Thank you for your question regarding using Rigid Conduit as a grounding conductor, it is our pleasure to help.
The National Electrical Code (NEC) has not placed a limit on the length of Rigid Metal Conduit (RMC) that can be used as a grounding conductor. Article 344.60 simply states “RMC shall be permitted as an equipment grounding conductor.”
Just because the NEC allows the use of RMC as a grounding conductor, does not necessarily mean that it is a good idea. Generally speaking, the further the conduit run, the greater the need for a copper grounding conductor. Keep in mind that the purpose of grounding in the National Electrical Code (NEC) is to ensure that Over Current Protection Devices (OCPD), such as fuses and circuit breakers, function properly. Rigid Metal Conduit (RMC) will generally allow enough current to pass through it to allow a breaker to trip, but especially in long runs, that is not always the case. There are a lot of connections in long conduit run, and it only takes one bad connection to break the ground fault path.
Now, that said, the Soares Book on Bonding and Grounding (see photo) does in fact list a maximum conduit run. But, this is not code per the NEC, it is a IAEI guideline.
There are other issues to consider as well. Copper is at least 12 times as conductive as steel (some charts list 17x), and is at least 250 times less magnetic. RMC is famous for having a high-impedance over distance and will allow magnetic fields to form that increase the likely hood of objectionable currents, harmonics, interference, and electrical noise of all kinds. This is why it is E&S Grounding Solutions official policy to recommend the use of copper conductors for the grounding conductor in all circuits. RMC should be considered a back-up system and not the primary. We also do not recommend the use of Edison circuits where you share a single ground wire between the loads on 3 different phases.
Additionally, there are many different standards and codes that actually require equipotential bonding of RMC! ANSI/TIA/EIA-J-STD-607-A, Motorola’s R56, numerous IEEE standards, and many more all require that the Rigid Metal Conduit itself, be bonded to ground and that jumper wires be used around each and every one of the connections. This is why you can purchase stock grounding bushings and grounding couplers, so that the grounding jumper wires can be easily installed. In the future, the NEC will not allow RMC to be used as a grounding conductor and will have many more requirements for equipotential bonding. Consider the equipotential bonding requirements for a swimming pool, and you will begin to understand where grounding is heading under the NEC in the future.
In conclusion, it is unfortunate that the NEC even allows the practice of using RMC as the sole grounding conductor, and it is a disgrace that it has no length limit for when it is used. There are no imaginable cases where using RMC as your primary grounding conductor is a good idea. Using copper grounding conductors each and every time improves the safety for people, and improves the way equipment functions.
We hope you have found this information useful. Please feel free to contact us again in the future should you have any further questions.
The Engineering Team at E&S Grounding Solutions
Photo Credit: IAEI