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Series circuits operate with a single wire – no ground, no neutral. They were commonly used from the 1920s – 1970s for street lighting. Why were they used? How do I know if I have a series loop?

  • Single wire meant lower wire cost

  • They could power and control hundreds of lights from one transformer

  • Unlike with line voltage, there is no voltage drop so all the lights are the same brightness. This was particularly important for incandescent lighting as it’s very sensitive to voltage drop.

  • It’s the same type of system that’s used for airfield lighting.

  • Series circuits provided modest efficiency increases for incandescent vs. line voltage


Overhead Series Circuit, Lubbock, TX


Series Regulator, Modesto, CA

​Where might I find series?

  • Older communities

  • Luminaires won’t have photocells

  • Remote ballasts frequently used because series ballasts were bigger (but not always)

  • Group switched incandescent lighting that switches on slowly (i.e. it dims up over 1-2 seconds)

  • Incandescent lamps with “V” shaped filaments

  • Locate overhead wired regulator (see picture at left). Sometimes these are located indoors at a “central station” or in underground vaults

  • Can be mercury or sodium or incandescent; most commonly mercury or incandescent.

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