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.
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