Download Voltage drop in cables

Marjohn Demanuele
[email protected]
Voltage drop in cables
In any cable that is carrying a current, it is inevitable that a certain amount
of voltage drop occurs. This is due to the resistance of the cable itself and the
current flowing in it.
Voltage drop in cable (V) = Resistance of cable (R) X Current through cable ( I )
Whenever a voltage drop occurs in a cable, power is being lost across that
voltage drop. This power must be paid for as well, so it is in the interest of the
consumer to minimize power loss in cables.
Effects of Voltage drop in cables
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Power wasted in cables
Reduction in the efficiency of lamps and heaters
Difficulty for fluorescent lamps to start up
Improper speed attainment of motors
Inability of motors to change position of centrifugal switches (lack of
speed)
Ex 1: A load of 15.5A at a terminal voltage of 455V is supplied through cables of
total resistance 0.32Ω. Calculate:
i.
The voltage at the supply end
(459.96V)
ii.
The total power supplied
(7129W)
iii.
The power lost in the cables.
(76.88W)
Efficiency ή (nita)
Efficiency is a ratio between output quantities and the input quantities of a
process. Such quantities include; work done (energy), voltage, power etc.
The input into a process is always greater than the output from the process;
hence efficiency is always less than 1. Efficiency is either expressed as a P.U.
(per unit) value or as a percentage.
Hence efficiency ή = output power pu units or output power x 100%
input power
input power
Also efficiency ή = output energy pu units or output energy x 100%
input energy
input energy
Licence A
1of2
Voltage drop in cables
Marjohn Demanuele
[email protected]
Ex 2: The power output of a generator is 2600W and the power required to drive
is equivalent to 3500W. Determine the percentage efficiency of the
generator.
(74.28%)
Ex 3: The voltage at the terminals of a motor is 435V and the mains voltage is
450V. Determine the percentage voltage drop.
(3.33%)
Ex 4: The voltage at the terminals of a water pump motor is 234V whereas the
voltage at the supply end is 240V. Calculate the percentage voltage drop.
(2.5%)
Ex 5: A generator supplies a maximum of 20 lamps, each absorbing a current of
0.45A at 230V. The power input to the machine is 2.85kW. Calculate the
percentage efficiency of the generator.
(72.63%)
Ex 6: A 3kW immersion heater rated at 240V is used, in a certain installation
where the terminal voltage is 228V. Calculate:
i.
ii.
iii.
The percentage voltage drop
(5%)
The actual power consumed by the heater
(2707W)
The percentage variation in power loss caused by the voltage drop
(9.7%)
Ex 7: A twin copper cable having a cross sectional area of 75mm 2, supplies a
20kW load 1000m from source. If the terminal voltage is 400V when the
winter ambient temperature is 20oC, calculate;
i.
The resistance of the cable
(0.453Ω)
ii.
The current absorbed by the circuit
(50A)
iii.
The supply voltage
(422.65V)
iv.
The percentage power loss in cable
(5.36%)
During the summer period, the effective ambient temperature of the cable
is raised to 75oC and the supply voltage is kept constant. Calculate;
i.
The resistance of the cable
(0.55Ω)
ii.
The current absorbed
(49.4A)
iii.
The voltage across the load
(395.2V)
iv.
The voltage drop across the cable
(27.3V)
v.
The percentage power loss in cable
(6.45%)
Assume resistivity of copper as 0.017µΩm and temperature coefficient of resistance of copper at 20oC is 0.004/oC
Licence A
2of2
Voltage drop in cables