Thank you for your question regarding the 20-ft grounding rule for cable antenna grounding, it is our pleasure to help.
There are really no special rules or exceptions for cable and antenna grounding in the National Electrical Code (NEC). In the end, your cable must be bonded to the main electrical grounding system and be in compliance with NEC Article 250. The primary purpose of the grounding is to ensure that there are no hazardous differences in potential between your cable system and the building.
Now, the 20-ft rule is governed under NEC Article 820.100(A)(4) which states that you must bond the outer sheath of your cable to the building’s main ground with a conductor less than 20-ft. The intention is to ensure a low impedance path. However, there is a listed exception to the code at 820.100(A)(4) which states: “In one- and two-family dwellings where it is not practicable to achieve an overall maximum bonding conductor or grounding electrode conductor length of 6.0 m (20 ft), a separate grounding electrode as specified in 250.52(A)(5), (A)(6), or (A)(7) shall be used, the grounding electrode conductor shall be connected to the separate grounding electrode in accordance to 250.70, and the separate grounding electrode shall be connected to the power grounding electrode system in accordance with 820.100(D).”
What this means is that when you bring your cable in to the dwelling and it is further than 20-ft to the main grounding electrode, you must install a standard 10-ft grounding rod (8-ft below the 18-inch mark, and 6-inches above grade for bonding) at the cable building entrance, AND run a #6 AWG copper conductor whatever length is required to bond your system to the building’s main ground.
One more thing to keep in mind is that the 2011 version of the National Electrical Code (NEC) has some fairly significant changes over the 2008 code. You may want to make sure you have the latest code.
We hope you find this information useful. If you should have any further questions, please do not hesitate to call us at 310-318-7151 California time and someone will be glad to discuss your issues with you free of charge. You may wish to consider some of the great training seminars that E&S Grounding Solutions offers. These seminars can be conducted at your facility for as many personnel as you need trained. Please let us know if we can be of any assistance.
The Engineering Team at E&S Grounding Solutions
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