240V is the Standard U.S. Household Voltage (Kinda.)

OK, So I’m Being a Bit Facetious

Obviously I and most everyone else knows that the vast majority of American household stuff is powered at 120V. Almost all receptacles and (nearly) all lights in a home are indeed supplied at 120V.

But! It’s not as simple as that. So maybe the video title is a tiny bit of clickbait, but it’s also more or less true. Most Americans do indeed have 240V supplied to their home, and that is the line-to-line voltage. The transformer is rated for 240 Volts with a center tap that happens to be referenced to ground/earth, and it just so happens that the potential difference between the center tap (ground) and either of the two lines is 120V RMS.

Full(er) Transcript

What follows is the original script I wrote for the video. Everything you heard in the video is in the script, but I cut some of the more tangential stuff.

So if you’re interested in every last bit of what I could have said, read on…

Hi everybody, I’m Scott and in this video I want to talk about a slight misconception regarding electrical service in the U.S.

I watch a lot of YouTube videos about vintage and modern tech, by people both here in the U.S. and abroad. When talking about powering those devices, the consensus seems to be that in the States we have a 120V electrical system, with 220 to 240 Volts in Europe and 100 Volts in Japan, just to name a few.

So it might surprise you know that standard household electrical service here in the U.S. is 240 Volts, not 120.

Or, rather, not just 120.

(And in actuality it’s more like somewhere between 220 and 250 Volts, and 110 and 125 Volts respectively.)

Though I’m sure many people here already know this, I figured this subject might be of particular interest to people in other parts of the world.

So, let me show you my electrical service.

In my area we obviously have overhead power lines, which I’d say represents the majority of local delivery systems here in America. Many neighborhoods do have underground wiring though, but you’ll usually find those in warmer climates such as in Florida or California.

Underground wiring doesn’t play nicely with the freeze/thaw cycles seen here in New York where temperatures can go anywhere from -10 to 110 degrees Fahrenheit (-23 to 43 degrees centigrade).

Yes, there are buried power lines in my area too, but they’re definitely not typical, and generally speaking maintaining them is far more expensive than overhead distribution (and far more disruptive as it would require digging up a street rather than simply bringing a bucket truck.)

Here’s what you’re looking at: At the top is the primary distribution circuit, and those can operate at a wide variety of voltages. I can’t measure mine for obvious reasons, but anywhere from 2KV to 14KV is probable.

As you can see, there’s only one wire supplying us with power. The system voltage is referenced to ground, so the actual Earth is effectively the return conductor on the circuit. That’s a pretty common arrangement the world over.

The primary line connects to this pole mounted transformer, which steps the voltage down to something that’s usable in the home: Namely 240 Volts.

This transformer is shared by various houses in the neighborhood, with the 240V secondary run along here, to the right of the transformer. It’s then tapped anywhere a house needs service.

My house appears to tap power directly adjacent to the transformer, but this cable actually originates a few houses down that way.

My service drop runs from that tap point over my yard and into a weatherhead, then down this conduit to our meter.

The wiring continues on from the meter to what’s commonly called the service panel, breaker box, or circuit breaker panel.

I feel like the breaker panel should be a topic for its own video, and though there are a wide variety of brands and models of panels in the U.S., this is undoubtedly the most common in general appearance, at least for post-1960-ish homes that have a 200 Amp service (which is the common amperage for service to most detached homes).

For this video, the important thing to note is this switch at the top. That’s the main breaker that can cut power to all other circuits in the panel.

And with the cover off, you can see the two connections running into that breaker. Those are the wires coming from the meter outside.

If you didn’t believe me when I said that most homes in the U.S. are fed at 240 Volts (or thereabouts), here’s the proof.

OK, at this point I feel like I’d be remiss if I didn’t address the elephant in the room: Messing around in your service panel is dangerous. Especially if the panel is powered. And most especially if you’re shoving metal electrodes into the main terminal lugs, because those are powered directly from the street and have no meaningful overcurrent protection.

In other words, if my test lamps here were faulty or if I simply slipped and managed to create continuity between the two service lugs (or between one of them and ground), I could be electrocuted or start a very problematic house fire. Or both!

So what I’m saying is don’t try this at home, even though I’m being completely hypocritical because I’m quite literally trying this at home. Just be aware that it can kill you and your whole family.

Anywho, getting back to the topic at hand: 240 Volts.

OK, I’m being a bit sneaky by repeatedly mentioning that 240V is our standard household voltage. While that is indeed the voltage being supplied by the transformer, the vast majority of electrical outlets in an American home supply power at 120 Volts, as is well known.

So, what’s going on? Well, I purposely forgot to mention the neutral wire.

Because of the way the transformer behind my house is oriented, you can’t see the connection points for the low voltage side. But this is pretty much the same type of unit, and you’ll see that there are three secondary connections. Those are generally referred to as L1, N, and L2, for Line 1, Neutral, and Line 2.

L1 and L2 are both live (also called “hot”) conductors, while N is a grounded conductor. (Note that it’s not the grounding conductor, which in the house is always separate from the neutral – except in the service panel. Again, I’ll need to do a whole video on the panel at some point.)

L1 and L2 are out of phase with each other by 180 degrees, meaning their waveforms look like this.

The RMS (root-mean-square) difference – the way A/C voltage is typically measured – between these two waveforms is 240 Volts.

However, the difference between L1 and N is 120V RMS, as is the difference between L2 and N.

Let’s take a look at this much smaller AC transformer that’s rated for a 120V primary and a 12V secondary. In other words it’s a step-down transformer for operating 12V equipment such as relays or lights.

On the primary side it has two connections: One for live and one for neutral. This is just like the transformer behind my house, where the live conductor was operating at multiple kilovolts, and the neutral was quite literally the ground beneath it.

On the output side there are three connections, just like it’s bigger brother. We could call them L1, N, and L2.

So, let me connect this to a supply voltage and do some probing.

Again, like a scaled down version of its bigger brother, the two outer terminals measure 12V relative to each other.

If I measure from either of the outer lugs to the center lug, it’s 6V!

The oscilloscope also shows that, just like the 240 volts coming into my house, there are two sine waves that are 180 degrees out of phase with each other when measuring 12 volts on the outer two connections.

From either of those connections to the center tap there’s only a single sine wave.

And also like the primary side of the transformer outside, this transformer only has a single phase at 120V relative to neutral/ground.

As much as I’d love to take that transformer apart, I kinda need it, so let me just show you a diagram of what’s going on here:

A transformer like this consists of two coils of wire wound around a common core. A difference in voltage between one side of the transformer and the other is created by varying the number of times the wire is physically wound around the core on both sides.

The ratio of turns on one side versus the other is the same as the ratio of the voltage on one side versus the other.

So If both sides have exactly the same number of turns, the input and output voltage will be exactly the same. If one side has twice as many turns as the other, that side will have twice the voltage as the other side.

This transformer, to step down 120V to 12V, would have 10 times as many turns on the 120V side as the 12V side. Just for the sake of example, let’s say there are 1000 turns on the 120V side and 100 on the 12V side. What if we took a wire and soldered it right here, and then measured the voltage between these two points?

Well, in the space between those two connections there are 50 turns. The ratio is then 50 to 1000, and so the voltage here will be 20 times lower than the voltage here. Hence when I supply the transformer with 120V, you can see 6V!

And this is inherently bidirectional in principle: I could feed 12V into this transformer here and measure 120V here.

In other words, a simple transformer like this can be used to either step-up or step-down voltage. In fact, it’s the same for the transformer on the pole.

I should also note that in the real world transformers are not nearly as simple as I’m making them sound, but the general idea is correct.

And that’s how residential electrical supply in the US works.

It’s called a split phase system, because the single phase coming from the distribution grid is effectively split into two by the center tapped transformer.

The upshot of all this is that I have both 120V and 240V outlets in my house, as with most houses.

240V is widely used, but mainly for large permanently installed equipment like hot water heaters, air conditioners, clothes dryers, stoves, ovens, hot tubs, electric car charging and even whole-house heating. Basically in any situation where a lot of power is required.

However, you can call an electrician and have them wire a 240V outlet anywhere you might need it.

The only sticking point is that if you’re looking to run, for example, vintage European 240V computers, they may or may not be compatible with our 60 Hertz system. That being said, for equipment that solely uses DC voltage internally, the capacitors on the low voltage side would probably have an easier job of maintaining charge at the higher frequency. I think it’s potentially more problematic the other way around, using equipment rated for 60Hz on 50Hz systems.

But don’t take my word for it, double check that for your particular device before applying power.

As a side note, it’s pretty common (though far from universal) for US households to have a natural gas connection. I have one, and so therefore my water heater, clothes dryer, stove, oven, and so-called boiler are all gas-fed. That leaves a nearly ridiculous amount of amperage available for things like computers and electric cars.

So I hope you found that interesting. Split phase power has its advantages and disadvantages, and of course there’s a lot more to it than what I’ve outlined in this video. But the main advantage for me personally is that I can operate random 240V electronics at home, as well as all the usual American 120V stuff. In fact, most of the UPSes and servers here in my basement are running at 240V!

I’m getting off on a tangent here, but you may have noticed that this is decidedly NOT a smart meter, and though it’s a pretty old-school design these are still quite common here in the States. It does actually require a person to come and read it manually.

Where I live, we’re billed monthly, but the meter is only read every other month. The power company estimates usage for billing in the intermediate month, though I could manually report my meter reading if I was a stickler for paying the exact amount.

My power company (as many do) offers something called balanced billing, where they charge a fixed amount every month based upon your previous year’s usage patterns. That can be very helpful if, for example, you have a gas heating system and therefore use very little electricity in the winter, but have the air conditioning is running all summer. It wouldn’t be uncommon to have a bill four or five times the cost in the summer versus the winter, so that can help quite a lot when it comes to cash flow.

Well, that’s it for now. If you enjoyed this video, please subscribe and do the liking thing and all the regular YouTube outro stuff. You can also examine my website at s.co.tt, which is totally a real URL.

I tried whenever possible to make it clear that I was talking about the most common way in which power is delivered to an American home. There are exceptions, of course.

For example, if you live in an apartment in the US your building may be fed by three-phase power, which is quite standard for commercial and industrial properties. In that case you, as the tenant, will most likely only have access to 120V circuits and there probably won’t be any 240V circuits available anyway.

Even if your building is fed by split-phase power, your landlord might have no need to supply your apartment with any 240V circuits.

Power delivery in rural areas can be a little weird, so for all I know there might be single phase 120V systems out there.

And finally some older homes may still have single-phase 120V distribution panels (which are probably fuse boxes), even in neighborhoods where split-phase power is otherwise in place.

Also this video is intended as a general interest thing, and so don’t take my word as absolute gospel on any of these issues. I don’t have the capacity to bestow any magical powers of electrical licensing on any of you.

About Scott

I'm a computer guy with a new house and a love of DIY projects. I like ranting, and long drives on your lawn. I don't post everything I do, but when I do, I post it here. Maybe.
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