What's Under Your Feet in New York City?

[Note that this article is a transcript of the video embedded above.]

New York City is unlike any other city in the United States. It’s the most densely populated area in the country by far, absolutely packed with buildings, business, homes, and people. But for all that you can see walking the streets or flying overhead, there’s a whole other world of infrastructure that makes the city possible below the surface. What if you could peel back the paving and soil to see what one of my favorite authors, David Macaulay, called the city’s “massive root system.” Let’s take a tour underneath the Big Apple. I’m Grady and this is Practical Engineering.

Let’s pick a generic Manhattan intersection to start this journey. You’ve got cars, buses, bikes, buildings, sidewalks, traffic lights, fire hydrants, hotdog stands, manholes, and more. But open it up and there’s the whole other world below the surface. We’ll start with the water lines.

If you think about it, water pipes could run overhead like electrical wires. Digging trenches is a lot of work, after all. And fixing underground pipes is pretty disruptive to streets. Of course, New York isn’t unique in having them run underground. It’s standard practice across most of the world for a lot of simple reasons: water is heavy, so continuous support along the length of a pipe is a structural convenience. Water also freezes, so putting pipes underground below the frost line prevents them from freezing in the cold winters. It also protects them from a whole host of natural and human-caused hazards like car crashes and rogue parade balloons. That’s important because a broken water main can be a big problem.

When you see a huge rooster-tail of water spraying from the street, it’s easy to wonder why we need water mains to run at such high pressures. Of course, pressure helps move water through pipes to all the individual places where it’s needed across the city. But maybe more importantly, that pressure pushing out keeps contamination from getting in. You want any crack, hole, or break in a water main to be a one-way street. If anything’s moving from one side of the pipe wall to the other, it’s pretty important that it happens from in to out.

Like electricity, water lines typically run in somewhat of a grid pattern. This adds redundancy, providing multiple paths for water to reach a destination so that taking a line out of service doesn’t disrupt the flow to residents. It also makes sure that all the water in the pipes is constantly flowing. If you build a water distribution system like the branches of a tree, you end up with a lot of dead-ends where water can slow down or even stagnate, making it unsafe to drink.

New York City’s water system is famously gravity fed, with most of the source water coming from upstate at a higher elevation. It also requires no filtration because the source watersheds are fiercely protected to keep contamination out. But the City doesn’t just assume things are good. Dotted throughout the streets are more than 900 water sampling stations that let officials collect and test the quality of the water at the end of the distribution system to make sure it’s safe to consume.

If you could peel back the soil and look at the city’s water distribution system, you’d see water mains down nearly every street; shutoff valves used to isolate individual lines for maintenance or repairs, connections to street and wall hydrants where firefighters can hook up their engines, and service lines that tap into the mains to supply each individual building. You’ll notice that few utilities run under the buildings themselves. The main reason is that we need to be able to access them to fix them if needed. The other reason is that buildings often have their own underground structures, specifically piles, piers, or drilled shafts that serve as their foundation. I have a whole video on deep foundations if you want to learn more after this.

Unlike water pipes, it is pretty typical to see electrical distribution lines running overhead on utility poles everywhere across the globe. You won’t see this in most parts of New York City, though. Roughly 85 percent of the electrical lines are underground. Part of it’s about looks: lines clutter up the space and require dedicated rights of way that limits the use of that space. Another part is safety: keeping people and vehicles free and clear of distribution level voltages. And, of course, there’s reliability. When a heavy storm takes out an above-ground utility pole in a suburban neighborhood, the ensuing power outage is an inconvenience. That same outage in Manhattan could affect a lot more people.

There’s a lot of confusion about underground electrical service. You can kind of divide the grid into three distinct categories defined by voltage ranges: there’s transmission (where power moves over very long distances at hundreds of thousands of volts), distribution (where it’s carried throughout a populated area at a a few thousand volts), and finally service (the voltage at the plug). Putting service lines underground is pretty straightforward. You might even have an underground line at your house running to a lamp or a detached garage. Putting transmission lines at hundreds of thousands of volts underground is a pretty extreme engineering challenge because of insulation, heat buildup, and capacitance. Undergrounding distribution lines lies somewhere in the middle.

One of the big upsides of running lines above ground is the availability of air. Air is free and it works pretty well as an insulator if you keep enough space around energized conductors. You only need actual insulators at the pole. Putting lines at tens of thousands of volts underground requires pretty expensive insulation that prevents arcs to ground or other phases and resists the effects of water, a hazard that is inevitable for every underground utility.

We often call an electrical interconnection a “grid,” but that term mostly applies to the high-voltage bulk power system covering whole states or countries. It’s not really a good description at a city scale. Most urban areas use what’s called a radial system for distributing electricity, which is more akin to branches of a tree than a mesh. For a single-family residential home, you might share a transformer with a few houses. That connects to the distribution feeder, and you can follow that line all the way back to the substation. Each feeder is essentially a one-way dead end for power flow. There may be a crossover somewhere for redundancy, but it’s not an inherent part of the radial architecture. In New York City, it’s totally different.

Throughout the five boroughs, New York City operates about 70 separate so-called “secondary networks,” each of which is served by somewhere between 8 and 28 feeder lines from an area substation. Rather than individual transformers that serve one or two buildings, there are network transformers dotted around the city, usually in concrete vaults belowground, each connected to one of the redundant feeders from the substation. Because they’re underground, these transformers have to be capable of operating while fully submerged in water.

Those transformers drop the voltage to the service-level where a grid of conductors spread out to all the buildings in the area. These are true networks, actual grids of service-level voltage with multiple redundant pathways for energy to take (not like the branches of a tree at all). And there’s another way it’s not quite like the rest of North America.

A typical service transformer in the US gives you “split phase power”. It takes one phase from the grid and provides two energized lines we call hots. Each hot leg has a voltage sine wave between neutral that is 180 degrees out of phase. So between one hot and the neutral, you get 120 volts. That’s a typical wall outlet. Larger appliances and EV chargers use both hot lines to get 240 volts. In New York City, the service networks are d