[Note that this article is a transcript of the video embedded above.] I love the periodic table of the elements. I love it because it reveals the deeper order of what seems like an otherwise wildly disparate collection of atoms with different physical forms, chemical properties, and nuclear stabilities. I love it because, even before we actually found the elements that fit into each box, we knew that something did and could even predict some things about those elements before they were ever discovered. And finally, I love it because it’s a bit messy. Not everything lines up perfectly, and in some ways, it’s still a work in progress. In many ways, human-created standards follow that same form, and I want to try and convince you that they deserve the same affection. Let me present the periodic table of standard North American electrical connections. Isn’t it beautiful? I’m fascinated by stuff like this: a diversity of needs and purposes put into a relatively nice, neat order. But why do we need so many? And where do any of these actually get used? Well, I’ve spent the past month reading just about everything I could find on electrical plugs and receptacles to figure those questions out, and I even have a few of them here so I can show you what I learned. I’m Grady, and this is Practical Engineering. Electricity is something we really don’t want to be proprietary. It’s one thing if your charger doesn’t work on your buddy’s cell phone. It’s another thing entirely when you have to rewire your house because you bought a different brand of toaster. The National Electrical Manufacturers Association, or NEMA, was founded in 1926 as a coalition of companies making electrical equipment. Their members realized that life would be better with some standards, so that any company making an electrical device could be reasonably confident that the people who might want to buy that device would be able to use it, and more importantly, use it safely. This didn’t happen overnight. It took a diverse group of manufacturers, engineers, and testing labs to form a consensus around the system we use today. And it’s far from a perfect system. My friends Mehdi and Alec have covered receptacle-related topics on their channels, including the merits and disadvantages of the NEMA designs. But it works pretty well. Well enough that the NEMA connector standards have been adopted not just in the US, but all of North America, Central America, parts of South America, Japan, Taiwan, the Philippines, and beyond. Here’s that table again. You probably noticed that every type of plug and receptacle has its own special number. They seem a bit arcane at first glance, but it’s actually a handy naming scheme that’s pretty straightforward to understand. The first number is the configuration that defines the combination of voltage rating, wire count, and grounding style. These numbers are a bit arbitrary, but they kind of represent a certain class of receptacles and plugs. For example, NEMA 1 receptacles are rated for 125 volts and have just 2 poles (a hot and neutral) with no ground. The NEMA 1-15 was the classic North American outlet until the 1960s, and you still see these in older buildings. Lots of devices made today can still use them, especially low-voltage equipment like chargers, and, critically, those without external metal parts. If an energized wire inside the device comes loose and contacts the case, there’s still an insulating barrier protecting someone from being shocked. The reason NEMA 1 receptacles are mostly a thing of the past is what could happen when equipment didn’t have that protection. If a device with a metal enclosure or exposed metal parts had an energized wire come loose, that metal would be energized too. But, critically, it might not create a short circuit. With nowhere for current to flow, the device could just sit there, indefinitely dangerous, until someone happened to touch it, allowing current to flow through them to a lower potential. The ground wire we see in nearly all plugs and receptacles today fixes that specific hazard. Bonding exposed conductive elements and connecting them to ground makes sure that if they somehow become energized, current will flow, a short circuit will form, and protective devices like breakers will activate. Today we use the NEMA 5 standard for the vast majority of receptacles and plugs. Even if you’ve never heard of NEMA or seen the other plugs on the periodic table, you’re almost certainly familiar with this design. They have a 125 volt rating to handle the standard 120 volt service for most electrical devices with a little buffer. They have an energized pole, called the hot; a neutral pole to provide a return path, and a separate ground return that is bonded to the neutral line in the main electrical panel. The ground pin on most outlets is round instead of flat, and that’s the reason why nearly all electrical outlets kind of look like they’re screaming. Or at least they do to me. One thing about NEMA 5, and actually most of the NEMA configurations, is that the outlets have polarity. On the NEMA 5-15, the neutral slot is a bit wider than the hot, making it so the plug can only go in one way. In function, polarity often doesn’t matter for AC circuits. Current travels in both directions, so the equipment inside the device can’t really tell the difference. And some devices, like switch-mode power supplies, don’t care which direction they’re plugged in. Both blades are the same size. For safety, though, a lot of devices do. You really don’t want heating elements, motor coils, and circuit boards energized and waiting for a ground. It’s less hazardous to put the switch on the hot wire so that nothing beyond the cord is energized until it’s turned on. Enforcing polarity at the plug prevents “switched neutrals” along with other issues like electrical noise. The NEMA 5-15 plug and outlet were designed to be backward compatible with the older 1-15 standard. 1-15 plugs work just fine in the modern 5-15 outlets, and there are quite a few interesting compatibility cases like that in the NEMA standards. For example, the “15” in 5-15 refers to the current rating. Nearly every household device and appliance that runs on 120 volts is designed so that it never draws more than 15 amps, and actually, if the device is meant to run for more than 3 hours continuously, like a space heater, it can only draw 80% of that (which is 12 amps if you’re keeping score at home). That limit is obviously fine for most household appliances. But, especially in commercial spaces, it’s not quite enough power for certain devices like kitchen mixers, treadmills, copy machines, and power tools. Of course, we could just change the codes to require 20-amp circuits everywhere, but that has huge implications: larger circuit breakers, heavier-gauge wiring, and more expensive receptacles. And in many cases, it’s just not necessary. So instead, NEMA created a different receptacle and plug for 120-volt, 20-amp circuits, the 5-20. I have a bunch of these in the studio. You can see they have that T shape on the neutral slot. And 20-amp devices have the neutral blade rotated 90 degrees on the plug. But here’s the backward compatibility: regular 15-amp plugs fit into the 5-20 receptacle as well. NEMA 5 has 30 and 50 amp receptacles too, although they aren’t used very often these days because of a quirk about the historic availability of voltage. Today, split phase electrical service is basically standard for residential power. You get two 120-volt hot lines which can be used individually for smaller circuits or combined to get 240-volts for circuits that need more oomph. In the early 20th century, 240-volt service wasn’t always available, so you have these very-high-current 120-volt receptacles that could power heavy commercial cleaning equipment like floor burnishers and blowers, kitchen equipment like warming cabinets and steam tables, and large shop tools like table saws and compressors. Also, not all portable generators run at 240-volts, so older models used the larger NEMA 5 receptacles as well. These are still available and installed in places where, for whatever reason, a higher-voltage circuit is hard to come by. But in most cases, the more power-hungry devices are going to run on 240-volts. That brings us to NEMA 2. Like NEMA 1, these are ungrounded receptacles, but instead of a hot and neutral, they have two hots. Each is 180 degrees out of phase with its neighbor, so you get 240-volts across them, handled with a little cushion by the 250-volt rating. There were 20 and 30 amp receptacles, but, also like NEMA 1, these are mostly obsolete now that a ground is required by code. They’ve been replaced with NEMA 6, which has 15, 20, 30, and 50-amp receptacles and plugs. Of course, with double the voltage, you also get double the power compared to the NEMA 5 equivalents at the same current rating. The 6-15 is common for window or wall-mounted air conditioners. The 6-20 is used for heavier-duty air conditioners plus commercial kitchen equipment and shop tools. The 6-30 is used with large heaters, kilns, and heavy power tools. The 6-50 is kind of the standard welder outlet, plus it’s pretty common these days for level 2 EV chargers, capable of delivering nearly 10 kilowatts of continuous power through the receptacle. Like NEMA 5, the NEMA 6 has some backward compatibility, allowing 6-15 plugs to fit into 6-20 receptacles. This is kind of clever, but it doesn’t work all the way up the different current ratings. Of course a 50-amp outlet could easily handle a 15-amp device. And it would certainly be possible to design a series of outlets where each successive jump in current rating allowed those smaller devices to plug in. But there are two main reasons why they don’t: One is practicality. The blades on plugs aren’t all the same thickness. Designing a single receptacle slot that can safely grip both a thin, 15-amp blade and a massive 50-amp one would make manufacturing more difficult and increase the chances of developing loose connections inside the receptacle over time. Two is safety: circuit breakers are sized to protect everything downstream, including the plug and the appliance cord. If a thin cord on a low-current device develops an internal short, the resistance of that thin wire itself will cap the fault current so that a larger breaker might take much longer to trip or not trip at all. That could allow the wire to reach high enough temperatures to start a fire. Of course you don’t want a high-current device plugged into a lower-current-rated circuit, but if you trace out the things that can go wrong, it turns out that you also don’t want lower-current devices plugged into a high-capacity circuit. So, the plugs and outlets are designed to prevent both cases, except for the 15 and 20 amp situation, where the current is close enough that a breaker should still work as intended. 240 volts are useful to supply more power at the same current rating, but of course it comes at a cost. Higher voltage means more potential, literally, for arcs to occur. Equipment designed to handle the higher voltage needs better insulation and more careful design. Take a clothes dryer for example. You want the extra voltage for the power-hungry heating elements, but all the other stuff inside (like timers, controllers, and clocks) can easily run on 120 and those lower-voltage components are more affordable. That’s where NEMA 10 came in. You get three poles: two hots and a neutral. In that way, you get dual voltage: 240 between the hots and 120 between each hot and neutral. Of course, NEMA 10 receptacles also lack a ground connection, so they’re mostly obsolete. Plenty of houses still have them installed for clothes dryers and kitchen ranges, but since the 1990s, they’ve been supplanted with the NEMA 14 configuration. This is the most widely-used 240-volt standard in North America today. It’s versatile, providing both voltages. And there are a full range of current capacities, allowing you to design a circuit that’s well-suited for a device, from 15 all the way up to 60 amps. The 14-15 is pretty rare. I couldn’t even find someone making the receptacle. The 14-20 is also not that common. Some food service equipment uses this like certain coffee makers. The warmers rely on 240 volts while the fans and timers run on 120. Same with some jobsite heaters and specialized laboratory equipment. The 14-30 is the standard residential electric clothes dryer plug and is often used for EV chargers. Some server and mainframe equipment uses it as well. The 14-50 is the standard residential cooking range and oven plug. It’s also widely used for EV chargers and pretty common at RV campgrounds as well. The 14-60 is more of a commercial or industrial receptacle, used for large kitchen appliances and distribution of power at events like concerts. Single phase electrical service covers nearly all residential and lots of commercial buildings. But, the grid runs on three phases and it’s pretty common for larger commercial buildings and essentially all industrial facilities to have three-phase service. It’s particularly useful for devices that use large motors. And of course, if you have the service, you’re going to need receptacles and plugs for those devices, or at least the ones that aren’t hard-wired. NEMA 11 was the standard for up to 250V with receptacles and plugs ranging from 15 to 50 amps. Those have been replaced by the new NEMA 15, again because of grounding requirements. And this is going to almost always be relatively specialized industrial devices: woodshop and machining tools, laboratory testing equipment, grinders, pumps, dust collectors, heavy welders, plasma cutters, and so on. It’s not stuff most people see in everyday life, and in many cases, each receptacle is going to be custom-installed for a specific piece of equipment. And since hard-wiring equipment directly to the service panel is typically the default, that makes receptacles like these even more rare. You really only see them in places that need a high degree of modularity, allowing for rapid reconfiguration of workspaces like jobsites, certain manufacturing facilities, and short life-cycle equipment that needs to be easily swapped out. There are two main three-phase service classes used in most commercial and industrial buildings in the US. The most common is 208 volts phase to phase, which uses the NEMA 15 configuration. There’s also 480 volts phase to phase, but like I mentioned before, you can get a lower voltage between phase and neutral (in this case, 277 volts). So NEMA 7 has plugs and receptacles specifically for using just one phase from buildings wired with 480-volt, three-phase service. A lot of commercial and industrial lights use these receptacles, like warehouses, factories, and arenas, making them easy to swap out without hard-wiring. Commercial ventilation and air conditioning systems use them too. And just like the dual-voltage 240-volt plugs, there are also dual-voltage three-phase plugs, delivering equipment with all three hot phases plus a neutral so different components can run at different voltages. NEMA 18 has receptacles for 208-volt service, although they don’t have a ground, so they’re mostly obsolete. There are no straight-blade plugs that have replaced NEMA 18. Aligning and inserting a 5-blade plug would be tricky and take a lot of force. And I’ve kind of buried the lede here only talking about the straight-blade NEMA standards. The reality is that a large number of the NEMA receptacles and plugs have an equivalent locking version. These use curved blades that twist inside the receptacle so they can’t be easily pulled out. Actually the locking versions are more common than the straight-blade equivalents in many cases, especially when it comes to portable generators, jobsite equipment, and events where things are always moving around. If your vacuum cleaner unplugs itself because you’ve gone too far into the hallway, that’s usually not a big deal, but if a three-phase 600 volt plasma cutter does the same thing, you can get serious damage from arcing. That’s why the locking standards extend beyond the voltage ratings of the straight-blade ones up to three-phase 600-volt circuits. They even have receptacles for 400-hertz power used in aerospace, submarine, and military systems. Of course, sometimes the standards make themselves. When it comes to RVs and travel trailers, (from what I can gather) the industry had already developed a 120-volt, 30-amp receptacle before NEMA formalized its catalogue of standards. Instead of forcing an entire industry to retool, NEMA just adopted what everyone was already using, calling it the TT-30. TT for travel trailer and 30 for the current capacity. In function, it’s not any different than the NEMA 5-30 receptacle and plug, but you’ll almost never see one of those, because the TT-30 is far more common. It’s a face only an outlet enthusiast could love. I haven’t really talked about the smaller versions of the locking connectors used where space is an issue. And there are even more specialized standards like ship-to-shore power, aircraft, and military uses. Of course, when you look beyond NEMA, there are way more standards out there. But I feel like this is enough to get you excited about the weird, wide world of electrical receptacle standardization. There are all kinds of practical considerations that make it much more complicated than just a 2D chart with voltage on one side and current on the other. Just like the periodic table of the elements, the NEMA connection standards are a bit messy. And that’s what I love about them.

[Note that this article is a transcript of the video embedded above.] Flaming Gorge Dam rises from the Green River in northern Utah like a concrete wedge driven into the canyon, anchored against the sheer rock walls that flank it. It’s quintessential, in a way. It’s what we picture when we think about dams: a hulking, but also somehow graceful, wall of concrete stretching across a narrow rocky valley. But to dam engineers, there’s nothing quintessential about it. So-called arch dams are actually pretty rare. For reference, the US has about 92,000 dams listed in the national inventory. I couldn’t find an exact number, but based on a little bit of research, I estimate that we have maybe around 50 arch dams - it’s less than a tenth of a percent. The only reason we think of arch dams as archetypal is because they’re so huge. I counted 11 in the US that have their own visitor center. There just aren’t that many works of infrastructure that double as tourist destinations, and the reason for it is, I think, kind of interesting. Because an arch dam isn’t just an engineering solution to holding back water, and it’s not just a solution to holding back a lot of water. It’s all about height, and I built a little demo to show you what I mean. I’m Grady, and this is Practical Engineering. Engineers love categories, and dams are no exception. You can group them in a lot of ways, but mostly, we care about how they handle the incredible force of water they hold back. Embankment dams do it with earth or rock, relying on friction between the individual particles that make up the structure. Gravity dams do it with weight. Let me show you an example. I have my tried and trusted acrylic flume with a small plastic dam. Once this is all set up, I can start filling up the reservoir. This little dam is a little narrower than the flume. It doesn’t touch the sides, so it leaks a bit. The reason for that will be clear in a moment. And hopefully you can see what’s about to happen. This gravity dam doesn’t have much gravity in it, so it doesn’t take much water at all before you get a failure. I’m counting failure as the first sign of movement, by the way. That’s when the stabilizing forces are overcome by the destabilizing ones. And the little dam by itself could hold until my reservoir was about a quarter of the way to the top. Gravity dams get their stability against sliding from… you guessed it… friction. Bet you thought I was going to say gravity. And actually, it kind of is gravity, since frictional resistance is a function of just two variables: the normal force (in other words, the weight of the structure) and a coefficient that depends on the two materials touching. Engineers analyze the stability of gravity dams in cross-section, essentially taking a small slice of the structure. You want every slice to be able to support itself. That’s why I didn’t want the demo touching the sides of the flume; it would add resistance that doesn’t actually exist in a cross-section. The destabilizing force is hydrostatic pressure from the reservoir, which increases with depth. And the stabilizing force is friction. There are some complexities to this that we’ll get into, but very generally, as long as you have more friction than pressure, you’re good; you have a stable structure. So let’s add some normal force to the demo and see what happens. [Beat] You can see my little reservoir gets a little higher before the dam fails, about halfway to the top. And we can try it again with more weight. But the result gets a little more interesting… the dam didn’t actually slide this time, but it still failed. Turns out gravity dams have two major failure modes: sliding and overturning. Resistance to sliding comes from friction, which really doesn’t depend on how the weight of the dam is distributed. That’s not true for overturning failures. Let’s look back at our cross-section. For a unit width of dam, the hydrostatic pressure from the reservoir looks like this. Pressure increases with depth. And the area under this line is the total force pushing the dam downstream. We can simplify that distribution and treat it like it’s a single force, and it turns out when you do that, the force acts a third of the way up the total depth of water. Most dams want to rotate about the downstream toe, so you have a destabilizing force offset from the point of rotation. In other words, you have a torque, also called a moment. The dam has to create an opposite moment around that point to remain stable. Moment or torque is calculated as the force multiplied by its perpendicular distance from the point of rotation. So, the further the center of mass is from the downstream toe, the more stable the structure is, and the demo shows it too. Here’s where we left the weights the last time, and let’s see it happen again. The reservoir makes it about two-thirds of the way up the walls before the dam overturns. Let’s make a simple shift. Just move the weights further upstream and try again. It’s not a big difference. The reservoir reaches about three-quarters the way up before we see a sliding failure, but shifting the weights did increase the stability. And this is why a lot of gravity dams have a fairly consistent shape, with most of the weight concentrated on the upstream side, and usually a sloped or stepped downstream face. Interestingly, you can use the force of water against itself in a way. Watch what happens when I turn my little model around. Now the hydrostatic pressure applies both a destabilizing and stabilizing force, so you get more resistance for a given depth. A lot of deployable temporary storm barriers and cofferdam systems take advantage of this kind of configuration. You can imagine if I extended the base even further, I could create a structure that was self-stable just from its geometry alone. The weight of the water on the footing would overcome the lateral pressure. But there’s a catch to this. This is fully stable now, but watch what happens when I give the dam just a bit of a tilt. All of a sudden, it’s no longer stable. This might seem kind of intuitive, but I think it’s important to explain what’s actually going on. Hydrostatic pressure from the reservoir doesn’t only act on the face of a dam. With smooth plastic on smooth plastic, you get a pretty nice seal, but as soon as even a tiny gap opens, water gets underneath. Now there’s upward pressure on the bottom of the dam as well. If you’re depending on the downward force of a dam from its weight for stability, it’s easy to see why an upward force is a bad thing. And it’s so dramatic in the example with the upstream footing specifically. In that case, the downward pressure of the reservoir is acting as a stabilizing force, but if water can get underneath that footing, it basically cancels out. The pressure on the bottom is the same as the pressure on the top. But this isn’t only an issue in that case. The ground isn’t waterproof. In fact, I’ve done a video all about the topic. Soil and rock works more like a sponge than a solid material, and water can flow through them. That’s how we get aquifers and wells and springs and such. But it’s a problem for gravity dams, because water can seep below the structure and apply pressure to the bottom, essentially counteracting its weight. We call it uplift. Looking back at the cross-section, we can estimate this. Of course, you have the triangular pressure distribution along the upstream face. But at this point you have the full hydrostatic pressure also pushing upward. And at the downstream toe, you have no pressure (it’s exposed to the atmosphere). So, now you have a pressure distribution below the dam that looks like this. Of course, this part can get a lot more complicated since most dams don’t sit flush with the ground, and many are equipped with drains and cutoff walls, so definitely go check that other video out if you want to learn more. But let me show you the issue this causes with some recreational math on our cross-sectional slice of the dam. The taller the dam, the greater the uplift force. That happens linearly. In other words, the force is proportional to the depth of the reservoir. But look at the lateral force. Again, remember it’s the area under this triangle. Maybe you remember that formula: one-half times base times height. Well, the height is the depth of the water. And the base is also a function of the depth. More specifically, it’s the unit weight of water times depth. Multiply it together, and you see the challenge: the force increases as a function of the depth squared. So for every unit of additional height you want out of a gravity dam, you need significantly more weight to resist the forces, which means more material and thus a lot more cost. Hopefully all this exposition is starting to reveal a solution to this rapid divergence of stability and loads as a reservoir increases in height. Dams don’t actually float in space like my demonstration and graphics show. You know, by necessity, they extend across the entire valley and usually key into the abutments on either side. Naturally, that connection at the sides is going to offer some resistance to the forces dams need to withstand. And if you can count on that resistance, you can significantly lower the mass, and thus the cost, of the structure. But, again, this gets complicated. Let’s go back to the demo. Now I’m going to replace my gravity dam with something much simpler. Just a sheet of aluminum flashing, and, to simulate that resistance provided by socketing the structure into the earth, I’ve taped it to the bottom and sides… with some difficulty, actually. When I fill up the reservoir with water, it holds just fine. There’s a little leaking past my subpar tape job, but this is a fully stable structure. And I think the comparison here is pretty stark. When you can develop resistance from the sides you can get away with a lot less dam. But it’s harder than you might think to do that. For one, the natural soil or rock at a dam site might not be all that strong. The banks of rivers aren’t generally known for their stability, so the prospect of transferring enormous amounts of force into them rarely makes a lot of engineering sense. But the other challenge is in the dam itself. Take a look back at this demo. See how my dam is bending behind the force of the water. It’s holding there, but, you know, we don’t actually build dams out of aluminum flashing. Resisting loads in this way basically treats the dam like a beam, like a sideways bridge girder. Except, unlike girder bridges that usually only span up to a few hundred feet, dams are often much longer. Even the stiffest modern materials, like prestressed concrete boxes, would just deflect too much under load to transfer all the hydrostatic pressure across a valley into the abutments. Plus we usually don’t like to rely on steel too much in dams because of issues with corrosion and longevity. So where a typical beam experiences both tensile and compressive stress on opposite sides, we really need to transfer all that load, creating only compressive stress in the material. I’m sure you see where I’m going with this. How have we been building bridges for ages from materials like masonry where tensile stress isn’t an option? It’s arches! The arch is a special shape in engineering because you can transfer loads by putting the material in compression only, allowing for simpler, cheaper, and longer-lasting materials like masonry and concrete. You basically co-opt the geology for support, reducing the need for a massive structure. For completeness’s sake, let me show you how it works in the demo. I’ve formed a little arch from my thin sheet of aluminum. Now when I fill up the reservoir, there’s no deflection like the previous example. And again, side by side, it’s easy to see the benefits here. You get a lot more efficiency out of your materials than you do with an earthen embankment dam or a gravity structure. Of course, there are some drawbacks here. For one, arches create horizontal forces at the supports called thrusts that have to be resisted. Sites that use this design really require strong, competent rock in the abutments to withstand the enormous loads. And just like with bridges, the span matters. The wider the valley, the bigger the arch needs to be, so these dams generally only make sense in deep gorges and steep, narrow canyons. The engineering is a lot more complicated, too. You can’t use a simple 2D cross-section to demonstrate stability. The structural behavior is inherently three-dimensional, which is tougher to characterize, especially when you consider unusual conditions like earthquakes and temperature effects. And since they’re lighter, arch dams don’t resist uplift forces very well, making foundation drainage systems more critical. All this means that it’s really only a solution that makes economic sense in a narrow range of circumstances, one of the most important being height. For smaller dams, the additional complexity and expense of designing and building an arch aren’t justified by the structural efficiency. Gravity and embankment dams are much more adaptable to a wider range of site conditions. And there are other types of dams, too, that blend these ideas. Multiple-arch dams use a series of smaller arches supported by buttresses, dividing the span into more manageable components. Even what is perhaps the most famous arch dam in the world - Hoover Dam - isn’t a pure arch structure. Technically, it’s a gravity-arch dam, meaning it resists part of the water load through mass while also distributing the forces into the canyon through arch action. The proportions are carefully balanced to take advantage of the unique site conditions and relatively wider canyon than most arch dams are built in. And so, when you look at the tallest dams on Earth, one structural form dominates. By my estimation, around 40 percent of the tallest 200 dams in the world incorporate an arch into their design. There aren’t that many places where it makes sense, but when you compare what it takes to hold a reservoir back in a narrow canyon valley, I think the case for arches is pretty clear.