Below is given the table which shows the ratio of conductor-material in any system compared with that in the corresponding 2-wire DC system. Cos φ is the power factor in an AC system.
System | Same maximum voltage to earth | Same maximum voltage between conductors |
DC system: Two-wire | 1 | 1 |
DC: Two-wire mid-point earthed | 0.25 | 1 |
DC: 3 wire | 0.3125 | 1.25 |
Single-phase: 2 wire | 2/cos2ϕ | 2/cos2ϕ |
Single-phase: 2 wire mid-point earthed | 0.5/cos2ϕ | 2/cos2ϕ |
Single-phase: 3 wire | 0.625/cos2ϕ | 2.5/cos2ϕ |
2-phase: 4 wire | 0.5/cos2ϕ | 2/cos2ϕ |
2-phase: 3 wire | 1.457/cos2ϕ | 2.914/cos2ϕ |
3 phase, 3 wire | 0.5/cos2ϕ | 1.5/cos2ϕ |
3 phase, 4wire | 0.583/cos2ϕ | 1.75/cos2ϕ |
The following points may be noted from the above table:
(i) There is a great saving in conductor material if DC system is adopted for transmission of electric power. However, due to technical difficulties, DC system is not used for transmission.
(ii) Considering the AC system, the 3-phase AC system is most suitable for transmission as well as distribution due to two reasons. Firstly, there is a considerable saving in conductor material. Secondly, this system is convenient and efficient.
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