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 [music] 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 to 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
[00:01:00] and where do any of these actually get used? Well, I've spent [music] 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 [music] 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 [music] a coalition of companies making electrical equipment. Their members realized that life would be better with [music] some standards so that any company making an electrical device could be reasonably confident [music] that the people who might want
[00:02:00] 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 >> [music] >> and it's far from a perfect system. My friends Medhi 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.
[00:03:00] These numbers are a bit arbitrary, but they kind of represent a certain class of receptacles and plugs. [music] For example, NEMA 1 receptacles are rated for 125 volts >> [music] >> and have just two poles, a hot and a 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. [music] 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 [music] 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
[00:04:01] 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 [music] conductive elements and connecting them to ground make they somehow become energized, current [music] 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-V rating to handle the standard 120-V 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's bonded to the neutral line in the main electrical
[00:05:00] 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 [music] the switch on the hot wire so that nothing beyond the cord is energized until it's turned on. Enforcing polarity at the plug
[00:06:01] 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 V 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 [music] 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, >> [music] >> treadmills, copy machines, and power tools. Of course, we could just change
[00:07:00] the codes to require 20 amp circuits everywhere, but that has huge implications: [music] larger circuit breakers, heavier gauge wiring, and more expensive receptacles. And in [music] many cases, it's just not necessary. So instead, NEMA created a different receptacle and plug for 120 V 20 amp circuits, the 5-20. I have a bunch of these here in the studio. You can see they have that T shape on the neutral [music] slot. And 20 amp devices have the neutral blade rotated 90° on the plug. But here's the backward compatibility. Regular 15 amp plugs fit into the [music] 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 [music] availability of voltage. Today, split-phase electrical service is basically standard for residential power. You get 220-V hot lines, which can be used individually [music] for smaller circuits or combined to get 240 V for circuits that need more oomph. In
[00:08:02] the early 20th century, 240-V service wasn't always available, so you have these very high-current 120-V receptacles that could power heavy commercial cleaning equipment like floor burnishers and blowers, >> [music] >> 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 [music] V, so older models use 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 V. That brings us to NEMA 2. Like NEMA 1, these are ungrounded receptacles, but instead of a hot neutral, they have two hots. Each is 180° out of phase with its neighbor, so you get 240 V across them, handled with a little cushion by the 250-V rating. There were 20 and 30-A
[00:09:03] receptacles, but also like NEMA 1, these are mostly obsolete [music] now that a ground is required by code. They've been replaced with NEMA 6, which has 15, 20, 30, and 50-A receptacles [music] and plugs. Of course, with double the voltage, you also get double the power compared to the NEMA [music] 5 equivalents at the same current rating. The 6-15 is common for window or wall-mounted air conditioners. The 6-20 [music] 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 [music] these days for level two EV chargers capable of delivering nearly 10 kW of continuous power through the receptacle. [music] Like NEMA 5, the NEMA 6 has some backward compatibility allowing the 6-15 plugs to fit into the 6-20 receptacles. This is kind of clever, but it doesn't
[00:10:01] 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 [music] is safety. Circuit breakers are sized to protect everything downstream, including the plugs and appliance cord. If a thin cord on a low-current device develops an [music] internal short, the resistance of that thin wire itself will cap the fault current so that a larger breaker might
[00:11:01] take 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 [music] potential, literally, for arcs to occur. Equipment designed to handle the higher voltage needs better [music] 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
[00:12:00] 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. [music] 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-V 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
[00:13:01] warmers rely on 240 volts, while the fans and timers run on 120. Same with some job site 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 use 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 [music] to have three-phase service. It's particularly useful for devices that use large motors and of course, if you have
[00:14:00] the service, you're going to need the receptacles and plugs for those devices or at least the ones that aren't hardwired. NEMA 11 was the standard for up to 250 volts 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. Wood shop 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 [music] a specific piece of equipment. And since hardwiring 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 job sites, certain manufacturing facilities, and short life
[00:15:02] 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. [music] 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 [clears throat] 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 hardwiring. Commercial ventilation and air-conditioning systems [music] 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, [music] so
[00:16:01] 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 five-blade plug would be tricky and take a lot of force. And I've kind of buried the lead 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 easily be pulled out. Actually, the locking versions are more common than the straight-blade equivalents in many cases, especially when it comes to portable generators, job site equipment, and events where things are always moving around. If your vacuum cleaner unplugs itself cuz you've gone too far in the hallway, that's usually not a big deal, but if a three-phase 600-volt
[00:17:02] plasma cutter does the same thing, you can get some 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 Hz power [music] 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 V 30 amp receptacle before NEMA formalized its catalog 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.
[00:18:01] It's a face only an outlet enthusiast could love. I haven't really talked about the smaller versions of the locking connectors [music] 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. Receptacles and plugs may not technically be considered infrastructure, but I love them because they're our interface to it. I've covered a lot of topics on the grid, including some high-profile failures resulting from overgrown trees in
[00:19:01] transmission [music] line corridors. If you look at these on a map, they stand out for that exact reason. >> [music] >> They have to be kept clear of large trees that could cause a short circuit, and they have to be accessible for crews to do maintenance or repairs. But, that doesn't mean they have to be completely bare in stripes through the landscape. My friends at Planet Wild did a mission in Switzerland to prove it. Rewilding a 30-ha transmission line corridor to [music] improve biodiversity and create an insect highway. I've been a Planet Wild member for a year now, and I'm happy to have them as today's sponsor, because projects [music] like this are exactly why I joined in the first place. If you never heard of them, they're a not-for-profit organization focused on [music] environmental restoration, but they do things differently. Every month, their community [music] of over 25,000 members funds a mission related to protecting endangered species, oceans, or forests, and then they produce a video and
[00:20:01] release it on YouTube, so you can [music] see what your contributions actually achieve. They're one of my favorite partners. With Planet [music] Wild, I feel much more like a part of a group, and I look forward to the monthly videos when I get to see how my contributions are doing something positive. [music] They do a lot of infrastructure-related missions, like the one rewilding a transmission [music] line, and introducing beavers to a farm to convert a degraded stream [music] into better habitat for plants and wildlife. And they don't just document [music] each mission once, they go back and revisit later to give 1, 3, [music] and 10-year updates. If you've been looking for a neat way to give back, I think Planet Wild is a very cool way to do it. [music] And to prove it, for the first 100 people who sign up, I'll cover your first month. Just scan the QR code or click the link in the description, and use my code practical29 to get your first month free. No catches, you can cancel anytime, you can give whatever amount that feels right to you. [music]
[00:21:01] Check out some of their missions, like the transmission line rewilding video linked below, and I hope you'll consider joining. [music] Thank you for watching, and let me know what you think. >> [music]