5/100 x 150
= 0.7/1000 volts/ampere/meters

Reference to the voltage drop tables indicates that the cable size with a voltage drop of 0.7/1000 V/A/m
(0.7mV/A/m) OR LESS is a 70mm copper conductor.

Therefore, in order to transmit a 3 phase current of 100A per phase over a route length of 150m, with a total
voltage drop equal to or less than the statutory maximum 10.5 volts, the use would require a
70mm (cu.) multicore PVC.

Conversely
The user may have 150m of 35mm (Cu.) multicore PVC cable and require to know what maximum current
rating can be applied without exceeding the allowable voltage drop. The method is exactly the same as above,
viz:total drop = 16.6

= YxAxM
= 1.1 x A x 150/1000
from the tables Y = 1.1mV/A/m
=1.1/1000V/A/m
therefore A = 10.5 x 1000/1.1.x 150
=64 amperes

From the foregoing, it is apparent that knowing any two values of Y, A or m, the remaining, unknown value can
readily be calculated.

The advice is always available to check, clarify or suggest the most suitable size and type of cable for any particular, specified requirements.

Voltage Drop for Single Core LV Cables (mV / amp / metre)

Copper Conductor >Flat Arrangement Trefoil arrangement Aluminium conductor Flat arrangement Trefoil arrangement
4 7.83 7.770 16 3.343 3.283
6 5.287 5.226 25 2.161 2.100
10 3.184 3.124 35 1.602 1.542
16 2.086 2.008 50 1.222 1.162
25 1.357 1.297 70 0.890 0.830
35 1.034 0.971 95 0.686 0.623
50 0.793 0.732 120 0.569 0.509
70 0.595 0.534 150 0.490 0.430
95 0.469 0.408 185 0.420 0.360
120 0.410 0.349 240 0.353 0.293
150 0.354 0.294 300 0.312 0.252
185 0.312 0.252 400 0.274 0.214
240 0.272 0.211 400 0.245 0.185
300 0.247 0.187 630 0.222 0.162
400 0.224 0.164      
500 0.208 0.148      
630 0.194 0.134      

VOLTAGE DROP PER AMPERE PER METER (mV). Conductor operating temperature: 70ºC

Conductor Cross Sectional Area Two Core Cable D.C. Two Core Cable SIngle Phase A.C. Three or Four Core Cable Three phase A.C.
mm mV mV mV
1.5 29 29 25
2.5 18 18 15
4 11 11 9.05
6 7.3 7.3 6.04
10 4.4 4.4 3.08
16 2.8 2.8 2.04
    r x z r x z
25 1.75 1.75 0.170 1.75 1.50 0.145 1.50
35 1.25 1.25 0.165 1.25 1.10 0.145 1.10
50 0.93 0.93 0.165 0.94 0.80 0.140 0.81
70 0.63 0.63 0.160 0.65 0.55 0.140 0.57
95 0.46 0.47 0.155 0.50 0.41 0.135 0.43
120 0.36 0.38 0.155 0.41 0.33 0.135 0.35
150 0.29 0.30 0.155 0.34 0.26 0.130 0.29
185 0.23 0.28 0.150 0.29 0.21 0.130 0.25
240 0.180 0.190 0.150 0.24 0.165 0.130 0.21
300 0.145 0.155 0.145 0.21 0.136 0.130 0.185
400 0.105 0.115 0.145 0.185 0.100 0.125 0.160

VOLTAGE DROP PER AMPERE PER METRE (mV).  Conductor operating temperature: 70ºc

Conductor Cross-Sectional Area Two Core Cable D.C. TWo Core Cable Single Phase A.C. Three or Four Core Cable Three Phase A.C.
1 2 3 4
Mm Mv MV MV
16 4.5 45 3.9
25 2.9 29 0.175 2.9 2.5 0.150 2.5
35 2.1 2.1 0.170 2.1 1.80 0.150 1.80
50 1.55 1.55 0.170 1.55 1.35 0.145 1.35
70 1.05 1.05 0.165 1.05 0.90 0.140 0.92
95 0.77 0.77 0.160 0.79 0.67 0.140 0.68
120 - -     0.53 0.135 0.55
150 - -     0.42 0.135 0.44
185 - -     0.34 0.135 0.37
240 - -     0.26 0.130 0.30
300 - -     0.21 0.130 0.25

Tables derive from IEE copyright information

PVC INSULATED 600/1000 VOLT CABLES WITH COPPER CONDUCTORS SUSTAINED CURRENT RATING (AMP) (50Hz) PARAMETERS

Normal Conductor Area 600/100 VOLT
THREE-PHASE SINGLE CORE CABLES IN TREFOIL ARRANGEMENT
mm Direct Armoured Duct Armoured Air Unarmoured Air Armoured
50 203 199 184 193
70 248 241 233 249
95 297 282 290 298
120 337 311 338 347
150 376 342 338 395
185 423 375 450 452
240 485 419 537 532
300 542 459 620 607
700 600 489 722 690
500 660 523 832 776
630 721 563 957 869
800 758 587 1083 937
1000 797 621 1260 1010

APPROXIMATE CURRENT PER LINE OR PHASE TAKEN AT FULL RATED HP BY MOTORS OF AVERAGE EFFICIENCY AND POWER FACTOR

Motor Rating Direct Current Alternating Current
110V 220V 550V 240V 380V 415V 550V
HP amp amp amp amp amp amp amp
0.5 5.7 2.8 1.1 3      
1 10 5 2 6 1.9 1.7 1.3
2 18 9 3.6 10 3.6 3.3 2.5
3 26 13 5.2 15 5.1 4.6 3.5
5 42 21 8.4 24 8 7.3 5.5
7.5 60 30 12 35 11.6 10.6 8
10 80 40 16 46 15.1 13.8 10.4
15 117 59 23 67 22 20 16
20 154 77 31 88 29 27 21
25 190 95 38 110 37 34 26
30 227 114 46 130 43 40 30
40 300 150 60 180 59 54 41
50 375 187 75 210 73 67 50
50 445 223 89 253 87 80 60
60 520 260 104 291 102 94 70
80 600 300 120 332 117 107 81
100 740 370 148 412 145 133 100
125   460 184 515 181 166 125
150     220   217 199 150
175     256   253 232 175
200     292   288 264 199
250         353 323 244
300         421 385 291

Useful three phase formulae:

1. kW = kVA x power factor

2. kW =

Line amps x Line volts x 1.73 x p.f.

1000

3. kVA = kW
p.f.

4. Line amps = kW x 1000
Line volts x 1.73 x p.f.

5. Line amps = kVA x 1000
Line volts x 1.73

6. Line amps = h.p. x 746
Line volts x 1.73 x Efficiency x p.f.

7. kVA = Line amps x Line volts x 1.73
1000

8. kW = h.p. x 746
1000 x Efficiency

9. kVA = Line amps x Line volts x 1.73 x Efficiency x p.f.
746

10. h.p. =  kW x 1000 x Efficiency
746

11. h.p. = kVA x 1000 x Efficiency
746

CURRENT RATINGS OF CABLES CLIPPED DIRECT TO SURFACE OR TRAY BUNCHED AND UNENCLOSED

Conductor Size 2 ONe-Core D.C. 3 One-Core
4 One-Core
1 Two Core DV 1 Three Core
1 Four Core
One Phase A.C. Three Phase A.C. One-Phase A.C. Three Phase A.C.
R P R P R P R P
mm2 amp amp amp amp amp amp amp amp
1 16 13 15 12 14 12 12 10
1.5 21 16 19 15 18 15 15 13
2.5 29 23 26 20 24 21 21 18
4 38 30 34 27 31 27 27 24
6 49 38 45 34 40 35 35 30
10 67 51 60 46 56 48 48 41
16 90 38 81 61 72 64 64 54
25 115 89 105 80 96 71 84 62
35 145 109 130 98 115 87 100 72
50 205 175 185 160 170 140 150 125
70 260 220 235 200 210 175 185 155
95 320 270 285 240 255 215 225 190
120 370 310 335 280 300 250 260 215
150 420 355 380 320 335 285 300 250
185 480 405 435 365 385 325 345 280
240 570 480 520 430 450 385 400 335
300 660 560 600 500 520 445 460 390
400 770 680 700 610        
500 890 800 800 710        
630 1050 910 950 820        

CURRENT RATINGS OF CABLES IN CONDUIT OR TRUNKING BUNCHED AND ENCLOSED

Conductor Size 2 One Core D.C. 4 One Core D.C. Three Phase A.C.
One Phase AC Three Phase AC One Phase AC
R P R P R P R P
mm2 amp amp amp amp amp amp amp amp
1 14 11 11 9 12 11 10 9
1.5 17 13 14 11 15 13 13 12
2.5 24 18 20 16 20 18 17 16
4 31 24 27 22 27 24 23 22
6 40 31 35 28 34 30 30 27
10 55 42 49 39 47 40 41 37
16 73 56 66 53 61 53 54 47
25 94 73 89 71 80 60 70 53
35 115 90 110 88 97 74 86 65
50 170 145 145 125        
70 215 185 185 160        
95 265 230 225 195        
120 310 260 260 220        
150 350   300          

R = Heat-resisting rubber insulation
P = PVC insulation

MINIMUM SIZE OF EARTH CONDUCTOR (IF NOT CONTAINED IN CABLE)

Size of largest associated copper circuit conductor Size of Earthing Conductor Size of Earthing Continuity Conductor Size of Bonding Lead

1

6 1* 1#*
1.5 6 1* 1#*
2.5 6 1* 1#*
4 6 2.5 1#*
6 6 2.5 1#*
10 6 6 2.5
16 6 6 2.5
25 16 16 6
35 16 16 6
50 16 16 6
70 50 50 16
95 50 50 16
120 50 50 16
150 50 50 16
185 70 70 50
240 70 70 50
300 70 70 50
400 70 70 50
500 70 70 50
630 70 70 50

* 1.5 sq mm where earth conductor in unenclosed
# 2.5 sq mm for bonding other services at entry to premises.
 

DIAMETERS AND GLAND SIZES OF ARMOURED PVC INSULATED CABLES

Conductor Size Max. Diameter of Core No. of Cores Approximate Diameters Wire Recommended Gland Size#
      Bedding Sheath Armour Oversheath    
Sq. mm mm No. of Cores mm mm mm mm BS4121
14/8 26/8 2 7 9 11 6/8 7/8 5/8
    3 73/8 9 3/8 12 2/8 7/8 5/8
    4 8.1 10.1 13 0.9 3/4 S*
    5 8.9 10.9 13.8 0.9 3/4 S
    7 9.7 11.7 14.5 0.9 3/4 S
    10 12 2/4 15 18 1 1/4 3/4
    12 12 3/4 15 2/4 18 2/4 1 1/4 3/4
    19 15.1 17.8 21.1 1.25 1
    27 18.5 22 25.4 1.6 1
    37 21 24 2/4 17 3/4 1 2/4 1 3/4
    48 23 3/4 27 1/4 30 3/4 1 2/4 1 3/4
2.5 3.3 2 8.2 10.2 13.1 0.9 3 3/4 S*
    3 8.7 10.7 13.6 0.9 3 3/4 S
    4 9.6 11.6 14.5 0.9 3 3/4 S
    5 10.5 12.5 15.4 0.9 3 3/4
    7 11 2/4 12 2/4 16 2/4 1 3/4
    10 14.8 17.5 20.9 1.25 1
    12 15.3 18 21.4 1.25 1
    19 18.5 22 25.4 1.6 1
    27 22 25 2/4 29 1/4 1 2/4 1 3/4
    37 25 28 2/4 32 2/4 1 2/4 1 3/4
    48 29 33 1/2 37 1/2 2 1 1/2
4 4.3 2 10.2 12.2 15.1 0.9 3 3/4 S
    3 11 13 16 1 3/4
    4 12 14 3/4 17 3/4 1 1/4 3/4
    5 12 1/4 16 19 1 1/4 3/4
    7 14 2/4 17 1/4 20 2/4 1 1/4 1
    10 19 1/4 22 3/4 26 1 2/4 1
    12 19.8 23.3 26.8 1.6 1 3/4
    19 12 2/4 27 30 2/4 1 2/4 1 1/4
    27 28 1/2 33 37 2 1 1/2
6 5 2 11 2/4 13 2/4 16 2/4 1 3/4
    3 12 1/4 12 1/4 18 1 1/4 3/4
    4 13 2/4 13 2/4 19 1/4 1 1/4 3/4
10 61/4 2 14 16 3/4 20 1 1/4 3/4
    3 15 17 3/4 21 1/4 1 1/4 1
    4 16 2/4 19 1/4 22 3/4 1 1/4 1
16 Shaped Conductors 2 13 15 2/4 19 1 1/4 3/4
    3 14 2/4 14 2/4 20 2/4 1 1/4 1
    4 19 3/4 16 3/4 24 1 1/4 1

#Glands Type BW, CW, D1W, D2W, E1W, E2W.
• A cable made to minimum tolerance may be accommodated in a gland one size smaller.

GLAND SIZING CHART PVC/SWA/PVC CABLES

Size MM Sq Cores
1
2 3 4 5 7 10 12 19 27 37 48
1.5   16/20 16/20 20S 20S 20S 20L 20L 25S 25L 32 32
2.5   20S 20S 20S 20S 20L 25S 25S 25L 32 32 40S
4.0   20S 20L 20L 20L 20L 25L 32 32 40S    
6.0   20L 20L 20L                
10.0   25S 25S 25S                
16.0   25S 25L 25L                
25.0   25S 32 32                
35.0   25L 32 32                
50.0   32 32 40S                
70.0   32 40S 40L                
95.0 25S 40S 40S 50S                
120.0 25L 40S 40L 50S                
150.0 32 40L 50S 63S                
185.0 32 50S 50L 63S                
240.0 40S 50L 63S 63S                
300.0 40L 63S 63L 75L                
400.0 50S 63L 75S 75L                
500.0 50S                      
630.0 50L                      

The gland sizes tabulated are intended as a guide only and are based on the approximate under armour and
overall diameters.

Cables with aluminium wire must have aluminium glands.

DERATING FACTORS

GROUND TEMPERATURE DERATING FACTORS

AIR TEMPERATURE DERATING FACTOR

Air Temperature oC 25 30 35 40 45 50 55
PVC cables rated at 70oC 1.22 1.15 1.08 1.00 0.95 0.82 0.71

Burial Depth Derating Factor

Ground Temperature oC 25 30 35 40 45 50 55
PVC cables rated 70oC 1.13 1.07 1.00 0.93 0.85 0.76 0.65
Cables Cross Section
Depth of laying mt up to 70 mm sq 95mm sq – 240 mm sq 300 mm sq and above
0.5 1.00 1.00 1.00
0.60 0.99 0.98 0.97
0.80 0.97 0.96 0.94
1.00 0.95 0.93 0.92
1.25 0.94 0.92 0.89
1.5 0.93 0.90 0.87
1.75 0.92 0.89 0.86
2.00 0.91 0.88 0.85

SOIL THERMAL RESISTIVITY DERATING FACTOR

Soil thermal resistivity in °C cm / watt 80 90 100 120 150 200 250
Rating factor 1.17 1.12 1.07 1.0 0.91 0.80 0.73

PVC Rated Temperature Derating Factor

Type of PVC rated temperature oC 70 85 95 105
Rating Factor 1.000 1.195 1.309 1.414