Voltage drop(in volts)
Run Length (ft) X Conductor Current (amps) X Factor
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1000
Voltage drop (in percent)
Voltage Drop (volts) X 100
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Circuit Voltage (volts)
IIRCC
When calculating the voltage drop in 3 phase circuits, do the same as in the single phase but the voltage drop must be multiplied by 0.866
Below was taken from
>>>Clicky<<< and has some useful tables and formulas - worth having a look at
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 100A 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.