Additional Electrodes for Array Grounding

Article 690.47(D), new to the 2008 National Electrical Code®, has generated questions from PV system designers and installers. What is the impact of this new requirement for PV grounding electrode systems? Here Bill Brooks, Dick Ratico and John Wiles share their insights and clarifications on the topic.

Article 690.47(D) of the 2008 National Electrical Code

Additional Electrodes for Array Grounding

Grounding electrodes shall be installed in accordance with 250.52 at the location of all ground- and pole-mounted photovoltaic arrays and as close as practicable to the location of roof-mounted photovoltaic arrays. The electrodes shall be connected directly to the array frame(s) or structure. The dc grounding electrode conductor shall be sized according to 250.166. Additional electrodes are not permitted to be used as a substitute for equipment bonding or equipment grounding conductor requirements. 

The structure of a ground- or pole-mounted photovoltaic array shall be permitted to be considered a grounding electrode if it meets the requirements of 250.52. Roofmounted photovoltaic arrays shall be permitted to use the metal frame of a building or structure if the requirements of 250.52(A)(2) are met. 

Exception No.1: Array grounding electrode(s) shall not be required where the load served by the array is integral with the array. 

Note from NEC 2008 Handbook: Enhanced protection from lightning-induced surges may be afforded by connecting an array grounding electrode even on buildings with colocated loads (including inverters). 

Exception No. 2: Additional array grounding electrode(s) shall not be required if located within 6 ft of the premises wiring electrode.

Good Design Requirement in High Lightning Areas

By Bill Brooks

Many questions have arisen about the new requirement in Article 690.47(D) of the 2008 NEC related to additional electrodes for array grounding. It is helpful to clarify why such electrodes would be beneficial. First, an electrode in the vicinity of the array provides a short path to ground in the event of a lightning strike. Second, it provides a low resistance path to ground and offers additional protection for people should they come in contact with a module frame that might be inadvertently energized by a failed wiring system or damaged module.

The requirement is for all PV systems with very few exceptions. It calls for a supplementary electrode for all ground-mounted arrays (and polemounted, but then I thought poles got put in the ground) and requires an electrode as close as practical for arrays on rooftops. This separate electrode must have a conductor from the array frames to the electrode sized according to NEC 250.166, which is the requirement for sizing grounding electrode conductors for dc systems. Article 690.47(D) also makes it clear that this supplemental grounding system does not act as a substitute for the equipment grounding conductor required in 690.45.

If the support structure on a groundmounted system is 8 feet or more deep in the ground, as with most pole mounts, this meets the requirements of Article 250.52 for a grounding electrode. No additional electrode would be needed. Also, building steel that meets the requirements of Article 250.52(A) (2), common in newer commercial buildings, can be used as the grounding electrode.

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