The contractor's responsibilities include connecting wind farms to the Grid
Balfour Beatty has signed a new five-year alliance contract worth £550m with National Grid.
Under the agreement, Balfour Beatty will upgrade the electricity transmission network in the east of England and connect new infrastructure such as wind farms and new generation plants to the Grid.
The contract will come into force immediately and is one of two overhead line and four substation alliance contracts due to be signed by National Grid in the coming weeks.
Balfour Beatty has an existing £400m agreement with National Grid to replace the gas mains network in the north-west of England.