Grounding Electrode System for a New Building
A new commercial building has a 1200 A, 480Y/277 V service. The building has a concrete-encased electrode (CEE) with 4 AWG bare copper, a metal water pipe with 10 ft buried, and a ground ring with 2 AWG bare copper encircling the building. The soil is rocky with a resistivity of 500 ohm-meters. The electrical contractor proposes to use only the CEE as the grounding electrode, claiming it meets the 25-ohm requirement. The AHJ requires a grounding electrode system per NEC 250.50 and 250.53. The building also has a steel frame that is effectively grounded. The service disconnect is 50 ft from the water pipe entry.
1. Evaluate the proposed grounding electrode system and determine if it complies with NEC 250.50 and 250.53.
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NEC 250.50 requires all available grounding electrodes to be bonded together to form the grounding electrode system. Available electrodes include the CEE, metal water pipe, ground ring, and steel frame. The contractor cannot use only the CEE. The water pipe with 10 ft buried is an electrode (250.52(A)(1)) and must be bonded. The ground ring is also an electrode (250.52(A)(4)). The steel frame is an electrode (250.52(A)(2)) if effectively grounded. So all must be bonded. The 25-ohm requirement is for a single electrode (250.53(A)(2)), but with multiple electrodes, the resistance must be 25 ohms or less? Actually, 250.53(A)(2) requires a single rod, pipe, or plate electrode to have 25 ohms or less, or an additional electrode. But for the overall system, there is no explicit ohm requirement except for lightning protection? So the contractor's claim is insufficient. The system must include all electrodes bonded together. Also, the CEE must be at least 4 AWG copper (250.52(A)(3)) and the ground ring at least 2 AWG (250.52(A)(4)). So the proposed system is noncompliant because it omits the water pipe, ground ring, and steel frame.
2. What minimum size grounding electrode conductor is required for the service?
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The grounding electrode conductor (GEC) size is based on the service conductor size per NEC 250.66. The service is 1200 A, but we need the size of the largest ungrounded service conductor. Assume the service conductors are sized per 310.15(B)(16) for 1200 A, likely 600 kcmil copper or larger. For a 1200 A service, typical conductor is 600 kcmil Cu (ampacity 475 A? Actually, 600 kcmil at 75°C = 475 A, so multiple conductors per phase. For 1200 A, you might use 4 sets of 350 kcmil Cu. The GEC size is based on the equivalent area of the largest phase conductor. Table 250.66: for service conductors over 1100 kcmil Cu, GEC is 3/0 AWG Cu. But we need to determine the equivalent area. If using 4 sets of 350 kcmil, total area = 1400 kcmil, so GEC = 3/0 Cu. So minimum GEC is 3/0 AWG copper. However, if the service conductors are smaller, adjust accordingly. But the answer should state that the GEC is sized per Table 250.66 based on the largest ungrounded service conductor, and for a 1200 A service, it is likely 3/0 Cu or 250 kcmil Al.