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Thread: Low, Medium & High Voltage Definition

  1. #1
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    Low, Medium & High Voltage Definition

    I'm looking for a qualified definition for Voltage levels ie. Low Voltage < ? Medium Voltage > to < & High Voltage > ? There are various deinitions for these in the NEC & NFPA - Is there a definitive answer for construction, maintenance & utility electricians? Is there a standard and if not why not?

  2. #2
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    I think not. The "why not" is that it means different things to different sectors of the industry. In my world, "LV" is below 120 VAC, and applies to fire alarm, security, and similar systems. I use "MV" for anything above 480 VAC, up to 25,000 VAC or so, and "HV" for anything higher. But a person who works with fire alarm systems might consider 120 VAC to be "high voltage." So whenever I hear or read someone's use of these terms, I make sure to verify what that person means by the terms, before proceeding.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    FWIW, in my 1925 NEC, Low Voltage is 600 volts or less.

    High Voltage is 601-5000 volts.

    Extra-high voltage is over 5000 volts.

    But, that was 83 years ago.....

  4. #4
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    Although many use these terms incorrectly, because it seems to be all relitive to what you are used to, they are defined by IEEE for equipment desginations as"

    LV - 600V or less
    MV 601V-69,000V ( Changed a few years ago)
    HV - 69,001V-230,000V
    EHV 230,001V-800,000V
    UHV >800,000

    I feveryone used these properly alot of confusion could be avoided. I get alot of calls for service work and repairs on "High voltage" breakers, when asked if we work on HV breakers my answer is always, some of them, but only up to 115kV, the answer is usually, "I was talking about 4160V" or something like that.

  5. #5
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    Quote Originally Posted by spark2
    I'm looking for a qualified definition for Voltage levels ie. Low Voltage < ? Medium Voltage > to < & High Voltage > ? There are various deinitions for these in the NEC & NFPA - Is there a definitive answer for construction, maintenance & utility electricians? Is there a standard and if not why not?
    See attached IEEE-141-1993 Table 3-3
    Attached Images Attached Images

  6. #6
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    Thanks for your input !

  7. #7
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    I notice they don't list any voltage under 120 volts.

    Apparently 24 VAC is 'no voltage'.

  8. #8
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    Using too simplistic of a breakdown of just low, medium, + high voltage doesn't work out very well. . As an inspector, I perfer letting the NEC dictate the breakdown rather than IEEE. . But if your job deals with IEEE standards more often than you deal with the NEC, then I would use IEEE. . The category limits that you use should to provide you with usable categories for your particular job.
    In the classes for electrical construction apprentices that I teach, I list these categories:

    EQUIPMENT
    0-600 . . Standard
    601-2000 . . Low Industrial
    2001-35,000 . . Medium Industrial
    35.001 & up . . High Industrial

    The categories come from the NEC + manufacturers.

    You'll notice that T310.13(A) writes down what we already knew, that standard insulations are rated up to 600v.

    490.2 lists more than 600v as "high voltage" equipment but that category then gets subdivided in other places in the NEC. . Right from the beginning of 490 [490.21(A)(1)(a)], you see wording that shows that the primary application of over 600v is industrial. . For this reason, I find it easier and more compatible with the rest of the code, to call over 600v as "industrial" equipment voltage rather than "high voltage".

    328.2 def + 328.10 lays out the middle ground of these "industrial" equipment voltages by designating 2001-35,000 as medium voltage, or as I would say medium industrial. . T310.5 is an example of a code article that applies only to standard, low industrial, + medium industrial equipment voltages. . 310.6 + 310.7 are 2 examples of code articles that apply only to medium + high industrial equipment voltages.

    450.21(C) is an example of a code rule applying to high industrial equipment voltage.

    Of course, there are exceptions [such as T300.50], but these breaking points between voltages holds for most equipment addressed in the NEC.

    SYSTEMS
    0-49 . . Low Distribution
    50-1000 . . Medium Distribution
    1000-4160 . . High Distribution
    4160 & up . . Transmission

    0-49 . . Low Distribution is covered by 250.20(A)
    50-1000 . . Medium Distribution is covered by 250.20(B)
    1000-4160 . . High Distribution and 4160 & up . . Transmission are covered by 250.20(C).
    4160v is presented in many text books as the breaking point between distribution and transmission [such as Rockis + Mazur Electrical Motor Controls page 288]. . Industrial plants that are supplied by voltages of greater than 4160v to ground are considered as recieving transmission voltage service.

    Certainly there are other ways of categorizing voltages. . And I don't know how you could call one way right and another way wrong. . But I'm teaching electrical apprentices and use of the NEC is critical. . I prefer categories that highlight the breaks found in the NEC.

    Looking at it this way, you would understand 250.180 as recognizing 1000v as being a break because it's looking at a system category. . It states this limit as a bottom limit for "high voltage". . You would also understand the bottom limit of 600v "high voltage" in 490.2, 230.200, 240.100, + other places as being a break because it's looking at an equipment category.

    Whatever limits/categories you choose, make sure that they are useful in categorizing the information that you use everyday.
    David
    Inspector
    Medina County Ohio
    &
    PSECI

  9. #9
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    Huh??? no way is 4160V at all associated with "Transmission".

  10. #10
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    Quote Originally Posted by zog
    Huh??? no way is 4160V at all associated with "Transmission".
    Not associated with substation to substation transmission, I agree. . 4160v from substation to pole mounted kettle transformer is very common in my area. . But much more important is that it is listed as the breaking point between what is considered distribution and what is considered transmission levels in text books.
    David
    Inspector
    Medina County Ohio
    &
    PSECI

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