The short of this article is that U.S. transitioned from 110V to 120V due to safety, convenience, and practicality considerations. This negligible shift allowed for more power delivery without larger wires, reduced line losses, and improved compatibility with evolving appliances and lighting technology. The National Electrical Code (NEC) standardized the voltage in North America to 120V and 240V, reflecting the industry’s shift and ensuring compatibility across the country.
Why 60Hz over 50Hz? Why did Europe choose 50Hz and North America choose 60Hz? Well the 50Hz adoption goes back to Allgemeine Elektricitäts-Gesellschaft (AEG) decision in 1891 as various other systems used 40Hz or even as low as 25Hz. This decision was rooted in flickering caused as 40Hz and below. The prime mover would spin at a regulated speed of 3000RPM.
Westinghouse who was consulted by Nikolai Tesla used 60Hz in their power generation. This decision was also rooted in minimizing flicker, but when the difference in the prime mover speed of 3600RPM?
The technical tradeoffs – 50Hz would have less losses over long distance transmission compared to 60Hz – which one would think would be more important in the US with the size of the country (but I’ll get back to that in a moment.) Where 60Hz beet 50Hz is in the weight and size of generation and transformer equipment for the same power. New York City was the most population dense with 143.2/sq-mile in Manhattan to the Lower East Side at 335k/sq-mile compared to London topping out at 35.8k/sq-mile – this meant that there would be more economic advantages in the short haul cost associated with transformers – pushing 60Hz.
One conjecture I have which I cannot find proof of in the choice of 50Hz is in slicing a second into a 100 ticks for clocks. Specifically, in a rectified 50Hz AC input you will get 100 “DC” pulses. I wonder if AEG, Siemens, and the like considered this as a point for 50Hz.
Circling back to the transmission benefits of 50Hz, the west coast of the Americas did adopt 50Hz most likely due to the transmission efficiencies and shifted to 60Hz between 1936 and 1948 during the transition to the various North American interconnects – specifically in the formation of the Western Electricity Coordinating Council which is now known as the Western Interconnection. This was a very significant change for the end customers as clocks needed to be re-geared from 50Hz to 60Hz and whole factories were set up where end-consumers could bring their clocks to be re-geared.
When and Why Did the U.S. Transition from 110V to 120V Supply?
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