8.1 Equipment Grounding and System Bonding

Equipment Grounding and System Bonding

Safe and code-compliant photovoltaic (PV) power systems require two distinct grounding and bonding functions: system grounding and equipment grounding. While field personnel often group these concepts under the broad term "grounding," their electrical purposes, mechanical methods, and National Electrical Code (NEC) governing rules differ fundamentally. System grounding dictates how electrical circuits interact with the earth, while equipment grounding establishes a low-impedance bonding path to protect personnel and property from shock and fire hazards.

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1. System Grounding vs. Equipment Grounding: Core Principles and Code Intent

The National Electrical Code establishes a strict conceptual division between system grounding and equipment grounding across Article 250 and Article 690.

System Grounding (NEC 690.41)

System grounding involves the intentional electrical connection of one conductor of an electrical circuit to the earth (ground). The primary purposes of system grounding are to stabilize circuit voltage with respect to earth during normal operation, limit voltage surges caused by lightning or line transients, and facilitate the operation of overcurrent and ground-fault protective devices.

Under NEC 690.41, PV array DC circuits fall into three primary classifications:

1. Solidly Grounded Systems: One circuit conductor (historically the DC negative conductor in negative-grounded systems, or DC positive in positive-grounded sun-power systems) is bonded directly to the grounding electrode system with no intervening overcurrent device or impedance. Solidly grounded DC systems were standard in legacy central inverters with galvanic isolation transformers.
2. Reference Grounded / Functionally Grounded Systems: Defined in 2017 NEC 690.2 (with the permitted configurations listed in 690.41(A)), a functionally grounded PV system has an electrical reference to ground through electronic measurement circuits, ground-fault protection devices (such as a residual current sensing loop), or high-impedance paths, rather than a solid metallic bond. The overwhelming majority of modern grid-tied string inverters and microinverters operate as functionally grounded systems. In these transformerless (non-isolated) topologies, both the DC positive and DC negative conductors float relative to earth during operation, though their midpoint or reference potential is continuously monitored.
3. Ungrounded Systems: Systems where neither circuit conductor has any intentional connection to ground, such as floating arrays on isolated-transformer inverters. The 2014 NEC regulated these under 690.35 (requiring PV wire and overcurrent protection in both conductors); the 2017 NEC deleted 690.35 and lists ungrounded arrays as one of the configurations in 690.41(A).

Equipment Grounding (NEC 250 & 690.43)

Equipment grounding connects all exposed, non-current-carrying metal parts of PV equipment—including PV module frames, mounting racks, metal enclosures, conduit, junction boxes, combiner enclosures, and inverter chassis—to the grounding electrode system.

The core objectives of equipment grounding are:

• Shock Hazard Elimination: Prevent exposed metal structures from carrying dangerous voltage potentials if insulation fails or an ungrounded conductor contacts the frame.
• Fault Clearing Path: Provide an effective ground-fault current path with sufficiently low impedance to facilitate the rapid operation of overcurrent protective devices (OCPDs) or electronic ground-fault detectors.
• Equipotential Bonding: Maintain all metallic equipment at zero volts relative to earth and to adjacent structural metalwork.

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2. Equipment Grounding Conductors (EGC): Sizing and Installation Requirements

An Equipment Grounding Conductor (EGC) is the designated conductor installed along electrical circuits to bond non-current-carrying metal enclosures together and route fault currents back to the electrical source or ground.

Sizing the EGC (NEC 250.122 and 690.45)

In standard AC electrical circuits, the EGC is sized according to NEC Table 250.122 based on the ampere rating of the overcurrent protective device (fuse or circuit breaker) protecting the circuit conductors. However, in PV source and output circuits, specific adjustments apply:

1. PV Circuit Sizing Rule (NEC 690.45): An equipment grounding conductor for a PV array circuit must be sized in accordance with NEC 250.122 based on the rating of the circuit overcurrent device. Where no overcurrent device protects the circuit (such as one- or two-string systems without string fuses), the EGC is sized from Table 250.122 using an assumed overcurrent device rated per 690.9(B) (125% of the 690.8(A) maximum circuit current).
2. Minimum Size: The 2017 NEC 690.45 sets a 14 AWG minimum, and array EGCs smaller than 6 AWG must meet the physical-protection rules of 250.120(C) (690.46).

Voltage-Drop Upsizing: The PV Exception (NEC 690.45)

This distinction is frequently tested:

• General rule (NEC 250.122(B)): In ordinary ac branch circuits and feeders, when ungrounded conductors are increased in size beyond what ampacity requires, wire-type EGCs must be increased proportionately in circular mil area. This still applies to the PV system's inverter output (ac) circuit.
• PV dc circuits (2017 NEC 690.45): For PV source and output circuits, "increases in equipment grounding conductor size to address voltage drop considerations shall not be required." PV source currents are limited, so a larger EGC adds cost without improving fault clearing.
• Example: A PV output circuit protected by a 20 A fuse uses 12 AWG copper conductors, upsized to 8 AWG to cut voltage drop on a 250-foot run. Table 250.122 still calls for a 12 AWG copper EGC for a 20 A device, and 690.45 does not require it to grow. If the same upsizing were done on the ac inverter output circuit, 250.122(B) would require a proportional increase: 16,510 ÷ 6,530 = 2.53, so the EGC grows to about 16,500 circular mils (8 AWG).

Material and Physical Protection (NEC 250.120(C))

• Copper vs. Aluminum: Bare copper conductors are the industry standard for array EGCs. Aluminum or copper-clad aluminum conductors are permitted under NEC 250.118, but they cannot be installed in direct contact with masonry or earth, nor installed outdoors within 18 inches (450 mm) of the earth.
• Mechanical Damage Protection: Under NEC 250.120(C), equipment grounding conductors smaller than 6 AWG must be protected from physical damage by installing them inside an approved raceway (such as EMT, PVC, or metallic conduit) or armored cable armor. Bare 6 AWG or larger copper conductors may be secured directly to structural members or module racking without conduit, provided they are securely fastened and not subjected to severe physical damage.

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3. Module Frame and Aluminum Racking Bonding Mechanics

Modern PV modules are manufactured with an anodized aluminum frame. Anodizing creates an electrochemically generated aluminum oxide ($Al_2O_3$) layer that provides superior corrosion resistance and exterior durability. However, aluminum oxide is an electrical insulator with a dielectric breakdown voltage exceeding 1,000 volts.

Overcoming the Anodized Insulating Layer

Direct contact between smooth anodized aluminum surfaces does not establish an electrical bond. Fasteners, clamps, or mounting brackets bolted to an anodized frame will remain electrically isolated unless the anodized layer is mechanically penetrated down to the raw conductive aluminum substrate.

Methods of penetrating the anodized layer include:

1. WEEB (Washer, Electrical Equipment Bond) Clips: Thin, stamped stainless steel washers featuring precision sharp teeth or ridges. When torqued between the module frame and the aluminum mounting rail, the teeth pierce the anodized coating on both components, forming a gas-tight, corrosion-resistant electrical connection.
2. UL 2703 Integrated Grounding Mid-Clamps and End-Clamps: Standard modern mounting systems utilize mid-clamps engineered with integrated stainless steel bonding pins or teeth. As the installer tightens the clamp bolt to the manufacturer's specified torque, the clamp secures the modules mechanically while simultaneously bonding adjacent module frames directly to the underlying rail.
3. Lay-In Grounding Lugs: Certified tin-plated copper or aluminum lay-in lugs mounted directly to the designated grounding hole on each module frame or rail section. The lug bolt must incorporate a stainless steel star washer or serrated head to penetrate the anodization around the mounting hole.

UL 2703 Listing Standard

UL 2703 (Standard for Mounting Systems, Mounting Devices, Clamping/Retention Devices, and Ground Lugs for Use with Flat-Plate Photovoltaic Modules and Panels) evaluates and certifies:

• Integrated electrical bonding across module frames, clamps, rails, and splice plates.
• Mechanical load ratings (wind uplift, snow load).
• Fire classification of the combined module and mounting assembly.

When a racking system and specific module series are certified together under UL 2703, the racking rails serve as the equipment grounding conductor between modules. Installers need only run a single copper EGC from a certified rail lug back to the combiner or inverter, rather than running individual bonding jumpers to every individual module.

Dissimilar Metals and Galvanic Corrosion

Connecting bare copper directly to bare aluminum triggers severe galvanic corrosion in outdoor environments due to their wide electrochemical potential difference. In moist outdoor air, the aluminum acts as an anode and rapidly oxidizes, destroying the mechanical and electrical integrity of the bond.

• Bare copper conductors must never touch aluminum frames or rails directly.
• Connections must utilize tin-plated copper lugs, stainless steel star washers, or dual-rated AL9CU / CU9AL mechanical lugs with stainless steel separating hardware.

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4. Grounding Electrode Conductors (GEC) and Electrode Systems

A Grounding Electrode Conductor (GEC) is the conductor used to connect the system grounding conductor, the equipment grounding conductor, or both, to the grounding electrode (such as a ground rod, concrete-encased electrode/Ufer ground, or ground ring).

Sizing the GEC (NEC 250.66)

The GEC is sized according to NEC Table 250.66 based on the circular mil area of the largest ungrounded service-entrance conductor or equivalent area for derived systems:

• Up to 2 AWG copper service conductors: 8 AWG copper GEC
• 1 AWG to 1/0 AWG copper service conductors: 6 AWG copper GEC
• 2/0 AWG to 3/0 AWG copper service conductors: 4 AWG copper GEC
• Over 3/0 AWG up to 350 kcmil copper service conductors: 2 AWG copper GEC

Special limits apply to specific electrode types: A GEC connecting exclusively to a rod, pipe, or plate electrode is never required to be larger than 6 AWG copper (NEC 250.66(A)). A GEC connecting to a concrete-encased electrode (Ufer) is never required to be larger than 4 AWG copper (NEC 250.66(B)).

Common Grounding Electrode System (NEC 690.47)

Under 2017 NEC 690.47(A), a building or structure supporting a PV array must have a grounding electrode system installed per Part III of Article 250, and the PV array EGCs must be connected to it. For PV systems that are not solidly grounded, the EGC of the PV system output circuit, connected to the distribution equipment, may serve as that connection. All electrodes present at a building (ground rods, metallic water pipe, concrete-encased rebar, building structural steel) must be bonded together into a single, unified grounding electrode system per NEC 250.50.

Installers must never create an isolated, standalone ground rod for a rooftop PV system that is not bonded to the building electrical service grounding electrode. An isolated ground rod creates dangerous potential differences during lightning strikes or ground-fault events, leading to equipment destruction and flashover hazards.

Auxiliary Grounding Electrodes (NEC 690.47(B) and 250.54)

At remote ground-mounted arrays or detached pole-mounted structures, installers frequently drive an auxiliary grounding electrode (ground rod) at the array structure. Under NEC 250.54:

• The auxiliary electrode is optional and supplemental.
• It may be connected directly to the array frame or structure (690.47(B)) or to the equipment grounding conductor, and it does not have to be bonded to the building's electrode system or meet the 25-ohm rule.
• The earth shall not be used as an effective ground-fault current path. An actual metallic equipment grounding conductor must always run with the circuit conductors to ensure sufficient fault current can return to clear overcurrent devices.

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Lightning Protection Systems

Where a building has a lightning protection system (air terminals and down conductors installed under NFPA 780 and UL 96A), the PV array must be coordinated with it. NFPA 780 requires metal bodies such as racking that are within the calculated bonding distance of the lightning protection conductors to be bonded to them, or else kept beyond that separation distance, and air terminals may be added so modules sit within the zone of protection. NEC 250.106 requires the lightning protection system's ground terminals to be bonded to the building's grounding electrode system. Never cut or reroute lightning conductors without the lightning protection installer's involvement, and expect surge protective devices (NEC Article 285) where arrays connect to building wiring.

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5. Comparison: EGC vs. GEC Technical Specifications

The following reference table summarizes the code rules, sizing standards, and installation distinctions between Equipment Grounding Conductors and Grounding Electrode Conductors: