Power. We seem to take it for granted but are reminded during storm season that it can be extinguished with one strategic lightning strike or a fallen tree or broken power pole. I haven’t shared much about the electrical engineering side of the project, but it is an important design element to the Chickasaw Visitor Center. The electrical system powers the lights, outlets, pumps, heating/cooling equipment, elevator and more.
Power is often discussed in terms of voltage which is the energy required to move an electrical charge along a path where negative charged objects are pulled towards higher voltages and the flow is called current. Volts are measured in joules per coulomb. All perfectly clear, right? Well you could always refer to Kirchhoff’s circuit laws to get everything straightened out, but wait, there’s more. Where have I heard that expression? E-gads! OK, here’s the simple version.
Think of electrons flowing in a wire like water flowing in a pipe driven by a pump. The bigger the pump (the voltage) the more water flows. The bigger the pipe (the wire) the more water can flow as well. The flow of the water can perform work like turning turbines. And in a similar fashion the flow of the electrons in the wire can do work as well like turning motors and illuminating lights. So the electrical engineer needs to figure out how much electrical work needs to be done in the building and design the power distribution system accordingly which is not exactly as simple as it sounds.
So the power system is kind of like a tree with a trunk and lots of branches. So you start with the trunk which is the large power source of overhead lines distributed by the power company. And that’s where the power diagram above starts. Voltage comes in many sizes. The large towered transmission lines that cross the countryside are considered “high voltage” which can range from about 35,000 volts (35kV) and upwards. The “medium voltage” distribution lines that run alongside the major roads in our towns can range from about 5000 volts up to 35,000 volts but are usually around 15,000 volts. So when officials say don’t fly kites around power lines there’s a really good reason. To put things in perspective, a flashlight runs on 1.5 volts, a car battery is 12 volts and the common power in our homes is 120 volts.
After tapping into the medium power distribution line by the street, the power comes via what is called a primary feeder to a pad-mount transformer. That’s the large green box in the grass that you see just outside of buildings, about 4-5 feet square and 3-4 feet tall. Everything up to this point is typically supplied by the power company. Our transformer converts the medium power into 3 phase 400 amp service that provides 480/277 power; add up the phases and you get 332,000 volt amps to do work in the building. By comparison, the typical residence is a single phase 200 amp service with 240/120 power for a total of 48,000 volt amps to do work.
The power is brought from the transformer to a Main Distribution Panel (MDP) via 4 – 500 KCMIL wires which is EE code talk for 4 – ½ inch diameter copper wires consisting of 1 neutral and 3 hot wires each about the thickness of my thumb. The MDP is located inside the building and feeds a number of devices that are designed to run on 480/277 volt power which includes the elevator, condenser units (for air conditioning), the outside air heat pump, 2 gray water pumps and the lighting panel. The MDP also serves 2 other panels but goes through 2 step-down transformers first to convert the power to 208/120 volts. One panel is for mechanical equipment like the fan coil units that heat and cool the spaces with air, supply fans, unit heaters, 2 more gray water pumps and the water feature pumps. The other panel is for all the receptacles (wall plugs) located throughout the building.
The electrical engineering process starts in reverse of how the power flows. You add up all the low voltage (208/120) power needs so you can size the panels. You then add in all the higher voltage needs (480/277) to that so you can size the MDP. Make sure you add spare breaker spaces in all the panels for any future needs, take into count some other things like voltage drop, throw some salt over your left shoulder (just kidding) and from that you can determine the size of the transformer you need outside. Because of this reverse approach to designing the electrical system, the electrical engineers are typically waiting on everyone else before they can finalize their work. Even though, we still can’t resist giving them a hard time for always finishing last. There you have it, the family tree of power for the Visitor Center.
I need to provide credit to the master electrical guru without whose guidance and interpretations of EE (electrical engineering) speak and ability to convert complex things like big wire sizes to something simple like the diameter of my thumb I would still be chasing down electrons. Thanks Pat…
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Design Matters 