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Thread: sizing of "additional" ground conductor

  1. #1
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    sizing of "additional" ground conductor

    We are doing an install of some equipment that has one board that is pretty fussy about the ground reference. The equipment is served with 1-1/4 EMT and #4 conductors. The conduit is serving as the ground. As an additional item, we were going to run a ground directly to the circuit board to make sure it has a good ground reference. It seems that since the conductor is not required at all, we could just use a #10 since it's just for the circuit board. However, I could also see an argument that if the conductor is present, it has to be sized for the conductors. Any thoughts and or code references to confirm either way?

    Thank you

  2. #2
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    I don't see anything that relieves you from the requirements of 250.122 even for an isolated grounding conductor.
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

  3. #3
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    It has to be sized for the overcurrent device.
    The more they overthink the plumbing, the easier it is to stop up the drain

  4. #4
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    Technically the equipment grounding conductor you're installing is not an "additional" ground it is an isolated ground. Of course that is assuming that all the requirements of an isolated ground are met. However for instance if your equipment is being fed from a subpanel and you only run this ground from the equipment back to the sub panel you do not have an isolated ground. It would just be another equipment grounding conductor. Either way the sizing is still based on 250.122 based on the rating of the overcurrent protection device ahead of the circuit.

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  5. #5
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    In general, I don't see anyway you can avoid running a full size EGC while meeting code requirements.

    Maybe you could run a #10 EGC to a bonding bushing where the EMT enters the device.

    Personally, I think these so called requirements for ground references like this rarely if ever serve any useful purpose other to give some level of cover to the manufacturer when mysterious malfunctions occur.

    OTOH, you might be able to make an argument that this conductor may not actually be an EGC since it appears it might not meet the definition of an EGC found in the code. If so, you cannot run a green wire for whatever this wire actually is.

    Grounding Conductor, Equipment (EGC). The conductive
    path(s) installed to connect normally non–current-carrying
    metal parts of equipment together and to the system grounded
    conductor or to the grounding electrode conductor, or both.
    It also does not seem to me that this conductor, whatever it is, is not the EGC associated with the #4 power wires.

    I don't know where you go from there.
    Last edited by petersonra; 05-19-17 at 01:28 PM.
    Bob

  6. #6
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    Bob brings up a very good point. If this thing is not an EGC, then the rules for sizing an EGC would not apply. What do the manufacturer's instructions have to say?
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  7. #7
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    Thanks for all the replies. I guess the question I'm really asking is more like the following. If you ran 3 #4 THHN Two ungrounded conductors, a grounded conductor, and an EGC, that would be perfectly compliant. If you chose to run an ADDITIONAL #10 as a ground reference for one of the circuit boards, does that ground also need to comply with 250.122? I think the real issue may be what Bob brought, namely that it isn't really an EGC because that requirement is already met. This particular situation is a little unique in that we are working with the manufacturer to meet the needs of their equipment and also to maximize the efficiency of future installations. Some of their engineers were fine with the conduit being the ground path and other were not. The idea was thrown out that all of the engineers were fine with the conduit being the NEC required EGC, they just wanted a "better" ground reference for one of the circuit boards. That is how this question came up.

  8. #8
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    Quote Originally Posted by sfav8r View Post
    Thanks for all the replies. I guess the question I'm really asking is more like the following. If you ran 3 #4 THHN Two ungrounded conductors, a grounded conductor, and an EGC, that would be perfectly compliant. If you chose to run an ADDITIONAL #10 as a ground reference for one of the circuit boards, does that ground also need to comply with 250.122? I think the real issue may be what Bob brought, namely that it isn't really an EGC because that requirement is already met.
    It is an interesting point, what do you have when you have a conductor that is not a grounded nor a grounding conductor? What color insulation do you use for it?

    This particular situation is a little unique in that we are working with the manufacturer to meet the needs of their equipment and also to maximize the efficiency of future installations. Some of their engineers were fine with the conduit being the ground path and other were not. The idea was thrown out that all of the engineers were fine with the conduit being the NEC required EGC, they just wanted a "better" ground reference for one of the circuit boards. That is how this question came up.
    It's like a cult for these guys. I'm surprised they don't spec animal sacrifices take place when ground rods are driven.
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

  9. #9
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    Quote Originally Posted by ActionDave View Post
    It is an interesting point, what do you have when you have a conductor that is not a grounded nor a grounding conductor? What color insulation do you use for it?


    It's like a cult for these guys. I'm surprised they don't spec animal sacrifices take place when ground rods are driven.
    In Louisiana the appropriate earth ground modifier is chicken blood. Kills two stones with one bird.


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