Carillion's power contracts have been flogged to privately-owned rival J Murphy, saving 22 jobs.
National Grid’s electricity overhead lines, substation and underground cable framework contracts are to be transferred as part of a deal for an undisclosed sum.
Chief exec John Murphy said the acquisition represents "an excellent strategic fit for the business".
The three frameworks include a 20km line between Richborough and Canterbury that is attached to an interconnector between Belgium and the UK.
Strategy director David Brewer said: “At Murphy, we believe that private ownership creates a natural alignment of the interests of our customers, owners, employees and suppliers."
Headquartered in Kentish Town and founded by Irish businessman John Murphy, the contractor remains in the family and is run by his grandson.
Similar to many other contractors the firm was boosted by the 2012 London Olympics, winning several awards to build sporting venues.