CODE CORNER: Grounding & Bonding


A favorite topic for PV installation professionals, or anyone in the electrical trade, is grounding and bonding. That the topic is one of the hardest to effectively navigate within the National Electrical Code (NEC) leads to fun and, sometimes, heated debates. We’re moving closer to the adoption of the 2017 NEC for a number of jurisdictions—so let’s discuss some of the biggest impacts of the Code affecting PV installations.

Grounding and bonding has been covered in many previous “Code Corners,” including HP149, HP152, HP153, and HP155. Each of those articles has good background information to better understand Code definitions and rules specific to PV installations. In HP149, for example, the definitions for the different types of grounding and bonding conductors are discussed to help differentiate requirements.

Changes in inverter and module-level power electronics have made navigating the grounding and bonding requirements for modern PV arrays difficult as we have moved away from technologies that were prominent when the 2014 Code was written. For example, back then, the majority of the grid-tied PV systems used inverters with isolation transformers. These transformers isolated the AC and DC sides of the system and required the need for a DC grounding electrode system as defined in 690.47(C). Since that time, non-isolated inverters have become the norm, changing the need for DC grounding electrodes.

Equipment Bonding Requirements

Now that transformerless inverters constitute the majority of grid-tied PV installations, the requirements for grounding and bonding require closer examination. In 690.43 of the 2014 Code, subsections A through F outline the equipment-grounding requirements and 690.45 dictates the size of the grounding conductor. Regardless of system voltage, there is a requirement that “all exposed non-current-carrying metal parts of PV module frames, electrical equipment, and conductor enclosures shall be grounded in accordance with 250.134 or 250.136(A).” All of the metal components need to be bonded together. This equipment grounding eliminates the difference in voltage potential, thereby reducing the shock risk. This requirement pertains to systems both with and without an isolation transformer in the inverter.

Section 690.43 lists the rules for bonding the different components and the acceptable components that can be used. Since the inclusion of racks that use mechanical attachments to also make the module-to-rail bond under UL standard 2703, this aspect of the installation has become much easier for installers. One section that may trip up installers is 690.43(F), which requires that an equipment-grounding conductor (EGC) be run with the circuit conductors when they leave the array vicinity. Common mistakes are running the EGC exterior to a conduit containing circuit conductors or in a different path from the conductors.

Section 690.47 (“Grounding Electrode System”) may be confusing for installers working with non-isolated inverters. This section is divided into four subsections. Prior to the widespread use of non-isolated inverters, installers focused on subsection C, since it pertained to systems using inverters with isolation transformers. “Code Corner” in HP155 focused on how to meet that subsection.

For PV systems with non-isolated inverters, the requirement for DC grounding electrode conductors is eliminated, effectively making 690.47(C) inapplicable. But what is required to comply with this section when using non-isolated inverters?

A look back to 690.47(B) helps. The first sentence of 690.47(B) refers you back to 250.169 for ungrounded systems. But when you read 250.169, the section applies to separately derived systems. Going into the definitions of the Code, a non-isolated PV array does not meet the definition of a separately derived system—therefore, 250.169 does not apply and this grounding electrode conductor is not required.

Comments (1)

bornfreeee's picture

Being grounded has never been so much fun! In a case with a roof mount array on an outbuilding some 400' from the main home where the service entry, and existing grounding electrode are....It seems like an additional grounding electrode at the outbuilding would be desired (maybe 2 with the dry clay soil, on the same side of the building). Bonded to the ground bar of the subpanel of the outbuilding, along with the equipment ground running from the service entry. And the equipment ground from the array racking could also be bonded here, or directly to the outbuilding additional grounding electrode shall be a better approach?

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