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Thread: Should mains be 110v or 120v?

  1. #1
    Product R&D for a powertool manufacturer dgold's Avatar
    Join Date
    Dec 2006
    Carlsbad, CA

    Default Should mains be 110v or 120v?

    This question is purely for curiosity's sake...

    I've searched all over the internet, but get conflicting information. Is US mains power supposed to be 110v (220v if pulling from both sides), or 120v (240v if pulling from both sides).

    I remember hearing somewhere that it's really supposed to be 110v, but there's conflicting info all over the place. I doubt it really matters for most appliances, but I'm very curious.

  2. #2
    Electrical Contractor/Instructor jwelectric's Avatar
    Join Date
    Jun 2007
    North Carolina


    220.5 Calculations.
    (A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.

    Table 430.248 Full-Load Currents in Amperes, Single-Phase Alternating-Current Motors
    The following values of full-load currents are for motors running at usual speeds and motors with normal torque characteristics. The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges of 110 to 120 and 220 to 240 volts.

    to answer your question one would have to say


  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Sep 2004
    New England


    My local service averages 118vac. That gives them a little wiggle room if the load increases. Most devices in the US are designed for 110-120vac operation, and many, if designed for international operations can operate with a much wider range (I've got one TV that is spec'ed for 80-250vac input!).

    As an aside, you can buy lighbulbs spec'ed at 130vac. They won't be as bright, but they will last a very long time compared to 120vac rated bulbs.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014


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