Giving a Dam Its Due

Each year, the National Spa & Pool Institute offers special programs in conjunction with its International Expo. Most years, these programs include tours of local places of interest, such as notable museums, historical sites, outstanding examples of local architecture and the like. With the Expo in Las Vegas last December, NSPI took advantage of the location and included a tour of Hoover Dam and Lake Mead, a scant 30 miles from the glitz and glitter of The Strip.

More than100 Expo attendees took the tour and learned a great deal about flood control and irrigation, Hoover Dam’s primary reasons for being. The tour guides also reported that the dam structure alone required 3.25 million cubic yards of concrete – and that there are no workers buried in the pour, despite lots of old and persistent rumors.

Of greater interest to readers of this column, attendees learned as well about the generation of hydroelectric energy, a lucrative by-product of this massive land-reclamation project. In fact, the total electricity output of Hoover Dam serves more than 1.3 million people in the Southwest, with current contracts sending 19% of the output to Arizona, 56% to California and 25% to Nevada.

STANDING AT THE SOURCE

It’s a great tour all in all, but for those of us with an interest in things electric it is simply a great starting point.

As we’re standing in the powerhouse looking at those huge generators, we recognize them as the origination point of all of the electrical energy that we require every day to maintain our lifestyles. How do we direct all this energy into the malls, factories, schools and office buildings? How do we guide this essential ingredient to the backyard of a home where a newly constructed watershape is awaiting its electrical connection?

To answer those questions, let’s take a closer look at this amazing feat of 20th-century engineering and the transmission of electricity from source to backyard. We’ll use some actual numbers from the Hoover Dam/Southern California link, working our way from the generators at the dam to a typical backyard in a Los Angeles suburb.

All of the generators at Hoover Dam produce three-phase power, as is true with virtually every generator in the United States. This means that there are three output conductors leaving each unit. (Only later, at the user end of the circuit, will we see how the single-phase power is derived.) These generators operate at 16,500 volts, so each of these output conductors is operating at a transmission voltage of 16,500 volts.

Does that sound high? Not when you remember Ohm’s Law, which shows us that as volts increase in a circuit, amps will decrease in direct proportion for a given amount of power to flow. This higher voltage allows smaller conductors to be used and improves the operating efficiency of the system.

These three conductors from each generator don’t go very far. In fact, they terminate at a switchyard/transformer center a few miles away in Boulder City, Nev. There, the outputs from various generators can be connected together in various combinations to suit the customer’s requirements. One of the outputs from the switchgear will be the group of three conductors heading for Southern California.

Each conductor is connected to the primary of a step-up transformer, the one final step required before the precious power heads out across the mountains on its 270-mile trip to Los Angeles. These transformers increase the voltage on each phase conductor from 16,500 volts to 230,000 volts.

This high transmission voltage allows a tremendous amount of electrical energy to be transmitted over relatively small conductors. This is important when you consider that the aluminum/steel conductors weigh about two pounds per foot and that the span between the transmission towers is from eight hundred feet to one thousand feet. All that weight adds up quickly.

A HIGH-WIRE ACT

You can see these towers in many places while driving between Los Angeles and Las Vegas. The three conductors hang from more than 200,000 large porcelain insulator sections. There is also a single, lightning-rod conductor running along the very peak of the towers.

If we could follow these towers far enough, we would eventually come to a utility company’s transmission substation in the Los Angeles suburbs. The very large towers used to carry the conductors across the countryside cannot continue into the built-up areas of a city – the cost of the real estate on which to build them would simply be too high. So the three conductors can be seen leaving the last tower and entering the substation.

Now the substation uses step-down transformers to reduce the 230,000 volts down to more manageable values for local use. Various voltages are used at this point. The 230,000 volt, three-phase supply coming in from the dam may, for example, be transformed down to several three-phase circuits of 4,600, 12,000, 33,000, or, maybe, 69,000 volts. There might even be a feeder circuit heading off to another substation at 138,000 volts.

(Please note: I have never seen the specific transmission substation I am referring to here. Some are now totally underground, in which case the following paragraphs don’t apply, but everything I say from here on does apply to the thousands of aboveground substations still out there. Onward to the backyard!)

If you were sitting in your car across the street, you could see these various circuits leaving the substation. Because everything at this point is still three-phase, each circuit will still consist of three conductors. Look for those groups of three conductors. You can get a pretty good feel for the voltage of each departing circuit: The taller the pole or tower, the higher the voltage.

Now pick a group of three conductors to follow. The group you want is probably on wooden poles 30 or 40 feet off the ground. The individual conductors won’t be spaced too far apart – maybe two feet. In all likelihood, this will be a 13,200-volt, three-phase circuit.

Several interesting things can happen to these three conductors as they travel from pole to pole. All three can branch off in a new direction (but you want to try to follow the original group), or all three conductors can branch off and connect to a transformer mounted on the pole. (The transformer looks like a slender metal trash can.) The transformer will reduce the voltage to some value such as 480 or 2,300 or 4,600 volts to provide power to an office building, a group of stores or maybe a group of small factories. But don’t stop there: Keep following the original group of three conductors.

Sometimes the three conductors might dive into a metal pipe running down the pole. They may have gone underground, never to be seen again. In this case, your chase is over – unless you want to backtrack, pick up another leg of this group and head off in another direction. Or the three conductors may have simply crossed the street, underground. Look around: You may find another pole with a pipe on it on the other side of a wide intersection.

DOWN TO ONE

Eventually the chase will end in a residential neighborhood. We do not distribute three-phase electric power in residential neighborhoods in this country, so there is no longer any need to keep the three individual conductors of our group together. Now each single-phase leg can take off on its own to provide 120/240 single-phase power to a large number of homes.

You’ll see it happen, usually at an intersection: One of the three conductors will take off to the right, another will go to the left, and the third will go straight ahead. From this point, these individual single-phase conductors travel along the very top of the poles.

At every tenth or twelfth house, you’ll see a step-down transformer mounted on the pole, and a group of conductors will connect the output of the transformer to the service entrance panel of each house. Through those conductors, each house will then have an almost endless supply of 120/240 volt, 60 hz, single-phase electric power ready to do the owner’s bidding.

When you’re standing in the backyard of the house, you can look (don’t touch) at the wires with new respect – you know that at the other end, maybe hundreds of miles away, an enormous dam is connected to those wires – and you’ll have a pretty good idea of how those conductors reached you.

Although I have been known to follow electrical lines on more than one occasion, I don’t honestly expect any of you to do so. I’ve done it because everyone needs new hobbies – and believe me, finding areas where everything is still on poles is becoming more difficult every day. And in taking this practice to extremes, you may find that setting off alarms, making dogs bark and bite and setting up the occasional ardenaline-pumping confrontation with a homeowner is hardly worth the effort.

But I do recommend that you look upward now and then and appreciate those wires whenever you spot them. Those towers and poles may not be beautiful, but they carry conductors and power that make so much of what we value in our daily lives possible.

Jim McNicol was a technical consultant to the swimming pool, jetted bath and spa industries. He worked on development of equipment standards for pools and spas throughout his career and was honored for his service by the National Spa & Pool Institute.