VOLTAGE DROP CALCULATIONS

The voltage drop of any insulated cable is dependent upon

1. the route length under consideration (in meters),
2. the required current rating (in amperes) and
3. 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 = 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 150/1000from 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 canreadily 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.

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

Resourced from : http://www.csedistributors.co.uk/cable/voltage-drop-pf.htm