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Thread: Minimum wire size for lightning protection

  1. #1
    Join Date
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    Minimum wire size for lightning protection

    I do not have a copy of NFPA 780-2004 which explains lightning protection procedures. Hope somebody can answer a few questions. I am connecting a lightning ground rod to a cross ontop of a church. The print specifies that the lightning ground wire be run INSIDE the building through emt and that the emt should be spaced 6ft from any other wiring or metal structure. It also says that the ground wire be #1AWG. Now first of all, I didn't think lightning protection was allowed to be run indoors. Nor does it even sound like a good idea. And the #1AWG seems a bit undersized. Anybody have any input on this?

  2. #2
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    Quote Originally Posted by bthouse View Post
    Anybody have any input on this?
    Only that I agree with your thoughts on the location and size of the wire.

    I don't know any lightning protection regulations off the top of my head, so I can't comment further.
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  3. #3
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    Lightning protection is in fact allowed to be run inside the building structure. Federal Court House I worked in had mutiple roof penetrations to down wires thru the structure to the ground level.
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  4. #4
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    Quote Originally Posted by bthouse View Post
    I do not have a copy of NFPA 780-2004 which explains lightning protection procedures. Hope somebody can answer a few questions. I am connecting a lightning ground rod to a cross ontop of a church. The print specifies that the lightning ground wire be run INSIDE the building through emt and that the emt should be spaced 6ft from any other wiring or metal structure. It also says that the ground wire be #1AWG. Now first of all, I didn't think lightning protection was allowed to be run indoors. Nor does it even sound like a good idea. And the #1AWG seems a bit undersized. Anybody have any input on this?
    The 1/0...is that what they are specifying for the lightning protection "down conductor" or the actual grounding grid?
    If it is for the ground grid that should be fine.

    If your structure is less 75' or less in height it is a Class I system. Otherwise it is a Class II. The material requirements vary between each. All you really need to know, assuming this is a Class I structure is whether or not your materials are Class I rated. I don't have NFPA 780 handy otherwise I could help you more.

  5. #5
    Join Date
    Aug 2007
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    St. Louis, MO
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    The grid wire is 3/0. The #1AWG is for the connection from the lightning rod to the grid. As far as the structure height, I'm not sure. The top of the cross (lightning rod) is somewhere close to 60-70ft.

  6. #6
    It is acceptable to conceal the LPS downleads within the structure.

    Per NFPA 780-2008
    Copper Class I materials
    Main conductor, solid strip
    Thickness 0.051 in.
    Cross section area 57,400 cir. mils

    Copper Class II materials
    Main conductor, solid strip
    Thickness 0.064 in.
    Cross section area 115,000 cir. mils

    I didn't include the aluminum requirements.

    The minimum Class I conductor is a #2, however, if a #1 tail is provided with the steeple continue using #1 or larger to grade. If metal conduit is used be sure to bond the conductor to the tubing at the point where it enters and exits the tubing.

    Side note: If the steeple is only grounded with one downlead to grade it's not a complete system and will not meet any standards. At least 2 downleads are required along with additional bonding and probably additional protection for the balance of the structure.

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