(3) Subject to subsection (4), the system of distribution of electricity shall be 3 phase 4 wire for 415 volts between lines and 240 volts between line and neutral, single phase 3 wire for 480 volts between lines and 240 volts between lines and neutral, the neutral in each case being earthed, or single phase 2 wire for 240 volts between lines with one line earth and designated “the neutral”, all or any of such systems to be used, the choice in any particular case being by the Company according to load conditions and the most economical method of supply.