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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












    List of Other Technical Tables & Useful Links

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