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Thread: SE, SER Service and Feeder Ampacity

  1. #1
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    SE, SER Service and Feeder Ampacity

    I have viewed many forums and no one can seem to put this issue to bed. The revised 310.15(B)(7) seemed to try, but there seems to still be the grey area in regard to accessory buildings of a dwelling unit such as garages, pole buildings, pool houses that are FED with FEEDERS from a dwelling unit.

    What is the ampacity rating requirement for these conductors in question?? I'm not really interested in what many incompetent inspectors will pass. I want to know what the intent of the code is on this issue.

    Although its not common practice in our trade, as an electrician i feel like I should be installing 4/0 copper SER for a 200 amp sub-panel; fed from and through a dwelling; going to a detached structure; based on the 60 degree column in 310.15(B)(16) (4/0 Copper SE/SER rated at 195 amps at 60 degrees) because its not main power feeder, and because of article 338.10. I also wonder if you start under the 60 degree column with the SER under that premise, would you not have to continue that sizing when you converted wiring methods to go underground outside the dwelling going to the same detached structure based on the 60 degree rating even though THHN has a 90 degree rating.

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    Let's start with which code cycle you're referring to. 2017?

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    All responses based on the 2014 NEC unless otherwise noted

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    Well I see they did get rid of the statement allowed for conductors of feeders need not be larger than service entrance conductors.

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    Well I am in Indiana which is still one of the last states to carve out parts of the NEC related to ARC fault breakers, tamper resistance receptacles etc; but mostly operating under the 2008 code cycle. However, I like to keep up with the codes intent and usually go above and beyond what the code's minimum requirement is if its cost permissive. Most of the AHJs in my area will allow newer code cycle recommendations and newer printed language that clears up any ambiguity.

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    Under the 2008 code, Article 310.15(B)(6) states that service and feeders shall be permitted, and includes all loads that are part of, or associated with the dwelling unit. Table 310.15(B)(6) indicates that 2/0 CU or 4/0 AL is acceptable for a 200 amp service. As an inspector, this is what I accept if a detached structure is fed from the dwelling service. If there is a load center (farm load service) that has separate disconnects for a dwelling and detached structure, then I feel that Table 310.16 kicks in and it requires 3/0 copper for the service conductor or 250 AL. Maybe I am interpreting this wrong, but that is how I read it. 4/0 CU is allowable, but exceeds what is allowable by these sections. Hopefully I am understanding you question correctly.

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    The reduction in ampacity is for dwelling units not for detached structures such as garages, shed, work shops etc. Those structures need to be fed using the table in 310.15(B16)
    They say I shot a man named Gray and took his wife to Italy
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    Ser is another issue. The ampacity of these conductors have changed many times thru many cycles. Best thing to do is use pvc and install 4/0 aluminum for 200 amps.

    In conduit, these conductors are rated 180 amps and since 240.4(B) allows us to use the next size breaker it is compliant. We do, however, have to make sure the calculated load is 180 amps or less.
    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



  8. #8
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    Indiana doesn't require TR receptacles? Interesting... Makes fixing aluminum wiring a lot easier I suppose... if the poster coppersmith is from there I owe him an apology

    Welcome to The Forum

    Eta: although quite uncommon, if you were to switch to thwn conductors and go underground, those conductors could be used at the 90-degree table if you used 90 degree connectors, like Polaris, when connecting them to your other cables.
    Last edited by JFletcher; 04-17-18 at 07:37 AM.
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