In case of underground system, the maximum stress exists between the line conductors. Hence the various assumptions for such comparison are,

1. The maximum voltage (V

_{m}), between the conductors is same.
2. The power (P) transmitted in all the systems is same.

3. The distance ( l ) over which the power is transmitted is same.

4. The copper losses (w) are same in all the systems.

__1.1 Two Wire D.C. System__

The voltage between the two conductors is V

_{m}. It can be seen that there are no changes in the condition which we have discussed for two wire D.C. system for overhead system earlier in (previous post section 1.1).
This is the base of comparison of other systems,

__1.2 Two Wire D.C. System With Midpoint Earthed__

Thus the volume of copper required in this system is same as that required for two wire d.c. system.

__1.3 Three Wire D.C. System__

Assume balanced load so there is no current through the neutral and there are no copper losses in neutral. So total copper loss is,

Let area of cross-section of neutral wire is half of the area of cross-section of each conductor i.e. 0.5 A.

Thus the volume of copper required for this system is 1.25 times that that required for 2 wire d.c. system. So 25% more copper is required in this system.

__1.4 Three Phase Three Wire A.C. System__

Consider a star connected three phase three wire a.c. system as shown in the Fig. 4. Note that the voltage between the line conductors is V

_{m}. Thus the line voltage is V_{m}.Fig. 4 |

Thus the volume of copper required in this system depends on the load power factor and it is 1.5/cos

^{2}Φ times that that required for two wire d.c. system.
The resultant remains same whether the system is star or delta connected.

__1.5 Three Phase Four Wire A.C. System__

The system is shown in the Fig. 5. The neutral is available for the load connection. As the load is assumed to be balanced, the neutral current is zero and the copper losses in neutral are zero. Note that the voltage between the lines is V

_{m}.Fig. 5 |

Let the cross-section of the neutral wire be half of the cross-section of each line conductor i.e. 0.5 A.

Thus the volume of copper required in this system is 1.75/cos

^{2}Φ times the volume of copper required for two wire d.c. system.
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