With PV systems and other electrical systems, grounding and bonding is one of the most discussed and contested topics. The preceding article on this topic in HP152 covered general requirements for equipment grounding and some of the applicable rules in National Electrical Code (NEC) articles 250 and 690. This second part dives deeper into the equipment grounding rules and gives an overview of the other half of the discussion—the grounding electrode system.
A PV system’s metallic, noncurrent-carrying components must be bonded together and then bonded to earth through the use of an equipment-grounding conductor (EGC). Article 250 establishes multiple ways in which this bond can be established. For PV systems, the most common method is to use a conductor to bond the materials together, especially from the PV array.
Sizing this conductor is determined by referring to Section 690.45 in the Code. The general rule, 690.45(A), states that the “EGC in PV source and output circuits shall be sized in accordance with Table 250.122.” Knowing the exact circuit designations and locations is important for properly applying the rules. If necessary, revisit the definitions section of Article 690 to familiarize yourself with the definitions of PV source and output circuits.
Table 250.122 indicates that the EGC size is determined by the rating of the overcurrent protection device (OCPD) that’s protecting the equipment. For PV source circuits, this will typically be the OCPD located in the combiner box. For PV output circuits, this will typically be the OCPD located at the inverter, recombiner, or fused DC disconnect, depending on the installation methods used (see also "Sizing Equipment Grounding Conductors").
If there are no OCPDs in the PV source circuits, as allowed by the exception in 690.9, the circuit’s rated short-circuit current shall be the assumed overcurrent protection rating for the table. In no case can the EGC be sized smaller than a 14 AWG conductor, the corresponding conductor size for a 15 A OCPD. The 2011 690.9 exception for OCPDs applies to source-circuit conductors. If an external combiner box (i.e., not integrated into the inverter) is used, then OCPDs need to be provided for the PV output circuits—this is accomplished via a fused disconnect shown in the diagram.
Section 690.45(A) does not require that the EGC be upsized if the current-carrying conductors are increased in size to account for voltage drop. Section 250.122(B)’s general rule requires that the EGC size be increased in proportion to the circular area of the current-carrying conductors. In PV installations, the rules set in 690 take precedence over the general rules in 250, and increasing the diameter of the EGCs is not required. The second subsection of 690.45 sets the requirements for the uncommon systems that do not use ground-fault protection, which include increasing the EGC’s size to meet safety requirements of 690.45(B).
The final EGC sizing requirement is in 690.46. It references 250.120, which requires protecting EGCs smaller than a 6 AWG conductor in places where the conductor may be “subject to physical damage.” This may include the back of an array, even on rooftops, according to some authorities having jurisdiction (AHJs). Some jurisdictions have also applied a general ruling that any EGC associated with PV systems must either be a minimum 6 AWG or protected in a raceway, regardless of the location. Discuss this with your AHJ early in the design process, since changing an EGC after the array has been installed can be a difficult task.
For PV systems that use grounded current-carrying conductors, i.e., systems using transformer-based inverters, Section 690.47 is the primary reference for the grounding electrode system. In grounded systems, a connection must be made between the DC circuit grounding point and earth, as required in 690.41 and 690.42. This required connection consists of the grounding electrode (GE) and the grounding electrode conductors (GECs). It is worth noting, regardless of the inverter’s use of a transformer, that the EGC requirements discussed always apply. It is also very important to read the installation instructions included with all inverters to make sure all of the grounding instructions specific to that inverter are being followed.
Over the past few Code cycles, the changes to 690.47 have been dramatic. In the 2011 Code, the changes made to 690.47 seemed well-received by the PV community, primarily due to the deletion of the most difficult and controversial requirement, 690.47(D), which called for additional electrodes for array grounding. The 2011 NEC’s 690.47 has three subsections, with the third being the most widely used. The first two are for systems that exclusively produce and use DC or AC. The third subsection, 690.47(C), applies to systems that have both DC and AC electrical requirements—the majority of PV systems installed today. That third subsection is divided into three additional subsections for different installation scenarios.
All three of 690.47(C)’s subsections require that the DC grounding system be connected to the AC system’s grounding electrode. For grid-direct PV systems, this is generally the existing grounding electrode—a ground rod, water pipe, concrete-encased electrode, etc. A new DC grounding system connection to earth can be accomplished by:
Each of these provisions has its own unique set of rules to follow. Grounding and bonding is always a complicated case. Future articles will explore these 690.47 requirements in more detail to help decipher the grounding electrode system requirements.
Ryan Mayfield is the principal at a Corvallis, Oregon, design, consulting, and educational firm with a focus on PV systems. He is currently working on international projects that prove that grounding and bonding issues know no borders.