Solar PV Grounding System Design: EGC, GFP & NEC 250 Compliance

Grounding is the most frequently failed category in solar PV inspections. A NYSERDA NY-Sun inspection review of New York installations found grounding deficiencies in 53% of systems inspected (NYSERDA, Solar PV Permitting and Inspecting, NY-Sun Program) — the single largest cause of permit re-inspection, PTO delay, and liability exposure. Yet most solar design guides treat grounding as an afterthought, tacked onto the end of a wiring chapter.

This guide covers every layer of a compliant solar PV grounding system design: equipment grounding conductors (EGC) and their NEC 250.122 sizing rules, grounding electrode conductors (GEC) and the 690.47 connection options, ground fault protection detection logic for both transformer-isolated and transformerless inverters, array frame bonding with UL 2703-listed hardware, and the 10 specific mistakes that trigger AHJ failures. If you use solar design software to generate single-line diagrams and electrical documentation, understanding these rules tells you exactly what your software should be checking automatically.

In this guide:

• The two-part grounding system in solar PV — what EGC and GEC each do
• NEC 690.41 configurations — which one applies to your project
• EGC sizing by OCPD rating (Table 250.122) with residential examples
• GEC design under NEC 690.47 — connections, materials, and sizing
• Ground fault protection — how GFDI fuses and isolation monitoring detect faults
• Array frame bonding — WEEBs, UL 2703, and the anodized aluminum problem
• 10 common grounding mistakes and how they map to inspection failures

Two Types of Grounding in Solar PV Systems

Solar PV grounding is not one system. It is two, with different purposes, different sizing methods, and different NEC sections. Confusing them is one of the most common design errors and a direct path to inspection failure.

Equipment Grounding (EGC path) bonds all exposed non-current-carrying metal parts — module frames, racking sections, enclosures, conduit — to an equipment grounding conductor that runs back to the source. Its purpose is safety: during a ground fault, the EGC carries fault current back to trip the OCPD and clear the fault. It does not connect to earth directly.

System/Earth Grounding (GES path) establishes the voltage reference to earth through a grounding electrode conductor (GEC) connected to a physical electrode (ground rod, building steel, concrete-encased rebar). Its purpose is system stability and lightning protection. It carries surge energy, not fault current.

NEC 690.41 System Grounding — Which Configuration Applies?

NEC 690.41(A) defines five system grounding configurations for PV arrays. The one that applies to your project determines whether a DC conductor is grounded, whether a GFDI fuse is needed, and how the inverter must be listed.

The 2023 NEC updated NEC 690.42 (retitled “Point of PV System DC Circuit Grounding Connection”) to clarify that when GFP is provided, the bond is made internally by the GFP device and must not be duplicated externally. The 2023 edition also added a note recommending the ground connection be located close to the PV source to limit lightning surge exposure.

Sizing the Equipment Grounding Conductor (EGC) Under NEC 250.122

The Basic Rule

NEC 690.45 directs you to size the EGC per Table 250.122, using the rating of the overcurrent protective device (OCPD) protecting the circuit. When no OCPD is installed in the circuit — common for DC homerun circuits between the array and combiner — treat the OCPD as though it were rated at 125% of Isc per NEC 690.8(A).

The absolute minimum EGC is 14 AWG copper. This floor applies even when Table 250.122 would otherwise allow a smaller conductor for very low OCPD ratings.

Common Residential EGC Sizes

Always reference the current edition of Table 250.122 directly. Values depend on conductor material and temperature rating, and some OCPD ranges map to the same AWG entry.

What NEC 690.45 and 690.46 Add

Physical protection: An EGC smaller than 6 AWG must be protected from physical damage per NEC 250.120(C). Run it in conduit or a listed cable assembly wherever it is exposed.

Routing with circuit conductors: The EGC must run with the circuit conductors in the same raceway (NEC 250.134(B)). A separate raceway is a code violation and creates an inductive ground loop that can cause nuisance AFCI/GFCI tripping.

No voltage-drop upsizing: When you upsize ungrounded conductors for voltage drop, the EGC does not need to increase proportionally. This is a common misconception that inflates copper costs on larger residential and commercial jobs.

Solar Grounding Electrode System — GEC Design

What the GEC Does

The GEC connects the electrical system to a physical grounding electrode in the earth. It establishes a voltage reference, provides a path for lightning-induced surges, and limits potential differences during utility disturbances. It is sized to handle surge energy, not continuous fault current.

Material requirement: The GEC must be copper. NEC Article 250 prohibits aluminum for grounding electrode conductors because aluminum corrodes at buried connections and the joint resistance rises over time.

Splices: The GEC may not be spliced at intermediate points. If a splice is unavoidable, use an irreversible compression connector or exothermic weld (Cadweld or equivalent). Wire-nutted splices inside a junction box fail inspection and violate NEC 250.64(C).

GEC Sizing

For a residential system fed by a 200 A service (2/0 AWG Al), Table 250.66 requires a minimum GEC of 2 AWG Cu on the AC side. A 10 AWG array homerun maps to 8 AWG Cu minimum on the DC side per Table 250.166.

GEC Connection Options (NEC 690.47)

NEC 690.47 gives three paths for integrating DC and AC grounding electrode systems:

Option (C)(1) — Bonding jumper between electrodes: Bond the DC array grounding electrode to the AC system grounding electrode with a bonding jumper sized per 250.166. Both electrodes stay in place.

Option (C)(2) — DC GEC run to AC electrode: Run the DC GEC directly to the AC grounding electrode. If the AC electrode is not accessible, terminate at the AC GEC with a listed connector.

Option (C)(3) — Combined AC EGC and DC GEC: The most common approach for modern residential. The AC equipment grounding conductor — sized to the larger of Table 250.122 or Table 250.166 — serves as the DC GEC as well. This eliminates a separate DC electrode conductor and is acceptable for grounded PV systems under 2023 NEC.

Ground-Mount Array Electrode

When the array is more than 6 feet (1.8 m) from the building’s grounding electrode system, NEC 690.47(D) requires a supplementary electrode at the array:

• Standard ground rod: 5/8-inch diameter, 8-foot length, copper-clad steel
• Array steel structure: If driven piles or poles meet NEC 250.52(A) — specifically 8 feet of metal in contact with earth — the structure itself qualifies as the electrode

Ground Fault Protection (GFP) — How It Works

Where GFP Is Required

NEC 690.41(B) requires ground fault protection for PV circuits with voltage above 30 V or current above 8 A. This covers virtually every grid-tied system. The narrow exception applies to off-building circuits that are solidly grounded and have no more than 2 PV source circuits in parallel.

Transformer-Isolated Inverters — GFDI Fuse

Older and some commercial string inverters use galvanic transformer isolation between the DC array and AC grid. In these systems:

1. One DC conductor is grounded (bonded to the GEC) via a GFDI (Ground Fault Detection and Interruption) fuse.
2. During a ground fault, current bypasses the normal GFDI fuse path, the fuse opens, and the inverter shuts down.
3. Typical fuse ratings: 1 A for systems ≤15 kW; up to 5 A for systems above 250 kW.
4. The fuse is the GFP device — it must be rated, accessible, and replaceable.

The limitation of GFDI fuses is sensitivity. A 1 A fuse cannot detect a 0.9 A ground fault — the fault burns continuously until it escalates. This was the root cause of several PV array fires in the 2010s on large commercial systems.

Transformerless Inverters — Isolation Monitoring

Modern transformerless (TL) inverters use active isolation resistance monitoring instead of a GFDI fuse. The detection logic works as follows:

1. The inverter continuously monitors insulation resistance between both DC conductors and the equipment ground.
2. Before connecting to the grid each morning, the inverter runs a self-test — checking isolation resistance while the array is energized but the inverter output is open.
3. During operation, the monitor detects both slow degradation (resistance drift) and sudden faults.

UL 1741 CRD thresholds (current standard for grid-tied inverters ≤30 kW):

TL inverters are approximately 3× more sensitive to ground faults than transformer-isolated inverters on residential circuits. For commercial-scale installations with thin-film modules — which have higher array-to-ground parasitic capacitance — the isolation monitoring topology matters. Topologies with constant common-mode voltage (NPC/half-bridge) generate less leakage current than full-bridge designs.

Both DC Conductors Must Be Switched

Because no DC conductor is grounded in a TL system, both poles carry potential. This has three design consequences:

• The DC disconnect must interrupt both DC conductors simultaneously (2-pole).
• The OCPD must protect both ungrounded conductors.
• Wire color coding changes: white and gray are reserved for grounded conductors. In TL systems, both DC conductors are ungrounded — use red/black or other non-white/gray colors.

Array Frame Grounding — Racking and Bonding

The Anodized Aluminum Problem

PV module frames are extruded anodized aluminum. Anodization creates a thin, electrically insulating aluminum oxide layer on all surfaces. This means:

• Bolting a module to a rail does not create an electrical bond.
• A standard hex-head bolt through an anodized hole provides no measurable continuity.
• Continuity through a bolted connection will fail a basic resistance test.

This is the source of the most common array grounding failure: an installer mounts 20 modules, runs a green EGC to the rail, and passes every visual check — but the frames are electrically floating because the oxide layer was never penetrated.

Listed Bonding Hardware

WEEB clips (Washer Electrical Equipment Bond): Stainless steel washers with raised teeth that bite through anodized coatings when torqued to specification. WEEBs are listed to UL 467. When installed per the listing, they provide a verified ground path from module frame to rail.

• Torque per manufacturer spec: IronRidge/Burndy WEEBs require 13.5 N·m (10 ft-lb); Wiley WEEB-PMC requires 20.5 N·m (15 ft-lb). Check the listing for each product.
• WEEBs are single-use. Retorquing after repositioning damages the bite teeth and voids the listing.

UL 2703-listed clamps and mounting systems: Many modern rail clamps (IronRidge UFO, Unirac SolarMount, Schletter) integrate bonding teeth directly into the clamp geometry. If the product bears a UL 2703 listing, the clamp simultaneously mounts and grounds the module — no separate WEEB needed.

What does NOT work:

• Standard star washers — not listed to UL 2703 or UL 467 for module bonding
• Self-tapping screws into anodized surfaces — no continuity guarantee
• Module-frame-to-clamp contact without listed bonding means

EGC Routing from Array

The EGC must leave the array in the same conduit or cable assembly as the DC current-carrying conductors. At the array, the EGC connects to a bare copper bus on the racking system — typically 6–10 AWG bare copper — collected at the combiner box or directly at the inverter input.

For ground-mount systems, the bare copper EGC run from combiner to inverter must be protected from physical damage if exposed (conduit or direct-burial listed cable). This is a frequent oversight on commercial ground-mount projects where the combiner-to-inverter run crosses open terrain.

Common Grounding Mistakes and Inspection Failures

NYSERDA’s NY-Sun inspection review found grounding deficiencies in 53% of systems inspected (NYSERDA, Solar PV Permitting and Inspecting, NY-Sun Program). Massachusetts DOER reported that 18% of critical violations were at the array and 16% were at the inverter — the two locations where grounding is most likely to be incorrectly implemented.

Here are the 10 specific mistakes that drive those statistics:

Grounding errors are invisible during installation. They surface at inspection or, worse, during a fault event. Solar design software that validates these checks during the design phase prevents the re-inspection cycle before it starts.

Grounding for Transformerless Inverters — Special Rules

Over 90% of US residential inverters installed today are transformerless (non-isolated) per industry design literature. They are 2–4% more efficient than transformer-isolated units and significantly lighter — but the grounding rules differ substantially from the GFDI-fuse era.

Side-by-Side: Isolated vs Transformerless

In TL systems, the DC array conductors are not connected to ground (system grounding), but all metal enclosures, module frames, and racking must still be EGC-bonded (equipment grounding). These are separate requirements and both are mandatory.

SLD Implications

A single-line diagram for a TL system looks different from an isolated system diagram:

1. No GEC terminal shown between the inverter DC input and the grounding electrode.
2. Both DC conductors are shown interrupted by a 2-pole disconnect.
3. The EGC runs from the array racking to the inverter enclosure, then to the panel EGC bar — but never from the inverter DC input to the electrode.

Solar software that understands inverter topology generates the correct SLD automatically. Manually drawing an SLD without topology awareness commonly results in incorrect GEC placement — a mistake that inspectors catch immediately on commercially submitted drawings.

For projects where shading analysis informs string layout and inverter selection, shadow analysis tools can help you confirm whether a given inverter topology suits partial-shade conditions before grounding design begins.

Frequently Asked Questions

What size ground wire do I need for solar panels?

The EGC is sized per NEC Table 250.122 based on the OCPD rating protecting the circuit. For a 20 A OCPD, use 12 AWG copper minimum. For a 60 A OCPD (typical residential homerun), use 10 AWG copper. The absolute minimum is 14 AWG. If there is no OCPD in the circuit, calculate as if the OCPD equals 125% of Isc per NEC 690.8.

What is the difference between EGC and GEC in solar PV systems?

The EGC creates the fault-current path from equipment enclosures and module frames back to the source — sized per Table 250.122. The GEC creates the earth reference connection between the system and the grounding electrode — sized per Table 250.66 (AC) or 250.166 (DC array). They serve different functions and cannot substitute for each other.

Do solar panels need a separate ground rod?

Only if the array is more than 6 feet from the building’s grounding electrode system. NEC 690.47(D) requires a supplementary grounding electrode at the array when it is more than 1.8 m from the nearest building electrode. For roof-mounted systems, the building electrode satisfies the requirement. For ground-mount arrays farther than 6 ft from the premises, install a 5/8-inch by 8-foot copper-clad ground rod, or use the array steel structure if it meets NEC 250.52.

Can you ground the DC conductors on a transformerless inverter?

No. Transformerless inverters lack galvanic isolation. If you ground a DC conductor, a circulating current loop forms through the grid neutral, causing DC injection into the grid, inverter damage, and continuous fault current. Equipment grounding (EGC bonding of frames and enclosures) is still required — that is separate from system grounding and remains mandatory in all TL installations.

Conclusion

A complete solar PV grounding system design requires two separate, correctly sized conductor paths — the EGC for fault return and the GEC for earth reference — combined with a ground fault protection strategy that matches the inverter topology. Getting either wrong produces a system that looks correct on the surface but fails at inspection or creates a latent fire hazard.

Three actions before you submit any solar permit:

• Verify EGC sizing against Table 250.122 for every circuit — string homerun, combiner output, and AC disconnect feed.
• Confirm inverter topology (isolated vs transformerless) and apply the correct GFP and wire color rules. A transformer-isolated SLD submitted for a TL inverter will be rejected.
• Check every module frame for listed bonding — WEEBs or UL 2703-listed clamps only. Star washers do not pass.

The solar proposal software and electrical design tools in SurgePV handle EGC sizing, GEC routing, GFP compatibility checks, and bonding hardware specifications within the same workflow that generates your permit package. The generation and financial tool models system output alongside electrical design, so the full project picture is consistent before it goes to the AHJ.

Grounding does not have to be the most confusing part of NEC 690. With the right design process, it becomes a checklist — one that SurgePV completes automatically.