Southwest Connecticut was the only part of the state that was not served by the 345-kV power supply system. This regionally interconnected system moves large blocks of power from major generating plants to local substations. The Bethel-Norwalkline extended this system into the growing Stamford/Norwalk area of southwest Connecticut, thus providing additional transmission capacity needed to assure reliable uninterrupted service, particularly during peak use hot and cold weather days.

Completion of this 345-kV system enhances service to the area and lessens the area's dependence on expensive power generation. This level of transmission capacity also supports a competitive market for electric energy.

The Bethel-Norwalk Project consists of a new 345-kV line and re-routed segments of an existing 115-kV line. About seven miles of the 345-kV line were constructed overhead on new structures, requiring the relocation of the 115-kV line from the existing right of way to under-street routes in southern sections of Bethel and Wilton, and northern sections of Redding and Norwalk. Also, for about one mile in Redding, the new 345-kV and existing 115-kV lines were combined on taller structures. About 12 miles of under-street 345-kV cables were installed in two segments - one in Bethel, the other in Wilton and Redding. At the Bethel Educational Park, a short length of the existing 115-kV line was also placed under ground.

Construction began mid-March 2005 and was completed in October 2006.

The project cost $350 million.

There is much more to putting transmission lines underground than just burying them. Overhead and underground lines make use of very different technologies.

Underground lines typically cost more, but can more easily be routed through highly developed urban areas, ideally within tunnels beneath city streets. But a disadvantage to underground installations is that their construction requirements, unlike an overhead line, necessitate access to every foot of the route, increasing construction impacts on roadways, traffic, wetlands, and wildlife habitats.

Electric power system engineers recognize that when it comes to working with higher transmission voltages, underground lines have important differences from overhead lines. For instance, because the heat created by resistive losses moves more slowly away from underground cables, underground transmission lines commonly require more or bigger conductors to deliver the same amount of power. Because of the thick insulation around underground cables, installed cable lengths are shorter and many more splices (made in large underground vaults) are necessary. While a failure may happen less frequently on an underground line, when it does its location and repair typically take a couple of weeks to complete, during which time the remaining transmission system must operate reliably without the cable. Also, the substations at the ends of an underground cable will often need to have large shunt reactors installed to compensate for the high charging currents associated with these cables.