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Thread: Sizing dwelling service NEC 220.82

  1. #1
    Join Date
    Jun 2011
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    6

    Sizing dwelling service NEC 220.82

    I am new to doing residential design and have gotten confused by the optional calculations in the back of the NEC for sizing the service of a dwelling unit. I am used to sizing the service by getting the load in amps and then multiplying by 125% and selecting the next standard size. It looks like the optional calculations allow you to get the load from 220.82 and then just round up to the next size for the service. Is this correct? Are they considering the load the calculations to be non-continuous and therefore the service size doesn't need to be 125% increased? Thanks for your help.

  2. #2
    Join Date
    Dec 2010
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    Tennessee NEC:2008
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    2,941
    Quote Originally Posted by beaucephas View Post
    I am new to doing residential design and have gotten confused by the optional calculations in the back of the NEC for sizing the service of a dwelling unit. I am used to sizing the service by getting the load in amps and then multiplying by 125% and selecting the next standard size. It looks like the optional calculations allow you to get the load from 220.82 and then just round up to the next size for the service. Is this correct? Are they considering the load the calculations to be non-continuous and therefore the service size doesn't need to be 125% increased? Thanks for your help.
    There are 2 methods in Article 220, the standard method and the optional method located in section IV. You only need to size the largest motor used in your calculations at 125%, not the entire load. The back of the book you refer to is Annex D and they are only examples. They certainly help in your calculations, but use 220 for your calculations and use Annex D for help.
    If you aim at nothing, you will hit it every time!

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