A Tale of Two Phases and Tech Inertia

What kind of power service is in the United States? You probably answered 120-volt service. If you thought a little harder, you might remember that you have some 240-volt outlets and that some industrial service is three phase. There used to be DC service, but that was a long time ago. That’s about it, right? Turns out, no. There are a very few parts of the United States that have two-phase power. In addition, DC didn’t die as quickly as you might think. Why? It all boils down to history and technological inertia.

Split Phase Power by Charles Esson CC-BY-SA 3.0

You probably have quite a few 120-volt power jacks in sight. It is pretty hard to find a residence or commercial building these days that doesn’t have these outlets. If you have a heavy duty electric appliance, you may have a 240-volt plug, too. For home service, the power company supplies 240 V from a center tapped transformer. Your 120V outlets go from one side to the center, while your 240V outlets go to both sides. This is split phase service.

Industrial customers, on the other hand, are likely to get three-phase service. With three-phase, there are three wires, each carrying the line voltage but out of phase with each other. This allows smaller conductors to carry more power and simplifies motor designs. So why are there still a few pockets of two-phase?

When Electricity Was New

It is easy to look back and realize that AC power transmission has advantages and why three-phase is used. But back when electricity was a new service, none of these things were obvious. Edison, Tesla, and Westinghouse famously battled between using AC and DC current. Back then, AC didn’t mean three-phase AC, though. Two-phase, where the phases were 90 degrees apart, was an easier system to analyze and generate. The famous generators at Niagara Falls, for example, produced two-phase. You can see ten 5,000 HP generators at the falls, below.

It was 1918 before mathematical tools for dealing with polyphase AC readily came about. By then, two-phase was pretty well entrenched. In many cases, once the superiority of three-phase was realized, things were just rewired. But high rise buildings were not always easy or practical to rewire.

Big City, Old Power

This was a similar situation with DC power. Did you know that Con Edison — New York City’s power company — still provided DC to some buildings until late 2007? Even then, the buildings didn’t switch everything to AC. They just installed converters so the DC motors that run infrastructure like the elevators didn’t need replacing. The conversion to AC started in 1928 and was supposed to take 45 years. Like most projects, it ran long and took nearly 80 years.

In the case of two-phase, though, there are still pockets of it in Philadelphia and Hartford Connecticut. This makes being an electrician in those cities a bit interesting and you can find services advertising their mastery of two-phase work. Incidentally, there are some breathtaking photographs of Philadelphia’s early twentieth century infrastructure. Take a look a the book Palazzos of Power: Central Stations of the Philadelphia Electric Company, 1900-1930.

You might wonder if the power companies in those two cities actually still maintain two-phase generators. As far as we can tell, no. They just convert from three-phase to two-phase using a Scott-T transformer (named after [Charles F. Scott] who worked for Westinghouse). You can see a typical configuration here.

March of Progress

We think of the march of technology as progressive, but it is amazing how many things hold on because of historical precedent. We still have AM radios, for example. My desktop computer can still boot MSDOS. There’s a lot of inertia even as new tech pushes out the old.

Why 120V? Because Edison’s first generators produced 110V (although, in fairness, 110V DC). After World War II, the nominal voltage kept creeping up until it settled on 120V by 1967. In 1899, a power company in Berlin decided to switch to 220V to increase its ability to distribute power. This took over Europe where 230V (raised up from 220) is the usual voltage.

Thanks to [Tom Frobase] who lived in Pennsylvania for suggesting this topic

97 thoughts on “A Tale of Two Phases and Tech Inertia

    1. While I love traveling around Europe I don’t do any line-related electronics work over there, so I didn’t know ;-) I can barely sort out all the plugs in my kit ;-)

      1. Something I have been curious about, does the average residential unit have just a three wire service, hot, natural, ground?
        Here in the US most homes get 4-wire split phase power so you can get either 240V pole to pole or 120V pole to neutral, as the article says. I have always wondered if you could get 400V or 460V out of the panel your average house or apartment building would have.
        I work on cruise ships built in Europe and they actually commonly use delta power rather then wye power. So your average 230V outlet like you’d have in your cabin actually have two hots at 127V relative to ground; two of three phases.

        1. For the curious: ships use delta power this way because in the old days water-tight cabling was rather poor and unreliable. So from my understanding it was considered a safety feature to not have the power have a path to the hull. They would meter each pole relative to the hull, and if they saw any current flow, they knew they had a short to the hull somewhere. This practice stuck with ships even to today, though slowly many seem to be moving to wye.

        2. The average residential unit has normally a 2-wire service. Line and neutral and a local ground. On some zones there’s a 230V delta so one has live,live and local ground. Some industrial areas have a 230V delta, but normally use a 230V wye. A car maker factory in Turin has both in the same buildings. Electricians are thrilled
          Another interesting thing is that in Italy there was in the ’60 a dual voltage distribution, this is why you find 10A sockets and bigger 16A socket, they were at a different voltage, like 220V and 110V like in the US. Now they are all at 230V but sometimes there’a a separate GFCI for them, so in case of a short of the dishwasher one has still the lights on. Nowadats especially in office and norhern Italy you’ll find 16A Shucko sockets.

      1. They still do. The 230V/400V is to 10%/−6% which means from 216 V to 253 V for single phase.

        In other words, they standardized the EU grid voltage simply by grandfathering all the national standards in and telling the appliance manufacturers to deal with it. Nobody actually had to change anything, and as a consequence the voltage may vary significantly from country to country, and within countries.

        It does wonders to ESL bulbs in the countryside when the voltage fluctuates by 20-30 Volts due to the relaxed regulation.

        1. Sorry, the nominal EU standard is 230 V +-10% which means from 207V to 253V. Apparently some areas in the UK still use 250V power, and the low 207 V limit is for countries that used to have 220V so they could suffer some transmission losses over long lines.

          So, in the worst case you can get 46 volts of sway in the line before the power company has to look into the issue.

          In comparison, the US standard is 120V +-5% from 114 V to 126 V

        2. Hi, I work in UK power distribution. This voltage variation also now gives us some wiggle room. to distribute power that is now being generated locally (Solar Farms, etc) , we use auto-tapping transformers on our primary substations.

          buy varying the voltage a lil bit in certain areas, or everywhere at once. There is less power demand and we can eek out our on-demand generating capacity to the max. this is because as were having trouble building proper power stations these days and everyone thinks solar will help. even in north England

        1. The switch in Europe to 230V start in the second half of the eighties, not 2003…
          With a transistion period with asymetrical tolerance (-10/+6% for country switching from 220V, -6/+10% for country switching from 240V) but the goal was 230V

      1. Actually figures just came from my experience in France (measured consistently over time in different area). I checked and it is indeed advertised as 230V/400V 50Hz. The figures observed are within the french regulation:
        “As required by law, the electricity delivered to your premises through the
        distribution system will be supplied: (a) if you have a single-phase supply, normally
        at 230 volts declared alternating voltage, with an allowed range of voltage
        variation from plus 10% to minus 6%; (b) if you have a three-phase supply,
        normally at 400 volts nominal alternating voltage, with an allowed range of
        voltage variation from plus 10% to minus 6%; and at either of the above voltages
        at a frequency of 50 hertz, with a permitted nominal variation of plus or minus
        1%.”

        My takeaway is not to assume the main voltage as the general assumption includes a broad acceptable range.
        YMMV.

        1. If you set a range you obviously aim for the centre and allow for unwanted drift either way
          You don’t set it at the top of the 10% margin since then any drift upwards will go over the margin (and people will massively sue you for damaged devices).

  1. In an industrial enviroment in the UK 125A 3 phase is quite common using connectors like this:
    https://www.stage-electrics.co.uk/shop/sales/connectors/power/400volt-three-phase/125a-red-3-phase
    Quite a lot of generator sets have power lock connectors which do up to 660A, 1000V:
    http://www.ittcannon.com/p/549/powerlock/
    Houses usualy have single phase 230V 60 – 100A. Although I have lived in one that had 3 phase with different areas on different phases.

  2. I know you wrote “We still have AM radios, for example” with the idea of commercial radio. But despite that AM still has a huge advantage over FM for aviation. There AM-radio is preferred over FM not because of quality but safety. Why you may ask? Well hold on, I’ll explain.
    In case of 2 airplanes transmitting on the same frequency, the stronger signal would win when both transmit using FM. But when both airplanes transmit using AM the two signals would mix. and both transmitters could be heard. Sure quality suffers, but quality isn’t the issue here.

      1. They were more effective as a telephone as well. Considering nobody uses a modern phone anymore to speak directly into it. Because somehow these things seem to be designed for text messaging through popular apps. Though for some reason a modern phone may not be suited as a hammer any more… from time to time I do like to hit it as the darn thing seems to be busy with many things except responding to my virtual button presses.

        1. And there was that anecdote of the police officer that often used his Motorola handheld as a billy club.
          It was already in his hand, so the clubee had no idea it was coming, and it was as strong as a billy club too!

        2. I have VoIP service at home and I prefer to use the landline when I am home, using a 1990’s phone (this one if you’re curious https://it.wikipedia.org/wiki/Sirio_(telefono)) because the sond quality is better and the overall construction is way better than the average phone: if falls on the floor there’s no problem, and there are phones like this still in use after 30 years. And the user interface is simple, reliable, and I can answer calls in the dark, and even make them.

      2. While your comment is funny, using AM for aircraft communication is vital for aircraft safety . Even if both transmitters are not understandable because of interference, it is easy for the listener to know there is interference, and can notify an aircraft to change frequency or stop transmitting.

        As explained in my “ancient” electronic text…
        An FM transmitter needs only to have twice the receiving power over another transmitter to completely override the other transmitter’s signal. With AM, a transmitter being received at only 5% of the power that another transmitter being received, is enough to interfere.

          1. Digital packet radio doesn’t degrade gracefully. It goes from working to “what?!” in an instant.

            Same reason why DAB never took off. You drive under a bridge or round a tall building and the radio cuts off, whereas with regular AM/FM would hiss and crackle but you’d still make out the commentator’s voice.

          2. Also, the packet radio errors are mode annoying than on analog. You get this chirp-blurb-stutter-blubber that completely masks the transmission when the signal is getting closer to the digital cliff where nothing gets through.

            It’s this effect which bit the DVB-T networks as well, because the marketing spiel was that digital television can do with lot less transmitting power since it’s more robust to errors, but because it has this rapid fall-off in reception quality with catastrophic consequences when the error correction fails, it actually needs more transmitting power to ensure that random dips in the S/N ratio don’t lead to peoples TV sets hanging with a frozen picture or green squares all over every few seconds.

            I mean, when the wind is shaking your antenna you got a bit of snow on an analog TV. WIth Digital TV the entire program becomes unwatchable because it skips and stutters, and eventually your decoder crashes and you need to reboot your TV.

      3. In the ’60s one wouldn’t use telephone receiver as a hammer because we would be criminally charged with destroying private property. Today our tempers are tempered by the fact our phones phone are our property paid for out of pocket

    1. I wouldn’t say the quality is bad on air band… AM may not be as high fidelity as WFM, but compared to straight FM it’s at best on par, and in marginal conditions, can be significantly better. It will do much better range than FM.

      SSB is pretty good too in the VHF. The Gap to Ipswich is a distance of roughly 40-50km. From my bicycle transmitting 30W FM through a ¼wave whip, between 144-148MHz, I can just make it over the hills. I’ll be practically unreadable at the other end. SSB however, that same 30W gets through just fine. As a bonus, I can wind the radio up to its full 50W because my license, whilst restricted to 30W average power, will allow me to transmit up to 100W peak.

      AM would suffer from the fact that the carrier absorbs most of the power, but would still work quite a bit better than FM.

    1. Thanks to the Americans and British splitting the job of post WW2 electrification of Japan. Thus any line frequency dependent appliance for the Japanese market must have a 50/60 Hz switch or be able to automatically detect and adapt.

    2. Or 200v 2-phase/split phase.

      I got a 200v 100amp (50 amps per phase) supply to my house when we built it to allow for burning hot metal in my future workshop.

      I’m used to the 230v single phase / 415v three phase in Australia though.
      So much power for activities when you have 415v in your shed… :)

      1. I can just hear an old timer telling a newbie,
        “Yeah, we keep the electricity in those vaults over there. About once a year we have to open them up, and swap out the old electricity for new stuff.”

    1. That was my favorite of the pictures too. Looked like bank vault doors. I wanted to include some of the pictures, but the rights were not clear so I figured the link would have to do.

    2. People had more common sense back then, knew not to touch things like exposed connections carrying lots of power. Units like that if built today would be all enclosed, a blank face with nothing but a wheel and digital display. Workers assigned to them might not even have a clue what’s inside.

      1. The reason why the modern units are enclosed is to stop birds and wrenches from flying into the equipment. Saves a whole lot of trouble and cleanup/maintenance work.

        If it was just for worker safetly, they’d rather put a mesh fence around it.

    1. Nope. Nominal 120V for many years. The allowable variance is 5% (or +5%,-13%) as per ANSI C84.1, and is generally at 120V or a bit higher when the power company can manage. If it sits more than a couple percent lower or higher routinely, that is a design or implementation issue, and needs to be addressed.

      Nameplate voltage on devices is typically lower (115V or, on older gear 110V), with a typical +/-10% allowable, since the voltage at the point of utilization is never higher than the supply, and, in typical applications, will be a bit lower due to line and connection losses. The 1500W heater in my shop came with 16AWG cord, which gives a drop of nearly 1V, and about another 0.25V at the stabs, worst case. Similar losses in a 25 foot run of 14AWG (15A circuit) or a 40 foot run of 12AWG (20A circuit) lead to lower than nominal at the outlet.

      Since so many devices are effectively constant power (a resistive heater being the exception these days, unlike in the days of incandescent lighting), most suppliers work hard to keep the service voltage at or a little above the nominal.

  3. Back in the 70’s there was still DC in some of the outlets in the Pfister hotel. Plug in an overhead projector or slide projector and it would light up fine. The fan would not do anything, and the projector would overheat. We lost a few projectors that way. Ah the memories.

  4. Of course, the other thing with the Falls is the 25Hz power. This is still used in some applications (electric railway, for example). For the most part, this is produced using converters from 60Hz supply these days.

      1. There were several industrial plants in Niagara Falls Ontario that still used 25Hz power when I was growing up there circa 50 years ago – if memory serves, two of the companies were Norton Abrasives and Lionite, which also made abrasives. So perhaps your comment about aluminum smelting is right, given the use of aluminum oxid in abrasive products. And before that, 25Hz power was used in homes in the Niagara area. We still had a few appliances that had been converted from 25Hz to 60Hz – IIRC the motors were replaced, although it’s entirely possible they were simply re-wound. Our old tank of a GE fridge started out its life as a 25Hz appliance before I was born, and it was still going strong on 60Hz power when I was past 30 and my dad sold it.

        1. Forgot to mention the stories I heard as a kid, about how badly the lights used to flicker in the 25 Hz days. Keep in mind that we’re talking about incandescent bulbs here…

          1. Maybe a grampa-story, because old style carbon filament bulbs have too much mass in the filament to respond to 25 Hz. It takes about half a second for the bulb to light up in the first place.

            Flickering only really became a problem in lightbulbs when they started using the double-coiled tungsten filament, because the filament is so absolutely thin that it loses heat instantly, which is why it’s double coiled to conserve heat. In the older bulbs with straight wires, the filament was under tension to prevent it from sagging onto itself, and that required stronger and longer filaments, which consequently took time to heat up to a glow.

            Most likely the flickering was just poor power regulation.

  5. A little quibble, the last of the DC was decommissioned around the new millennia. At least the stuff in NYC (which I think was the last station). For some that might be a long time ago but I’m not sure that’s the meaning you gave it.

    1. Stripped a 2phase motor from a feed mill on a farm here in the UK about 10 years ago. It was in use until around a decade before that (same farmer for the last 60 years).

    2. Two phase power is used in Europe but it is a safety measure on (some) building sites, mines etc. The 230 V single phase supply is reduced to 120 V center tapped (60 0 60) and the center tap connected to ground. Power hand tools working on 120 V are easy to find and in case of damage to the power cable the highest voltage to ground is only 60 V.

    3. Belgium has madness and still has old 132/230V networks going. But there is no null provided, so you can only use the 230V phase-phase voltage. Of course, they do actually have wye/star transformers with a null tied to ground. So meters have to meter all phases even if only two are provided residentially as people have used American devices (or transformers) between phase and ground with those setups.

  6. Here in Canada, sometimes houses are wired with the 2 phases 120V each on a regular 2 outlet wall plate (mostly in the kitchen). With a frankencable (2 plugs which use only the hot wires are connected to an European receptical Schuko type, earth ground connected as well of course) European 240V appliances e.g. a good vacuum can be used here. The 50 Hz motor might be complaining a bit about the 60 Hz.

    1. If you already know it, then reading it will bea usefull refresher course. If you didn’t know it, then you should educate yourself in basic power distribution engineering, and if you just don’t find these things interesting -always keep one hand in you pocket when playing with high voltages!

  7. “Why 120V? Because Edison’s first generators produced 110V (although, in fairness, 110V DC).” That is a pretty light weight reason. How about the AC is 120V RMS to provide the same power as DC? It peaks much higher. And everyone used to call it 110.

  8. we run 240V in most of Aus, the standard for my state is nominal 240v, lower 216v, high 264v. I’m normally about 250v on a single phase – but have three phases onto the house (which is good for the garage :-) )
    I always find it a bit strange that the usa has such low voltages, which means everything they plug in has to take so much more current…

    1. I wonder if there is a statistic that tells the electrocution tale, AUS vs North America? Contact with 120V vs 240VAC. Either can be lethal, especially with wet fingers. But my experience is that I’m still alive after getting buzzed a few times with 120V, starting with the old fork in the outlet trick and then later in the electrical trade with hands where they shouldn’t be (its rude to point).

      1. you have a point – 120V is much safer. But then again, it is not as useful, and all your motors etc have to be able to handle twice the current – which is a lot harder than twice the voltage!..
        We are bought up to respect the power coming out of the socket, I’ve seen lots of people in the usa treat it with indifference – I’ve even seen bare wires running the mains power… Nobody, who doesn’t want to commit suicide, does that in Aus..

  9. Here in my part of Kansas USA, split phase service is 240 V.\120 V., Three phase service 440 Volts. In the county west of me the 3 phase service is 762 V., with the split phase 240/120. Last time I had a need to check the voltage in my home they where higher than nominal, but still in tolerance.

  10. As long as 3-phase discussion is on the table…

    A farmer friend of mine used to wire up his 3 phase motors using a socketed light bulb with a couple of test leads.
    He said the “wild phase” was when the bulb shone brightest between two of the wires, and swapping those two around would change the direction of the motor.
    I only had a RadioShack analog meter at the time, and wasn’t eager to be using it to read 240 (or whatever) volts…

    Does someone have a “better” (i.e. a bit more technical) explanation of what he was talking about?

    1. wild phase is when you don’t have real 120 volt source on a delta wired 3 phase system. in a wye config you can just get a leg to ground to get your 120 from a phase to ground, but a delta may not be ground referenced. So you ground a center tap of one of the phases, shazam you have 120 avail for lighting in an industrial environment. however now one of your phases is fun and exciting, hence wild or high leg delta

  11. Why do I still have a landline? Because when the power grid goes down all those electronic phones lose power, the battery operated cellphones stop working because everyone is calling simultaneously and the cell phone system isn’t designed to handle that volume of traffic. My landline needs no power from the grid because the telephone provider still maintains a 48 VDC system to power landline phones. They will have to pry my “old” telephone from my cold hands when….

    1. Cherish it while you can, our service has switched to fiber which means that the power for the “land line” now comes from a minuscule UPS where the the data and phone lines split. Copper land lines are disappearing fast in the US.

  12. I understood when a home is wired for 240volts that some recepticals are on one phase delivering 120volts and other were on another phase. Is this wrong? Also wasn’t the 60cycles set and before atomic clocks, utilities had to guarantee 60cycles plus or minus a small amount to keep clocks on time. Was this true?

    1. In the USA, 240 split phase is delivered to most houses. The Breaker Panel typically has two rows of breakers,
      those on one side of the panel use one phase, and those on the other side use the other phase. 240 volt breakers
      latch onto both feeds.

  13. The nice thing about U.S. split phase is that you can run one length of 12/3 cable to a duplex outlet to get two separate 20 amp circuits (cut the tab on the line side of the outlets, obviously). Very handy in. say, a kitchen, where you plug in lots of high-power gear! Because the two hots are out of phase, the 12AWG neutral lead never sees more than the 20A that it can handle, no matter what the load balance between the two hots. You’re required by code to use a tandem circuit breaker if you do this, just as you would for a 240 volt outlet, but they don’t cost much more than a couple of single breakers. I’ve done many of the outlets in my house (especially in the workshop) this way. It confused my dumbass electrical inspector at first, but I showed him why it works and he accepted it. As a bonus, you can wire a 240 volt outlet to the same cable and mount it in the same junction box, but good luck finding a cover plate for this arrangement (someday I’m going to laser cut some for my combo outlets, but for now I just don’t use covers.)

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