ACADEMIA DO CONHECIMENTO

Cálculos de queda de tensão (Inglês) – Voltage Drop Calculations

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 ArrangementTrefoil arrangementAluminium conductorFlat arrangementTrefoil arrangement
47.837.770163.3433.283
65.2875.226252.1612.100
103.1843.124351.6021.542
162.0862.008501.2221.162
251.3571.297700.8900.830
351.0340.971950.6860.623
500.7930.7321200.5690.509
700.5950.5341500.4900.430
950.4690.4081850.4200.360
1200.4100.3492400.3530.293
1500.3540.2943000.3120.252
1850.3120.2524000.2740.214
2400.2720.2114000.2450.185
3000.2470.1876300.2220.162
4000.2240.164
5000.2080.148
6300.1940.134

 

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

Conductor Cross Sectional AreaTwo Core Cable D.C.Two Core Cable SIngle Phase A.C.Three or Four Core Cable Three phase A.C.
mmmVmVmV
1.5292925
2.5181815
411119.05
67.37.36.04
104.44.43.08
162.82.82.04
rxzrxz
251.751.750.1701.751.500.1451.50
351.251.250.1651.251.100.1451.10
500.930.930.1650.940.800.1400.81
700.630.630.1600.650.550.1400.57
950.460.470.1550.500.410.1350.43
1200.360.380.1550.410.330.1350.35
1500.290.300.1550.340.260.1300.29
1850.230.280.1500.290.210.1300.25
2400.1800.1900.1500.240.1650.1300.21
3000.1450.1550.1450.210.1360.1300.185
4000.1050.1150.1450.1850.1000.1250.160

 

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

Conductor Cross-Sectional AreaTwo Core Cable D.C.TWo Core Cable Single Phase A.C.Three or Four Core Cable Three Phase A.C.
1234
MmMvMVMV
164.5453.9
252.9290.1752.92.50.1502.5
352.12.10.1702.11.800.1501.80
501.551.550.1701.551.350.1451.35
701.051.050.1651.050.900.1400.92
950.770.770.1600.790.670.1400.68
1200.530.1350.55
1500.420.1350.44
1850.340.1350.37
2400.260.1300.30
3000.210.1300.25

Tables derive from IEE copyright information

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

Normal Conductor Area600/100 VOLT
THREE-PHASE SINGLE CORE CABLES IN TREFOIL ARRANGEMENT
mmDirect ArmouredDuct ArmouredAir UnarmouredAir Armoured
50203199184193
70248241233249
95297282290298
120337311338347
150376342338395
185423375450452
240485419537532
300542459620607
700600489722690
500660523832776
630721563957869
8007585871083937
100079762112601010

 

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

Motor RatingDirect CurrentAlternating Current
110V220V550V240V380V415V550V
HPampampampampampampamp
0.55.72.81.13
1105261.91.71.3
21893.6103.63.32.5
326135.2155.14.63.5
542218.42487.35.5
7.56030123511.610.68
108040164615.113.810.4
15117592367222016
20154773188292721
251909538110373426
3022711446130434030
4030015060180595441
5037518775210736750
5044522389253878060
605202601042911029470
8060030012033211710781
100740370148412145133100
125460184515181166125
150220217199150
175256253232175
200292288264199
250353323244
300421385291

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 Size2 ONe-Core D.C.3 One-Core
4 One-Core
1 Two Core DV1 Three Core
1 Four Core
One Phase A.C.Three Phase A.C.One-Phase A.C.Three Phase A.C.
RPRPRPRP
mm2ampampampampampampampamp
11613151214121210
1.52116191518151513
2.52923262024212118
43830342731272724
64938453440353530
106751604656484841
169038816172646454
25115891058096718462
35145109130981158710072
50205175185160170140150125
70260220235200210175185155
95320270285240255215225190
120370310335280300250260215
150420355380320335285300250
185480405435365385325345280
240570480520430450385400335
300660560600500520445460390
400770680700610
500890800800710
6301050910950820

 

CURRENT RATINGS OF CABLES IN CONDUIT OR TRUNKING BUNCHED AND ENCLOSED

Conductor Size2 One Core D.C.4 One CoreD.C.Three Phase A.C.
One Phase ACThree Phase ACOne Phase AC
RPRPRPRP
mm2ampampampampampampampamp
114111191211109
1.51713141115131312
2.52418201620181716
43124272227242322
64031352834303027
105542493947404137
167356665361535447
259473897180607053
35115901108897748665
50170145145125
70215185185160
95265230225195
120310260260220
150350300

R = Heat-resisting rubber insulation
P = PVC insulation

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

Size of largest associated copper circuit conductorSize of Earthing ConductorSize of Earthing Continuity ConductorSize of Bonding Lead
161*1#*
1.561*1#*
2.561*1#*
462.51#*
662.51#*
10662.5
16662.5
2516166
3516166
5016166
70505016
95505016
120505016
150505016
185707050
240707050
300707050
400707050
500707050
630707050

* 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 SizeMax. Diameter of CoreNo. of CoresApproximate DiametersWireRecommended Gland Size#
Bedding SheathArmourOversheath
Sq. mmmmNo. of CoresmmmmmmmmBS4121
14/826/827911 6/87/85/8
373/89 3/812 2/87/85/8
48.110.1130.93/4 S*
58.910.913.80.93/4 S
79.711.714.50.93/4 S
1012 2/415181 1/43/4
1212 3/415 2/418 2/41 1/43/4
1915.117.821.11.251
2718.52225.41.61
372124 2/417 3/41 2/41 3/4
4823 3/427 1/430 3/41 2/41 3/4
2.53.328.210.213.10.93 3/4 S*
38.710.713.60.93 3/4 S
49.611.614.50.93 3/4 S
510.512.515.40.93 3/4
711 2/412 2/416 2/413/4
1014.817.520.91.251
1215.31821.41.251
1918.52225.41.61
272225 2/429 1/41 2/41 3/4
372528 2/432 2/41 2/41 3/4
482933 1/237 1/221 1/2
44.3210.212.215.10.93 3/4 S
311131613/4
41214 3/417 3/41 1/43/4
512 1/416191 1/43/4
714 2/417 1/420 2/41 1/41
1019 1/422 3/4261 2/41
1219.823.326.81.61 3/4
1912 2/42730 2/41 2/41 1/4
2728 1/2333721 1/2
65211 2/413 2/416 2/413/4
312 1/412 1/4181 1/43/4
413 2/413 2/419 1/41 1/43/4
1061/421416 3/4201 1/43/4
31517 3/421 1/41 1/41
416 2/419 1/422 3/41 1/41
16Shaped Conductors21315 2/4191 1/43/4
314 2/414 2/420 2/41 1/41
419 3/416 3/4241 1/41

#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 SqCores
1
23457101219273748
1.516/2016/2020S20S20S20L20L25S25L3232
2.520S20S20S20S20L25S25S25L323240S
4.020S20L20L20L20L25L323240S
6.020L20L20L
10.025S25S25S
16.025S25L25L
25.025S3232
35.025L3232
50.0323240S
70.03240S40L
95.025S40S40S50S
120.025L40S40L50S
150.03240L50S63S
185.03250S50L63S
240.040S50L63S63S
300.040L63S63L75L
400.050S63L75S75L
500.050S
630.050L

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 oC25303540455055
PVC cables rated at 70oC1.221.151.081.000.950.820.71

 

Burial Depth Derating Factor

Ground Temperature oC25303540455055
PVC cables rated 70oC1.131.071.000.930.850.760.65
Cables Cross Section
Depth of laying mtup to 70 mm sq95mm sq – 240 mm sq300 mm sq and above
0.51.001.001.00
0.600.990.980.97
0.800.970.960.94
1.000.950.930.92
1.250.940.920.89
1.50.930.900.87
1.750.920.890.86
2.000.910.880.85

 

SOIL THERMAL RESISTIVITY DERATING FACTOR

Soil thermal resistivity in °C cm / watt8090100120150200250
Rating factor1.171.121.071.00.910.800.73

 

PVC Rated Temperature Derating Factor

Type of PVC rated temperature oC708595105
Rating Factor1.0001.1951.3091.414