Eversource capped construction of a new high-voltage transmission line from Londonderry to Tewksbury, Mass., Wednesday.
The Merrimack Valley Reliability Project aims to make service more reliable and a little cheaper for customers in both states.
The 345-kilovolt line stretches 25 miles along state right-of-ways. Eversource says it will power up to 400,000 homes at peak demand.
Eversource paid less than a third of the project's cost of $129 million, with National Grid covering the rest in Massachusetts and parts of New Hampshire.