Ground conductor bypassing ground bus in metal building
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  1. #1
    Join Date
    Oct 2009
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    Cartersville, Georgia
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    361

    Ground conductor bypassing ground bus in metal building

    My shop is a standard metal building (beams, purlins, exterior sheet). 200 amp single phase main service entrance. Also has 100 amp subfeed panel (installed correctly - ground not bonded to neutral).

    Two other meters feeding who knows what in the rest of the building (multiple tenants).

    All power in the warehouse is in EMT conduit, using the conduit as the equipment ground, with exception of conduit connecting main panel to sub panel. It's got plastic conduit with bare copper EGC connecting the two panels.

    I'm getting a brand new CNC press brake. Will be running it via American Rotary RPC, from the main panel. Despite running EMT between the panel and the RPC control box, and between control box and new machine, American Rotary recommended a separate EGC - so I'm running green THHN inside the EMT.

    Press brake manufacturer's tech highly recommended surge protection. No issue, picked up a Square D whole panel device. In researching that, I was told to inspect all of the GEC connections, as a surge protection device won't do very much with poor grounds.

    In doing so, I noticed that the #6 bare copper ground wire coming in with the service entrance bypasses the ground lug in the panel. Instead, it goes through one of the mounting holes in the back of the panel (surface mounted), and then bolts to one of the building's purlins. Following it backwards, outside, it goes into the service disconnect box, and is connected to a #4 that goes into the ground (actual ground - dirt), presumably to whatever rod would have been required in 1992. Between this disconnect box and the main shop panel is all rigid conduit.

    Obviously, the entire ground system is relying on conduit, and the #6 wire just enters the box as an easy way to get through the wall, to gain access to a purlin. Metal conduit, metal building, metal boxes, it all make sense.

    My question is, since I'm running ground wires to the ground bus for the phase converter and the surge protector (to not rely on conduit), should I also have the ground wire tie to the ground bus, or is the ground bus being attached to the panel considered appropriate? Logic tells me to tie it in, but I'm curious what the NEC tells me.
    Who is John Galt?

  2. #2
    Join Date
    Jan 2004
    Location
    Northern Cal., Shasta County
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    9,025

    Re: Ground conductor bypassing ground bus in metal building

    Don't know what the NEC says specifically, but if a bare ground passes through a metallic box that bare ground needs tied to that box so as to maintain the same (equal) ground potential for all exposed ground wires and metallic piece parts.
    "The things that will destroy America are prosperity at any price, peace at any price, safety first instead of duty first, the love of soft living and the get rich quick theory of life." -Theodore Roosevelt

  3. #3
    Join Date
    Feb 2012
    Posts
    3,723

    Re: Ground conductor bypassing ground bus in metal building

    Quote Originally Posted by bassboy1 View Post
    My shop is a standard metal building ...

    In doing so, I noticed that the #6 bare copper ground wire coming in with the service entrance bypasses the ground lug in the panel. Instead, it goes through one of the mounting holes in the back of the panel (surface mounted), and then bolts to one of the building's purlins.

    My question is, since I'm running ground wires to the ground bus for the phase converter and the surge protector (to not rely on conduit), should I also have the ground wire tie to the ground bus, or is the ground bus being attached to the panel considered appropriate? Logic tells me to tie it in, but I'm curious what the NEC tells me.
    If I understand; I would run the new ground to the purlin that has the #6 service ground and as close as practicable . I wouldn't trust the conduit connections if the manufacturer suggested good ground as essential . You won't get a better ground unless you connect directly to the #6. I think the same purlin grounding is all that's needed tho.

    I assume the panel bus bar was bypassed by design. The building is probly triple phase. Disclaimer , I'm just offering armchair advice so if your unsure then consult sparky to investigate.

  4. #4
    Join Date
    Oct 2014
    Posts
    373

    Re: Ground conductor bypassing ground bus in metal building

    They just did that because their destination was the service not the panel. That practice is not used as often today and the building ground can and often are run from the panel instead of the service. Either way works.
    Put the whole house surge on a breaker as close on the bus as you can to the most expensive equipment and make sure the panel is grounded properly. If the panel is bolted to the steel framework of the building it has a pretty good ground but you want a good proper sized copper wire from panel ground bar to the service disconnect neutral/ground bar and be sure your ground rods are connected properly and healthy. Never a bad idea to put down some new ground rods if the building is really old.

  5. #5
    Join Date
    Oct 2009
    Location
    Cartersville, Georgia
    Posts
    361

    Re: Ground conductor bypassing ground bus in metal building

    Quote Originally Posted by danielplace View Post
    They just did that because their destination was the service not the panel. That practice is not used as often today and the building ground can and often are run from the panel instead of the service. Either way works.
    Put the whole house surge on a breaker as close on the bus as you can to the most expensive equipment and make sure the panel is grounded properly. If the panel is bolted to the steel framework of the building it has a pretty good ground but you want a good proper sized copper wire from panel ground bar to the service disconnect neutral/ground bar and be sure your ground rods are connected properly and healthy. Never a bad idea to put down some new ground rods if the building is really old.
    That makes sense - they used the panel because it represented a hole through the wall. I'm not used to metal conduit. Was easy to understand using conduit as the ground back to the panel from devices, but was tough to get used to the idea that it would also be used from the panel to the disconnect.

    I'll tie it to the ground bus, so that I'll have copper all the way from the machine to the disconnect (and on into the dirt) in addition to conduit.

    I'll do some digging to see what exists for ground rods. All I know is there's a 4 gauge wire going into the dirt.

    I've heard to keep the leads on surge protector as short as possible - I'll rearrange a couple of things to get the breakers for the surge protector and the RPC next to each other.

    Good info. Thanks!
    Who is John Galt?

  6. #6
    Join Date
    Apr 2014
    Location
    richmond, va
    Posts
    411

    Re: Ground conductor bypassing ground bus in metal building

    Check the bolts that hold the panel to the frame. I use star washers in these applications so the can and frame can't corrode between them thus losing the bond to ground. Doing demo, I have run into this problem especially in minimally heated environments. You can use a split bolt to tie a piece of wire to the one that is going to the disconnect to the ground bar.
    Regards
    Dave

  7. #7
    Join Date
    Apr 2006
    Location
    Western, NY
    Posts
    226

    Re: Ground conductor bypassing ground bus in metal building

    Why not just drive a couple more ground rods near your table and loop a ground wire on them. Would seem much easier and then you have a dedicated grong for the table.

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