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Thread: Interconnections to switchboards and the 120% rule

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    Interconnections to switchboards and the 120% rule

    Since the change to 705.12 in the 2014 NEC the 120% rule for busbar loading only applies to panelboards. Has anyone had a project that back fed a switchboard at more than 120%? Have AHJs been a problem? Is anyone even doing it or just sticking with the 120% rule?

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    CMP-4 is striking "Panelboards" from the busbar language in 705.12(B)(3) for the 2020 NEC

    IMO, the busbar rule was intended for Switchboards too...

    Do you have reason to believe the 120% rule should not apply to Switchboards?

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    It was proposed as a change to the 2020 language to remove "panelboards" and add additional wording to allow engineered designs.

    The new language that is proposed for 705.12(B)(3) is:
    (b)Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar.

    (e) Connections shall be permitted on switchgear, switchboards, and panelboards in configurations other than those permitted in 705.12(B)(3)(a) through (d) where designed under engineering supervision that includes available fault-current and busbar load calculations.

    In the last two versions of the code, 2014 and 2017, the 120% rule only applied to panelboards. NEC 705.12(B)(2)(3): "Busbars. One of the methods that follows shall be used to determine the ratings of busbars in panelboards."
    It was implied that other than panelboards did not need to comply with the 120% rule since it specifically only called out panelboards but many people were still applying it to all distribution equipment, in 2020 it is being made more explicit. The reason for this is that panelboards are used in residential and small commercial systems that are less likely to have engineering input so the requirements were more prescriptive. Also, panelboards tend to have more thermal issues than switchboards and switchgear due to the smaller size and less airflow.

    I have to admit that I thought it was a mistake in the NEC for a long time and assumed that the 120% rule was intended to apply to all distribution equipment. It was only after I started asking around that I came to the conclusion that the NEC was written correctly, if unclearly, and it was not intended to apply to anything other than panelboards. The 2020 language clears this up.

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    Interesting perspective pv...

    So if switchboards are not subject to the 120% rule, who decides how much of an interconnected power production source can be added?

    And in 2020 when engineers can be responsible for determining how much of an interconnected power production source can be added under (e) (if accepted), what will be their formula for calculating how much a busbar can handle? How is an engineer qualified for determining what a specific manufacturer's switchboard busbar can handle?

    From what I've seen...engineers play it safe so they don't get sued. I would imagine they would use the 120% rule.

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    Quote Originally Posted by shortcircuit2 View Post
    Interesting perspective pv...

    So if switchboards are not subject to the 120% rule, who decides how much of an interconnected power production source can be added?

    And in 2020 when engineers can be responsible for determining how much of an interconnected power production source can be added under (e) (if accepted), what will be their formula for calculating how much a busbar can handle? How is an engineer qualified for determining what a specific manufacturer's switchboard busbar can handle?

    From what I've seen...engineers play it safe so they don't get sued. I would imagine they would use the 120% rule.
    In my case you imagine correctly, though it's a bit more than just that I don't want to get sued.

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    Quote Originally Posted by shortcircuit2 View Post
    Interesting perspective pv...

    So if switchboards are not subject to the 120% rule, who decides how much of an interconnected power production source can be added?

    And in 2020 when engineers can be responsible for determining how much of an interconnected power production source can be added under (e) (if accepted), what will be their formula for calculating how much a busbar can handle? How is an engineer qualified for determining what a specific manufacturer's switchboard busbar can handle?

    From what I've seen...engineers play it safe so they don't get sued. I would imagine they would use the 120% rule.
    The NEC acknowledges that licensed engineers have the training and expertise to sometimes go outside the prescriptive requirements of the NEC. It's up to the individual engineer to decide if they have the expertise to take on a specific task. You just have to find one who has the expertise and make it worth their time and exposure.

    I was told a long time ago that having an engineering license is having a license to be sued. If engineers only did work that would not get them sued they would do no work.

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    Quote Originally Posted by pv_n00b View Post
    The NEC acknowledges that licensed engineers have the training and expertise to sometimes go outside the prescriptive requirements of the NEC. It's up to the individual engineer to decide if they have the expertise to take on a specific task. You just have to find one who has the expertise and make it worth their time and exposure.
    The way the 2014 NEC is written, engineers can only expand from the restrictions in 705.12(D)(2)(3)(a, b, and c) in cases of center fed and multiple ampacity busbars. The 2017 NEC, I believe, has widened that window.

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    Quote Originally Posted by ggunn View Post
    The way the 2014 NEC is written, engineers can only expand from the restrictions in 705.12(D)(2)(3)(a, b, and c) in cases of center fed and multiple ampacity busbars. The 2017 NEC, I believe, has widened that window.
    I was speaking more generically, there are many places in NEC that allow engineering supervision to be applied in lieu of the normal prescriptive requirements. In addition, an AMMR can always be put in with the AHJ to allow an engineer to design an alternative to any NEC requirements. The AHJ just has to agree to accept it. I've had AHJs that will accept anything an engineer signs, others who won't accept any variation from the NEC, and many who fall somewhere in between.
    Last edited by pv_n00b; 09-14-18 at 05:14 PM.

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