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Thread: Minimum Service Size

  1. #1
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    Minimum Service Size

    I know the NEC says that dwelling units must have a service size no less than 100 Amps.

    I just want to break this down a bit:

    1) If I have my main service going to a house panel first then off this house panel I have a branch breaker feeding an individual dwelling unit. Does this mean that this branch breaker cannot be less than 100A?

    2) Is this 100A min. service size limited to dwelling units only? IE- If I have a warehouse with tiny loads, can I put a 20A service on it?

    Thank You

  2. #2
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    230.79 is what you are looking for

    230.79 Rating of Service Disconnecting Means. The service
    disconnecting means shall have a rating not less than the calculated
    load to be carried, determined in accordance with
    Part III, IV, or V of Article 220, as applicable. In no case shall
    the rating be lower than specified in 230.79(A), (B), (C), or
    (D).
    (A) One-Circuit Installations. For installations to supply only
    limited loads of a single branch circuit, the service disconnecting
    means shall have a rating of not less than 15 amperes.
    (B) Two-Circuit Installations. For installations consisting of
    not more than two 2-wire branch circuits, the service disconnecting
    means shall have a rating of not less than 30 amperes.
    (C) One-Family Dwellings. For a one-family dwelling, the service
    disconnecting means shall have a rating of not less than
    100 amperes, 3-wire.
    (D) All Others. For all other installations, the service disconnecting
    means shall have a rating of not less than 60 amperes.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  3. #3
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    It's worth noting the code says service disconnect rating, not service size or rating, which is a bit odd....
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  4. #4
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    Quote Originally Posted by electrofelon View Post
    It's worth noting the code says service disconnect rating, not service size or rating, which is a bit odd....
    Yes, but then there is this

    230.42(B) Specific Installations. In addition to the requirements of
    230.42(A), the minimum ampacity for ungrounded conductors
    for specific installations shall not be less than the rating of the
    service disconnecting means specified in 230.79(A) through
    (D).
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  5. #5
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    Quote Originally Posted by Dennis Alwon View Post
    Yes, but then there is this
    Is that new dennis? I recall this debate but don't remember that section
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  6. #6
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    Quote Originally Posted by electrofelon View Post
    Is that new dennis? I recall this debate but don't remember that section
    It's been there forever but everyone seems to ignore it. I have mentioned it many times that is why I don't bring up the fact that 230.79 only refers to the disconnect. It is true but 230.42 clarifies. I don't know why they don't just say the feeder or service conductors need to be sized to 230.79 because, IMO, that is what it means. I know others will argue it.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  7. #7
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    Instead of going with minimums, why not go "Bigger is Better".
    "Just because you're paranoid, doesn't mean they're not out to get you"

  8. #8
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    Quote Originally Posted by kingpb View Post
    Instead of going with minimums, why not go "Bigger is Better".
    Because clients like to spend the minimum amount they can to get what they want.

  9. #9
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    Quote Originally Posted by kingpb View Post
    Instead of going with minimums, why not go "Bigger is Better".
    Because we like to debate theoretical code situations
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  10. #10
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    Quote Originally Posted by kingpb View Post
    Instead of going with minimums, why not go "Bigger is Better".
    Sometimes it just a matter of stating what the code states.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



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