Guidelines for Designing Grounding Systems for Solar PV Installations in Accordance With the NEC

Grounding and bonding is a subject area that can be confusing to many. In this blog post, we summarize key points according to the NEC. The NEC is the primary guiding document for the safe designing and installation practices of solar PV systems in the residential and commercial markets in the United States. The summary outlined below can be used by a solar PV practitioner; however, it is highly recommended that section 690.41, 690.42, 690.43, 690.45 and 690.47 always be read in conjunction with section 240 of the NEC.

Major points to remember:

1) Ground fault current always needs an effective return path back to the source. An equipment grounding conductor (EGC) provides such a path in most of the cases. In this regard, a main bonding jumper (MBJ) should be installed to connect the EGC to the neutral of the supply at one point only (see figure below). This is true for a solidly grounded system.

2) Connection of grounding and bonding of the equipment grounding conductor (EGC), grounding electrode conductor (GEC), and bonding jumpers at any point or mounting PV modules should be carried out through listed devices or systems (i.e. connectors, terminal bars, etc.).

3) It is recommended that the main bonding jumper (MBJ) and the grounding electrode conductor (GEC) are connected together at the same point (i.e. grounding bus-bar inside main disconnect or distribution panel of a residential or building facility). Connecting neutral to the EGC at multiple points will lead to objectionable current (see figure below). Objectionable current occurs when there are multiple paths for the return current and current starts flowing back to the source though the grounding conductor (EGC) or body of an equipment under the normal circumstances (no fault-current existing) as shown in the figure below. This happens due to having more than one main bonding jumpers.

4) Description and types of grounding electrode systems are provided in section 250.50 and 250.52 of the NEC. Effort should be made that grounding electrodes are buried below the permanent moisture level in the earth. Major types are:

i) Metal underground water pipe: Underground metal water pipe in direct contact with the earth for 10 feet or more

ii) Metal in-ground support structure: Metal in-ground support structure(s) in direct contact with the earth vertically for 10 feet or more

iii) Concrete-encased electrode (ufer): One or more electrically conductive steel reinforcing bars of not less than ½ inch of diameter and 20 feet in length, or bare copper conductor not smaller than 4 AWG of 20 feet or greater length (any of these should be encased at least 2 inches)

iv) Ground ring: A ground ring consisting of at least 20 feet of bare copper conductor not smaller than 2 AWG buried in earth

v) Grounding rod: This is the most commonly used type of grounding or earthing electrode. It must have at least 3/8 inch of diameter and 8 feet in length buried in the earth

vi) Plate electrode: Bare or electrically conductive coated iron or steel plate with not less than ¼ inch of thickness, or solid uncoated copper metal plate not less than 0.06 inch of thickness, with an exposed surface area of 2 square feet.

5) If a supplemental grounding electrode or rod is used, then it should be separated at least by 6 feet from the main grounding electrode.

6) Aluminum conductors are not allowed as the GEC.

7) Size of the GEC is found using the section 250.66 and Table 250.66 of the NEC. This essentially means that size of the GEC is dependent upon the size of the largest ungrounded current-carrying conductor in the system.

8) If the size of the GEC is smaller than 6 AWG, then it must be protected in a metal, intermediate metal or PVC conduit. If the size of the GEC is 6 AWG or greater and is exposed to physical damage, then again it should be protected in conduit. Moreover, splicing the GEC is not allowed (except the permitted methods).

9) If there are multiple buildings or structures having their own grounding system or GEC, then all should be combined or connected at one common place (i.e. bus bar) and eventually connected to a common grounding electrode.

10) Exposed metal parts of fixed equipment likely to become energized must be connected to the circuit equipment grounding conductor.

11) Types of EGC can be wire type, rigid metal conduit, intermediate metal conduit, electrical metal tubing or part of metal-clad cable as discussed in section 250.118 of the NEC.

12) The size of the EGC is determined through section 25.122 and Table 250.122 of the NEC. This essentially means that the size of the EGC is dependent upon the size of the overcurrent protection device (OCPD) in the PV circuit(s). However, it is not required to be larger than the size of the current-carrying circuit conductor(s). If the size of the current conductors is increased to cater the voltage drop, then the size of the EGC is not required to be increased, except as sized according to OCPD and Table 250.122.

13) An EGC smaller than 6 AWG should be protected from physical damage by installing these in a raceway or cable armor. It should run with the circuit or current-carrying conductor in the same raceway or cable when it leaves the vicinity of the PV array.

14) Nowadays, functionally grounded inverters or PV arrays not isolated from the grounded output circuit of inverter are used. This allows the EGC of the PV circuit to be connected to the grounding point provided by the inverter, eliminating the need for a separate DC grounding system. The grounding point of the inverter is connected onwards to the grounding system or grounding electrode of the residential facility or building (see figure below).

15) PV circuits having 30V or 8A more shall be provided with a ground-fault protection device (GFPD). Nowadays, in general, this is a built-in function of inverters. A GFPD should be listed for this purpose. A GFPD shall disconnect the faulted circuit from the rest of the circuit and stop the inverter supplying output power. In this regard, the inverter should be listed to UL1741.

16) A GFPD is not required for a PV circuit which is not installed on a building, is solidly grounded and there are not more than two PV circuits connected in parallel.

17) Exposed metal parts, equipment or supporting structure in the PV circuit likely to become energized should be connected to the grounding system or EGC in accordance with the NEC sections 250.104, 250.134 and 250.136. Devices and equipment which are used to support or mount the PV modules or equipment, and which eventually are required to be connected to the EGC shall be listed, labeled and identified for the purpose of bonding to the grounding system in accordance with UL2703. UL2703-listed bonding components help reduce installation time and ensure required bonding strength.

Note: Lightning protection system and associated grounding system should be designed in accordance with the NFPA 780 as stated in the 250.106 of the NEC.