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Thread: NEC 500 - C1D2 Question

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    NEC 500 - C1D2 Question

    My design shows a transformer in a room that is C1D2. A note on the drawing says that all electrical components (and that would include the transformer) are to be at least 18 inches above the floor. Am I right in believing this would allow the transformer to be provided as though it were in a non-hazardous location?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Not unless the documentation [Section 500.4(A)] indicates that it is not Division 2 above 18". However, transformers in Division 2 are only required to meet Sections 450.21 through 450.27, which they would have in any case. [501.100(B)] You may have some boundary seal issues.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2017 NEC unless otherwise noted.

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    Thanks, Bob.

    I have been trying to find the source that had led me to believe that the rules changed above 18 inches. Does anyone know where that is?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    It's not an NEC rule as such. Several reference standards listed in 500.4(B), like NFPA 497 or API RP500, indicate 18" or 24" above grade or finished floor in various applications. Articles 511 to 516 is Sections 51x.3 will also indicate 18" above grade in certain applications but only if the Article applies. Neither 18" nor 24" is a general rule.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    Thanks, Bob.

    I have been trying to find the source that had led me to believe that the rules changed above 18 inches. Does anyone know where that is?
    It depends on what the application is, who classified the area and what code was used. For instance if this is a repair garage, NFPA 30A has requirements for area classification and in certain areas the 18" distance applies.

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    The application is a facility for testing helicopter engines at a local air force base. The fuel tank is outdoors, and fuel is pumped into the building and into the engines. I did the electrical design, and the government is its own AHJ, so I am guessing that makes it my call. I declared the test bays to be C1D2 on the basis of a possible fuel spill. My plans instructed the contractor to install all electrical components 18" above the floor, but they did not explicitly state that the room is unclassified above that level. I think they could have; I think they should have. But it turns out to be a non-issue. Reading the citations Bob provided, I concluded that the only requirement for our transformer (112.5 KVA, dry-type) is that it be 12 inches from combustible materials.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    The application is a facility for testing helicopter engines at a local air force base. The fuel tank is outdoors, and fuel is pumped into the building and into the engines. I did the electrical design, and the government is its own AHJ, so I am guessing that makes it my call. I declared the test bays to be C1D2 on the basis of a possible fuel spill. My plans instructed the contractor to install all electrical components 18" above the floor, but they did not explicitly state that the room is unclassified above that level. I think they could have; I think they should have. But it turns out to be a non-issue. Reading the citations Bob provided, I concluded that the only requirement for our transformer (112.5 KVA, dry-type) is that it be 12 inches from combustible materials.
    Note as rbalex pointed out - whether some/all of the building is classified could also affect your conduit design/routing to get to/from the transformer, including seals.

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    Quote Originally Posted by nhee2 View Post
    Note as rbalex pointed out - whether some/all of the building is classified could also affect your conduit design/routing to get to/from the transformer, including seals.
    Agreed. But I think that falls under means and methods, and is not something I would have to state on the drawings. I gave the room classification, and the EC can take it from there.

    Just FYI, this project is in the Bid stage. I posted this thread because of a Bidder Inquiry that suggested putting the transformer outdoors (i.e., out of the C1D2 area).
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    The application is a facility for testing helicopter engines at a local air force base. The fuel tank is outdoors, and fuel is pumped into the building and into the engines. I did the electrical design, and the government is its own AHJ, so I am guessing that makes it my call. I declared the test bays to be C1D2 on the basis of a possible fuel spill. My plans instructed the contractor to install all electrical components 18" above the floor, but they did not explicitly state that the room is unclassified above that level. I think they could have; I think they should have. But it turns out to be a non-issue. Reading the citations Bob provided, I concluded that the only requirement for our transformer (112.5 KVA, dry-type) is that it be 12 inches from combustible materials.
    Depending on the piping and ventilation, I might not have classified the room at all; however, your reasoning is sufficiently conservative with no serious fiscal effect, so I would endorse it as your professional judgment. Depending on the overall construction, I might have considered Article 513 or 514 as well; but again, the installation is likely out of their scopes. Article 515 would apply to the storage tank.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2017 NEC unless otherwise noted.

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