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Thread: Service conductor ampacity and protection

  1. #1
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    Service conductor ampacity and protection

    In the past, I have been told that service conductors are permitted to be protected higher than their ampacity, as long as the connected load does not exceed the ampacity.

    Isn't that a conflict with this?:

    (From the '02 NEC) VII. Service Equipment — Overcurrent Protection
    230.90 Where Required. Each ungrounded service conductor shall have overload protection.
    (A) Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each un-grounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor. A set of fuses shall be considered all the fuses required to protect all the ungrounded conductors of a circuit.
    In other words, if I have a 200a main-breaker panel with 100a worth of load, where in the service can the conductors be sized for the load, and where must they be full sized?

    Let's use the example with a conduit on the wall of a 2-story building, from the point-of-attachment down to the meter base, which is back-to-back with the main-breaker panel.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  2. #2
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    Larry,
    Unless there is more than one service disconnect the service conductors must be protected at or below their ampacity. When there is more than one service disconnect the conductors can be sized for the load even where the total of the ratings of the service disconencts exceeds the ampacity of the conductor.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #3
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    Just to add to Don's comment, which is spot on, see 230.90(A) exception No. 3.
    Ryan Jackson, Salt Lake City
    Inspector, Instructor

  4. #4
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    Why does 230.90(A) overrule 240.4(B)?

  5. #5
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    Quote Originally Posted by don_resqcapt19
    Unless there is more than one service disconnect the service conductors must be protected at or below their ampacity. When there is more than one service disconnect the conductors can be sized for the load even where the total of the ratings of the service disconencts exceeds the ampacity of the conductor.
    Quote Originally Posted by ryan_618
    Just to add to Don's comment, which is spot on, see 230.90(A) exception No. 3.
    So, where I have a 200a, 1-ph main-breaker panel, and a 125a, 3-ph main-lug panel with a single 30a, 3-ph breaker (A/C unit), and the two panels share the single open-Delta 3-ph service, and the total calculated load of both panels is 117a (99a 1-ph and 18a 3-ph), what would be the minimum required service-conductor ampacity?

    If the answer is "only large enough for the 117a", why would (just for the sake of discussion) eliminating the 30a panel require jumping up to a full 200a ampacity (other than "because the NEC says so")?

    I'm asking because I'd like to submit a change to my plans to allow switching to aluminum conductors without having to up-size the conduit, which would require down-sizing the ampacity to maintain conduit-fill limits.
    Last edited by LarryFine; 05-16-07 at 05:39 PM.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  6. #6
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    Smile

    Quote Originally Posted by LarryFine
    (other than "because the NEC says so")?
    Once you eliminate that answer I don't know what to say.

    I think this is a good example of the one of the reasons for exception 3.



    There are six 100 amp breakers protecting the riser on this service.

    If I recall the service conductors are in the 200 - 250 amp range.

    And if I recall that was a fair amount larger than the calculated load needed. (A friend of mine did this service)

    It would have served no purpose to require 600 amp service conductors for this service.

    JMO, Bob

  7. #7
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    Quote Originally Posted by iwire
    Once you eliminate that answer I don't know what to say.
    Okay, I'll buy that. This is a case of a single meter supplying the two panels, though. I'm asking about the two conductor sets, from the single meter to the two panels. Any difference?.

    So the conductors supplying the 200a M/B panel can be smaller than 200a (4/0 al or 3/0 cu) as long as another panel is also being supplied, but if it's the only panel, they must be full-sized? That's so weird!

    The left half of this riser diagram is what is under discussion:

    http://fineelectricco.com/Riser.jpg

    Added: http://fineelectricco.com/1line.jpg

    Note: typo in 1-line diagram: it should be two 3/0's, not three.

    In both diagrams, where I had specified 3/0 cu and the 250mcm, I'm looking to see how small I can go, and if I can get small enough to move to aluminum without having to enlarge the conduits. For info: For commercial space: 117a calculated load; 200a M/B is why I chose 3/0 cu. For combined apartments: 262a calculated load; thus the 250mcm.

    While were here, what would be the best way to get the #4's from the meter to the panel? I have a few options I'm considering:

    1) As in the riser diagram, a separate conduit from the meter to the smaller panel. Pro: as in drawings. Con: two sets of conductors in a smaller-than-320a meter.

    2) Nipple smaller panel to larger, with two sets of conductors in through-wall conduit. Pro: only one hole in wall. Con: same as above.

    3) Nipple smaller panel to larger, tapping onto A and C phases, only requiring one #4 added to through-wall conduit. Pro: single conductor set in meter. Con: having to tap A and C phases in larger panel.

    The only problem with #3 is that the main breaker has single lugs that are part of the breaker, like a residential panel, so I'd have to tap the #4's to the #-whatever's to be determined in the discussion that started all this.

    If this is the best route, what would all y'all recommend for the splicing itself? I could use Polaris-type blocks, I suppose. If I did, now what two sizes would I need from meter-to-blocks, and blocks-to-main breaker? The #4 is not at issue.

    Whew! Thanx, guys. :smile:
    Last edited by LarryFine; 05-16-07 at 07:04 PM.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  8. #8
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    Larry,
    I'm asking because I'd like to submit a change to my plans to allow switching to aluminum conductors without having to up-size the conduit,
    You may be able to do that using a compact strand conductor. That was the original purpose of compact stranding...to permit you to install the same ampacity circuit using aluminum without changing the conduit size.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  9. #9
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    Quote Originally Posted by don_resqcapt19
    You may be able to do that using a compact strand conductor. That was the original purpose of compact stranding...to permit you to install the same ampacity circuit using aluminum without changing the conduit size.
    True enough. Okay, I'll add that smaller, non-compact aluminum wire is easier to bend and terminate, lighter and easier to handle, and cheaper.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  10. #10
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    Okay, here's a drawing I just made, showing a cut-away of the back wall of the building. Please also see my prior post, #7 in this thread, what I described in "option #3", and links there.

    The service is an open Delta, and the only 3-phase load is the AC unit in the smaller panel, so the hi-leg #4 would be run from meter to 3-ph panel unbroken, and A and C phases tapped.

    This would be my preference, as long as it's legal. The three conductors to the small panel will be #4 cu, but what size for the incoming A- and C-phase lines? Same as to the meter?

    By the way, the 1-ph panel has a 117a calculated load.

    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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