Voltage Drop Calculations

The voltage drop of any insulated cable is dependent upon the route length under consideration (in meters), the maximum impedance and voltage drop applicable to each cable at maximum conductor temperature and under a.c. conditions is given in the voltage drop calculations table on page 36 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

The values detailed in the tables are given in m/V/Am, Volts/100 per ampere per meter) 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 phase 3 working or 0.025 x 240 + 6.0 volts for a single phase working.

Consider a 3 phase system

The requirements 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, however 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 (page 36) in mVA/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 (0.7mVA/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.

Alternatively:

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,

viztotal drop = 16.6
= YxAxm
=1.1 x A x 150/1000
from the tables Y = 1.1 mV/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 by knowing any two values of Y, A or M, the remaining, unknown value can readily be calculated. The advice is to always check, clarify or suggest the most suitable size and type of cable for any particular, specified requirements.

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