Thank you for your question regarding EMT box grounding. It is our pleasure to help.
It sounds from your email, that you are either trying to ground a metallic conduit box that is in series with plastic conduit, or you are trying to route a ground wire from data system conduit over to an electrical power system conduit. We will answer both options.
If you are running data system conduit, and have a combination of PVC conduit with EMT boxes, you should absolutely run a ground wire in the PVC conduit to bond all of the metallic components found along the way. In fact, even if it was all PVC, it would be advisable to run copper ground wires in the conduit to ensure that a low-impedance path exists between the two connecting points, so that stray currents will travel on the ground conductor and not on the data lines.
Now, if you are trying to run a ground wire in data system conduit to fix an electrical power system grounding error, than it is possible to do so, but certainly not advisable or normal. If there is an existing EMT box in the ceiling that is currently not grounded, and someone is trying to run a ground wire in a data-line conduit system to provide the ground for the box, there is nothing that we are aware of that would violate the National Electrical Code (NEC). Grounding conductors are non-current carrying, and certainly fall in the under 50-volt range and as such there is no electrical requirement to place grounds in conduit at all (although one could argue that the NEC requires conduit for grounding to provide physical protection for the ground system). Ideally, the ground wires are routed in the electrical conduit system and bonded to each EMT box along the way, to ensure that the EMT system is grounded at some level. When the ground conductor is not in the EMT system, there is a greater concern that the EMT may have a higher resistance back to the first-service disconnect. The other concern is of course if the equipment that is fed from the ceiling EMT box is properly grounded. This is probably more up to your local inspectors and what they are willing to accept.
That said, there are other standards beyond the National Electrical Code to be worried about. if you are trying to meet specific Telecommunications standards such as the Motorola R56 or ANSI-TIA-EIA-J-STD-607-A standards, interconnecting the electrical ground with your data systems ground may be considered a no-no. You may wish to consult with whomever is designing your server rooms grounding systems to ensure that no violations to Telco standards will occur, if you decide to allow the ground connection to be made.
If you should have any further questions, please feel free to contact us directly at 310-318-7151.
The Engineering Team at E&S Grounding Solutions
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