The project in the Tawsfynydd area is designed to boost electricity supplies to more than 45,000 homes and businesses in the area.
The company is installing approximately 6km of underground 33,000 volt cable from Maentwrog towards Gellilydan.
The vast majority of the civil installation along the A496 is already complete.
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After this, a new 132,000 volt cable will be laid underground from Trawsfynydd grid substation towards Gellilydan.
At the existing Maentwrog grid substation an additional 60MVA grid transformer will be installed.
Initial preparations for the project will begin this week, with major developed starting in August.
Mark Sobczak, Head of the 132kV Programmes for SP Energy Networks, said: "This is one of the largest upgrades to the electricity infrastructure in the Trawsfynydd area for a number of decades. It is designed to make the network more robust and reliable.
"By adding additional cables and reinforcing the main substation, the network becomes more secure which aims to reduce faults, and make it quicker to restore supplies if we do experience any issues."
(LM)
