What Is Bonding (Electrical)? Definition & Guide

Key Takeaways

NEC Article 250 and Section 690.43 require all exposed metallic parts in a PV system to be bonded to create a low-impedance fault current path
Bonding connects metal parts to each other; grounding connects the bonded system to the earth — they serve different safety functions
Common bonding methods include WEEBs (Washer, Electrical Equipment Bond), lay-in lugs, bonding jumpers, and listed bonding clips
Improper or missing bonding is the single most common reason for solar inspection failures across U.S. jurisdictions
Module frames must be bonded to the racking system using UL 2703-listed hardware to maintain warranty and code compliance
An equipment grounding conductor (EGC) must provide a continuous path from the array to the main service ground bus

What Is Electrical Bonding?

Electrical bonding is the permanent joining of metallic parts — module frames, racking rails, junction boxes, conduit, disconnects, and enclosures — so they form a single electrically continuous path. If a ground fault occurs (live conductor contacts metal), the bonded path carries fault current back to the source with low enough impedance to trip the overcurrent protection device quickly, preventing electric shock and fire.

Bonding does not connect anything to the earth. That is the job of grounding. Bonding creates the conductive bridge between metal components so that fault current has somewhere to go before it reaches the grounding electrode.

In a properly bonded PV system, every piece of exposed metal that a person could touch is at the same electrical potential. If a fault energizes one component, current flows through the bonded path — not through the person.

For solar designers using solar design software, verifying that every metallic component in the array layout is part of a continuous bonding path is a non-negotiable step before generating permit documents.

Types of Bonding in Solar PV Systems

Array Level

Module Frame Bonding

Each module’s aluminum frame must be electrically connected to the racking system. This is typically achieved with WEEBs, bonding washers, or listed clips that pierce the anodized coating to make metal-to-metal contact. NEC 690.43 requires the bonding method to be listed and identified for the purpose.

Structural

Racking Bonding

All racking rails, splice joints, and mounting feet must be bonded together as a continuous assembly. Many racking manufacturers achieve this through UL 2703-listed hardware that bonds rail sections at every splice point. A single break in continuity can fail the entire array’s bonding path.

BOS Components

Conduit and Enclosure Bonding

Metal conduit, wireways, junction boxes, combiner boxes, and disconnect enclosures must all be bonded. EMT and rigid metal conduit can serve as their own bonding path if joints are made tight and continuous per NEC 250.118. Bonding bushings are required at service equipment.

System Level

Intersystem Bonding

NEC 250.94 requires an intersystem bonding termination that connects the PV system’s grounding to other building systems — telephone, cable TV, and communications. This prevents voltage differences between systems that could damage equipment or create shock hazards.

Bonding Methods Comparison

Bonding Method	Application	NEC Reference	Advantages
WEEB (Washer, Electrical Equipment Bond)	Module frame to racking rail	690.43, UL 2703	Self-piercing teeth cut through anodizing; fast installation; listed for purpose
Lay-in Lugs	EGC connection at racking, combiner boxes	250.8, 250.118	Accepts range of conductor sizes; tool-free installation; re-inspectable
Bonding Jumpers	Across expansion joints, flexible connections	250.102	Maintains continuity where rigid bonding is not possible; sized per 250.102
Listed Bonding Clips	Module frame to rail (manufacturer-specific)	690.43, UL 2703	Integrated with racking system; pre-tested and listed as assembly
Bonding Bushings	Metal conduit entering enclosures	250.92(B)	Ensures bonding at knockouts; required at service equipment
Star Washers / Serrated Washers	General metal-to-metal joints	250.12	Inexpensive; penetrates paint and coatings; widely available

Designer’s Note

Not all bonding hardware is interchangeable. A WEEB designed for one racking system may not be listed for another. Always verify that the bonding component is UL 2703-listed for the specific racking and module combination in your design. Solar design software with integrated equipment databases can flag incompatible bonding selections during the design phase.

Ground Fault Current Path Requirements

Core Requirement

Ground fault current path impedance must be low enough to allow sufficient fault current to trip the OCPD within the required clearing time.

For a bonding path to function as intended, it must carry enough fault current to operate the circuit’s overcurrent protection device (breaker or fuse) quickly. NEC 250.4(A)(5) states that the fault current path must be “effective” — meaning it has sufficiently low impedance to facilitate the operation of the overcurrent device.

In practice, this means:

Wire sizing matters. The equipment grounding conductor must be sized per NEC 250.122 based on the rating of the upstream overcurrent device. For a 20A circuit, the minimum EGC is 12 AWG copper.
Connections must be tight and corrosion-resistant. A loose lug or corroded splice increases impedance, potentially preventing the OCPD from tripping during a fault.
The path must be continuous from fault point to source. Any break — an unbonded rail splice, a missing bonding jumper, or a disconnected EGC — defeats the entire protective scheme.

Bonding: The #1 Inspection Failure

Inspection Reality Check

Across U.S. jurisdictions, bonding and grounding deficiencies are consistently the leading cause of solar permit inspection failures. Common issues include: missing bonding hardware at module-to-rail connections, unbonded conduit fittings, undersized equipment grounding conductors, and failure to bond all metallic components in the array. A single unbonded module frame can fail an entire rooftop inspection. Inspectors check bonding first because it is the most safety-critical element and the most frequently missed.

The most common bonding mistakes on solar installations:

Missing WEEBs or bonding clips on one or more modules — often the result of running out of hardware on-site
Unbonded rail splices where two racking sections join without listed bonding hardware
Paint or anodizing not penetrated at bonding contact points, leaving high-resistance connections
EGC not continuous from array to ground bus — a break anywhere in the run invalidates the path
Using non-listed hardware — standard bolts and washers are not acceptable as bonding means unless specifically listed per UL 2703

Practical Guidance

Bonding requirements affect design decisions, installation practices, and how systems are presented to customers. Here is role-specific guidance:

Specify listed bonding hardware in your BOM. Every module-to-rail connection needs a UL 2703-listed bonding device. Include the exact part number in your bill of materials so the install crew uses the correct component.
Size the EGC per NEC 250.122. Match the equipment grounding conductor to the largest overcurrent device in the circuit. For string inverter systems with 20A fuses, minimum 12 AWG copper. For larger commercial arrays, size accordingly.
Show bonding details on plan sets. AHJs increasingly require explicit bonding details on permit drawings — callouts showing WEEB locations, EGC routing, and grounding electrode connections. Use solar design software that generates these details automatically.
Account for dissimilar metals. Copper EGCs in direct contact with aluminum racking create galvanic corrosion. Specify bi-metallic lugs or stainless steel hardware at copper-to-aluminum transitions.

Count bonding hardware before you start. Verify you have one bonding device per module-to-rail attachment point, plus extras. Running short mid-install leads to skipped connections and failed inspections.
Torque all bonding connections to spec. Over-tightening damages hardware; under-tightening creates high-resistance joints. Use a torque wrench and follow manufacturer specifications — typically 8–10 ft-lbs for module clamp bolts.
Photograph every bonding connection. Inspectors may not be able to see all connections once modules are installed. Photos of WEEBs, lugs, and EGC terminations before module placement serve as inspection evidence.
Test continuity before calling for inspection. A simple continuity test from the farthest module frame to the ground bus confirms the bonding path is intact. This five-minute check prevents costly re-inspections.

Use code compliance as a trust signal. Homeowners rarely understand bonding, but they care about safety. Mention that your installations include full electrical bonding per NEC requirements — it differentiates you from less rigorous competitors.
Explain the inspection process. Let customers know that the local building department will inspect the bonding and grounding before the system is energized. This sets expectations and reinforces that your work meets code.
Address the “will it electrocute me?” concern. Bonding is the direct answer to this common fear. Explain that bonding ensures fault current flows through the wiring — not through anyone touching the system — and that the breaker trips within milliseconds.
Highlight warranty implications. Module and racking manufacturers require listed bonding hardware for warranty coverage. Non-listed bonding methods can void both the product warranty and the workmanship guarantee.

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Sources and References

NEC Article 250 — Grounding and Bonding. Covers general bonding requirements, EGC sizing (250.122), bonding methods (250.8), and effective ground fault current paths (250.4).
NEC Section 690.43 — Equipment Grounding and Bonding. Specific requirements for PV systems, including that exposed non-current-carrying metal parts of PV modules, mounting systems, and equipment shall be grounded and bonded.
UL 2703 — Standard for Mounting Systems, Mounting Devices, Clamping/Retention Devices, and Ground Lugs for Use with Flat-Plate Photovoltaic Modules and Panels. Covers listing requirements for bonding and grounding hardware used in PV racking systems.

Frequently Asked Questions

What is bonding in solar installation?

Bonding in solar installation is the process of permanently connecting all exposed metallic parts — module frames, racking rails, conduit, junction boxes, and enclosures — so they form a continuous electrical path. This path allows fault current to flow safely back to the source and trip the overcurrent protection device, preventing electric shock and fire. NEC 690.43 requires all non-current-carrying metal parts of a PV system to be bonded using listed hardware.

What is the difference between bonding and grounding?

Bonding and grounding serve different purposes. Bonding connects metallic parts to each other, creating a low-impedance path for fault current to flow. Grounding connects the bonded system to the earth via a grounding electrode (ground rod, Ufer ground, or ground ring). Bonding provides the path for fault current to trip the breaker. Grounding stabilizes voltage references and dissipates static charges and lightning energy. Both are required — a system can be grounded but improperly bonded, which is dangerous.

How do you bond solar panel frames?

Solar panel frames are bonded to the racking system using UL 2703-listed bonding hardware. The most common method is a WEEB (Washer, Electrical Equipment Bond) — a serrated washer placed between the module frame and the mid-clamp or end-clamp bolt. The serrations pierce the anodized aluminum coating to make direct metal-to-metal contact. Alternative methods include listed bonding clips specific to the racking manufacturer and lay-in lugs with a separate bonding jumper. Whichever method is used, it must be listed for the specific module and racking combination per NEC 690.43.