If every transmission line in the Wasteland has been ripped to shreds? I don't know if Obsidian was just lazy or what, but there is literally not a single functioning transmission line to be found anywhere. Why the heck does every place have power? There's no way the NCR had the equipment and manpower to install a new underground line system all over the wasteland.

Radiation magic.

Or maybe they just have big batteries that they fill up and ship out. (Doesn't seem as probable though)

So they used the existing underground one.

Haven't you noticed the abundance of Fission Batteries? I figure that's the answer. You can see the Jerry Rigged setups all over the place if you look. Check out Doc's porch in Goodsprings, for instance. They set up the explanation first thing.

As far as the dam is concerned, the infrastructure would be much less difficult to set up than the source, since much of it is still intact.

Its been a while since I've been to Vegas but I don't recall ever seeing an above ground transmission system.

The NCR have some impressive civic engineering skills. After all, they have rebuilt an entire railway and road network back in California. So it is entirely possible that they simply rebuild the pre-war underground energy grid and deliver the power generated by the dam back to the Core Regions.

Underground power lines are referenced in at least one previous Fallout title.

I'm sure it's possible that they're using the existing underground network when it comes to actually powering the city, but that's not what I mean. The transmission towers that connect the dam to vegas are completely destroyed, as are the lines connecting the El Dorado substation to Helios and elsewhere. In real life the Dam isn't connected to everything via underground lines lol. The power has to get to the underground lines first, and in this wasteland it seems rather impossible.

You hardly see them anywhere where I live. The only place I can really think of where you're guaranteed to see them is out in the countryside. In populated areas it's pretty much always underground. The only cables I can see above ground here are phone lines.

Its a video game and by default video games do not require logic....so hush and play the game....much bigger issues in the world...

The Fallout is set in an alternate reality, nuclear apocalypse occurring about 50 years from present time. Even if the underground lines don't exist to the same extent in our modern reality, in the Fallout universe it is still plausible.

I like to think that the overhead lines were primaries for residential and commercial purposes, while secondary underground lines were put in place for important businesses and utilities in and around Vegas.

They are powered by an underground network of glowing ones tied together.