There's been a significant amount of talk about the urgent need for more transmission lines recently with the infrastructure initiatives. There are two driving factors behind the talk - capacity and location.

Before getting into it, understand that transmission lines are the big towers with large wires spanning great distances. 

Generally speaking, they carry electricity from the source (e.g. power generating stations) to distribution centers (called substations) which in turn deliver the electricity to homes and businesses.

Let's talk about capacity...

The Energy Information Administration (EIA) is projecting total US energy demand to increase over time in each of the four energy consuming sectors - industrial, transportation, residential, commercial.

In addition, decarbonization relies on electrifying as much energy use as possible. [p. 336 of linked reference]  More electricity demand from organic electricity usage growth plus more electricity demand as part of the electrification movement means the capacity of electricity being carried by transmission lines must be increased - significantly.

Let's talk about location...

Since transmission lines begin where electric power plants are located, and with the majority of electricity in the US still coming from fossil fuels, most of the transmission lines are starting from fossil fuel plants. Future renewable energy farms (solar and wind) are situated in different locations from fossil fuel plants. So we are looking at increasing capacity by putting in new transmission lines where none existed before.

The major issues…

As noted in "The Decarbonization Journey", the path to decarbonization deals with more than just the technology of putting in new transmission lines but also politics, regulations, environment, economics, and culture. All of these facets come into play when it comes to building transmission lines.

Currently, most of the transmission lines fall under state oversight. Looking at transmission lines from a national perspective leads to getting the Federal Energy Regulatory Commission (FERC) into the picture. It also introduces fundamental federal vs state authority arguments. Where exactly transmission towers are placed introduces environmental concerns along with "not in my backyard sentiment". Money is also needed to build this infrastructure and lots of it. Where is it coming from and how are those investments recouped? Add to all of this are the regulations associated with the electricity industry. They can be complicated and burdensome. These issues are just a few of the many other issues raised with the building transmission lines.

So we need to build our electricity capacity beyond what is currently in place. And yet, the issues facing this need are daunting. That is why you are hearing about transmission lines when there is talk about infrastructure initiatives.