Eight circuits of the power cable network, which were built in the 1980s, will soon become too old to maintain and will be left in place while being gradually replaced with the new tunnel system.
These cables were installed just below roads, and repairs and replacement work would cause traffic jams and increase the risk of accidents - problems that the new underground tunnel network will avoid, said SP Group managing director of infrastructure and projects Michael Chin.
He added that the existing cables are so old that they need to be inspected once every three months, which adds to maintenance costs.
Singapore's land scarcity means that SP Group had "no choice" but to build the tunnel system 60m below ground, said Mr Chin.
He said that the 60m-deep tunnels mean that cable checks will not disrupt ground-level life.
Reflecting long-term planning, Mr Chin said the 40km tunnel project is designed to last 120 years. This refers to the concrete segments and the structural reinforcements.
However, he said that there is no knowing what will happen once the system ends.
"I will not be around and you will not be around," he chuckled, adding that Singapore could end up with a completely different system of transmitting electricity.