ACADEMIA DO CONHECIMENTO

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)

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

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

Tables derive from IEE copyright information

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

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

Useful three phase formulae:

1. kW = kVA x power factor

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

CURRENT RATINGS OF CABLES IN CONDUIT OR TRUNKING BUNCHED AND ENCLOSED

R = Heat-resisting rubber insulation
P = PVC insulation

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

* 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

#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

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

Burial Depth Derating Factor

SOIL THERMAL RESISTIVITY DERATING FACTOR

PVC Rated Temperature Derating Factor