Three-Wire DC Distribution System

Three-Wire DC Distribution System

When the voltage is increased the efficiency of the transmission line increases and the cost of conductor material in feeder cables gets reduced but in case of dc system it is not possible to change the voltage between feeders and distributors without using rotating machinery which is undesirable,so the feeders and distributors are operated at same voltage i.e consumer voltage is the same as the voltage at feeding points.

The above difficulty is largely overcome by employing 3-wire system which consists of two outers and an earthed middle wire known as neutral wire.In such system consumer apparatus or lamps are connected between the neutral and one of the outers. The voltage between the outers being twice that of consumer’s terminals increases the transmission efficiency and reduces the cost of conductor material.

If the load on the both side is equal ,no current flows through the neutral and such a load is known as balanced load.But if the load on two sides is different then difference of load current on two sides,known as out of balance current,flows in the neutral.Neutral wire is generally taken of half cross section of either outer and only out of balance current flows through the neutral wire.

If currents flowing through +ve and -ve outers are I1 and I2 respectively then current in the middle wire will be difference of currents I1 and I2 flowing in the outers.

When the load on +ve side is greater than on -ve side,current equal to difference of outer currents will flow in the neutral wire from load end to supply end as shown in figure below,

But when the load on -ve side is greater than on +ve side,current equal to difference of outer currents will flow in the neutral wire from supply end to load end as shown in figure below,

Typical layout of 3-wire dc distribution system

    Fig:Typical Layout of 3-wire DC Distribution System

Advantages of 3-wire DC Distribution

  1. Mainly two voltages are available i.e high voltage for power loads and low voltage for domestic and commercial loads.
  2. 3-wire dc distribution is able to deliver more power i.e twice of that with two-wire distribution without affecting the efficiency with  the same volume of conductor material.
  3. If both of the 3-wire and 2-wire feeders transmit the same amount of power over the same distance with the same efficiency,the three wire feeder,if load is balanced and the cross section of middle wire is half of the cross section of either outers,requires only 0.3125 times conductor material as required by two wire feeder.


Let power delivered to consumers be P watts,

voltage at power station and feeding point be V1 and V2 volts respectively.


In a 3-wire system the voltage is double i.e 2V2 and therefore,each outer will carry current of P/2V2   so long as the load is balanced.

Since for given length of a conductor,the area of cross section and therefore volume is inversely proportional to resistance and if volume of conductor material required in two-wire system is v units,v/2 in each wire,volume of each outer in the 3-wire system=((v/2)/4)=v/8 and volume of middle wire=v/16.

Disadvantages of 3-wire DC Distribution

  1. Requirement of additional wire partially reduces safety and necessity of a balancer set causing an increase in cost.
  2. In an overhead system,the cross-arms of greater length are required.


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