Thank you for your question regarding the 20-ft earthing rule for cable antenna earthing, it is our pleasure to help.
There are really no special rules or exceptions for cable and antenna earthing in the National Electrical Code (NEC). In the end, your cable must be bonded to the main electrical earthing system and be in compliance with NEC Article 250. The primary purpose of the earthing 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 earth 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 earthing electrode conductor length of 6.0 m (20 ft), a separate earthing electrode as specified in 250.52(A)(5), (A)(6), or (A)(7) shall be used, the earthing electrode conductor shall be connected to the separate earthing electrode in accordance to 250.70, and the separate earthing electrode shall be connected to the power earthing 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 earthing electrode, you must install a standard 10-ft earthing 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 earth.
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 Earthing 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 Earthing Solutions
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