Rooftop Antenna Grounding and Bonding
A wireless internet service provider is installing a 20 ft rooftop mast on a 6-story building in Buffalo. The mast supports a small antenna and a 24 Vdc radio. From the radio, a coax runs 90 ft to a cabinet; a Cat 6 cable also runs to the cabinet for management. The contractor proposes to drive an 8 ft ground rod next to the mast, run 8 AWG copper from the mast to that rod, and bond the coax shield to the cabinet chassis. The cabinet is plugged into a 120 V receptacle with a plug-in surge strip. The building has a concrete-encased electrode and a water pipe; the contractor says a separate ground rod is better because it "keeps lightning out of the building." The roof has no intersystem bonding termination.
1. Evaluate the proposed grounding electrode and conductor sizes. What must be changed to make the installation code-compliant?
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A separate ground rod at the mast is not prohibited, but it cannot be an isolated grounding system. The mast, antenna, coax shield, and equipment cabinet must be bonded to the building's grounding electrode system. The antenna mast grounding conductor is generally required to be at least 10 AWG copper under NEC Article 810, so the proposed 8 AWG is not automatically a violation for the mast. However, communications bonding conductors under Article 800 are generally required to be at least 6 AWG copper, and the coax shield and cabinet bonding should follow that requirement. The new ground rod must be bonded to the building grounding electrode system with a conductor sized per NEC 250.66; for a rod electrode, a 6 AWG copper bonding jumper is commonly used. The installation should also include an intersystem bonding termination per NEC 250.94 so communications and antenna systems have a compliant place to connect. The water pipe cannot be used as the only electrode, and the concrete-encased electrode should be part of the building electrode system. The plug-in surge strip is not a substitute for a listed antenna discharge unit or surge protective device.
2. What is wrong with using the cabinet's 120 V receptacle ground as the only ground for the radio and antenna?
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The equipment grounding conductor of the 120 V receptacle is intended to clear faults on the electrical circuit; it is not a substitute for the grounding and bonding required for an antenna or communications system. The antenna mast, coax shield, and radio must be bonded to the building grounding electrode system to provide a low-impedance path for lightning and surge currents. Using only the receptacle ground can create a difference in potential between the cabinet and the building grounding system, and it does not provide the required intersystem bonding. The correct approach is to bond the coax shield to the cabinet or a listed ground bar, bond the cabinet to the building grounding electrode system with the appropriate conductor, and install a listed antenna discharge unit or surge protective device on the coax. The grounding conductor should be as short and straight as practicable, with no sharp bends, and should be connected to the intersystem bonding termination.
3. The contractor says the separate rod "keeps lightning out of the building." How should the owner respond, and what tests or inspections should be requested?
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The owner should reject the idea that an isolated rod keeps lightning out. An isolated rod can actually increase the risk of side flashes and equipment damage because it is not bonded to the building grounding system. The contractor should bond the new rod to the building grounding electrode system, install the intersystem bonding termination, and provide a listed surge protective device for the coax and power. Before acceptance, the owner should request a grounding and bonding inspection, verification of conductor sizes and connections, and documentation that the antenna discharge unit is listed. The contractor should also perform a visual inspection of the coax shield bonding and a continuity test between the mast, cabinet, and building grounding electrode system. If the building has a lightning protection system, the antenna system must be bonded to it per the applicable standards. The AHJ should inspect the final installation, especially the intersystem bonding connection and the grounding electrode conductor route.