Grounding, Bonding, and Ground-Fault Protection
Objective. Students will install equipment grounding conductors and array bonding hardware on a bench-top array mock-up in compliance with NEC 690 Part V and verify correct ground-fault protection device (GFPD) placement.
This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.
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Why Grounding and Bonding Matter in PV Systems
A PV array sits outdoors, often on a roof, exposed to weather, lightning, and the possibility of insulation failure. Without proper grounding and bonding, a fault current has no controlled path to earth. That uncontrolled current can start fires, damage equipment, and electrocute workers or first responders. NEC Article 690 Part V establishes the requirements that give fault current a safe, predictable route and protect the system from the consequences of a ground fault.
Grounding and bonding are related but distinct concepts. Bonding means connecting all metal parts of the array—rails, module frames, racking hardware—together so they are at the same electrical potential. Grounding means connecting that bonded assembly to the earth through a grounding electrode system. Together, they limit voltage differences between conductive surfaces and provide a low-impedance path for fault current to travel, allowing overcurrent protective devices and ground-fault protection devices to operate.
Equipment Grounding Conductors (EGC)
The equipment grounding conductor (EGC) is the copper or aluminum conductor that carries fault current from the array back to the inverter and ultimately to the grounding electrode conductor (GEC) termination point. NEC 690 Part V, read alongside NEC Article 250, governs EGC sizing. For PV source and output circuits, the EGC must be sized based on the overcurrent protective device protecting that circuit, following the tables in NEC Article 250. For most residential string systems, a 10 AWG copper EGC is common for source circuits, while larger output circuits may require 6 AWG or larger. Always verify sizing against the actual OCPD rating and applicable code tables—never guess.
EGC routing matters. Conductors must be protected from physical damage where subject to it, properly terminated using listed lugs torqued to the manufacturer's specification, and kept continuous—no splices unless made with listed connectors rated for the purpose.
Array Bonding Hardware and Methods
Aluminum racking rails and module frames are the most common conductive surfaces that must be bonded. Bare aluminum forms a thin oxide layer that is a poor conductor. Listed bonding hardware—clips, washers with sharp teeth, or bonding lugs—are designed to bite through or displace that oxide layer and make reliable metal-to-metal contact. Using unlisted hardware or standard hardware store bolts does not satisfy the listing requirement and constitutes a code violation.
NEC 690.43 requires that all exposed non-current-carrying metal parts of module frames, equipment, and conductors be bonded. Hardware must be listed for the purpose and installed per the manufacturer's instructions. When bonding module frames to rails, the torque specification printed on the hardware or in the manufacturer's installation guide is not optional—it ensures the bonding clip seats properly. Torque wrenches are required; guessing by feel is not acceptable in professional installation.
Rail-to-rail bonding jumpers or listed splice-plate bonding methods connect rail segments to each other. Each splice must produce a continuous bonding path from the farthest module frame back to the EGC termination point at the inverter enclosure or combiner box.
Ground-Fault Protection Devices (GFPD)
NEC 690.5 requires ground-fault protection for grounded DC PV arrays to reduce fire risk. A GFPD monitors the array for current flowing on conductors that should carry no current under normal operation—specifically, current returning on the grounding conductor. When it detects a fault above its threshold, it de-energizes the array and provides an indication of the fault condition.
For a GFPD to function correctly, it must be installed in the location and orientation specified by the inverter or GFPD manufacturer and must monitor the correct conductors. A common installation error is placing the GFPD so that it monitors only part of the array, or wiring the current-sensing element around rather than through the monitored conductors. Either condition means the device will not trip on a real fault—a serious safety deficiency that must be corrected before the system is energized.
Modern string inverters and microinverters often integrate GFPD functionality internally. Even so, the installer must verify that the internal GFPD is enabled and configured per the manufacturer's instructions and that the array wiring conforms to the topology the GFPD is designed to monitor.
Safety Requirements for This Lab
PPE is mandatory throughout this lab. Wear safety glasses and gloves at all times when cutting, stripping, or terminating conductors. Aluminum railing edges and copper conductor ends are sharp.
Although the bench-top mock-ups used in this lab are de-energized, treat all wiring practices as though the system will carry live voltage. Good habits built in the lab prevent injuries in the field.
Lockout/Tagout (LOTO): In field conditions, before working on any grounding or bonding connections in an existing system, you must apply LOTO procedures per OSHA 29 CFR 1910.147 and the employer's energy control program. Verify absence of voltage with a calibrated meter before touching conductors. Even a "de-energized" PV array can produce voltage in daylight—cover modules or work before sunrise if possible.
Torque discipline: Over-torquing can crack lugs or strip threads; under-torquing leaves a high-resistance joint that can arc under fault current. Read the torque value, set the torque wrench, and stop when the wrench clicks or signals.
Verification: Continuity Testing
After completing the bonded assembly, a continuity tester or low-resistance ohmmeter is used to verify that every rail section and module frame has a continuous path to the EGC termination point. A passing result means the meter shows continuity (or a resistance value within the acceptable range for listed bonding hardware—typically near zero ohms). An open reading means a bonding connection was missed or improperly made. Identify the break, correct it, and re-test before the instructor signs off on the assembly.
Document every connection in the lab log: wire gauge, conductor material, hardware part number and listing, torque value applied, and the NEC section that governs each element. This documentation habit mirrors the as-built records professional installers provide to inspectors and AHJs (Authorities Having Jurisdiction).
Key terms9
Worked examples4
- Step 1: Identify the OCPD rating protecting the circuit — 15 A.
- Step 2: Look up NEC Table 250.122. For a 15 A OCPD, the minimum copper EGC is 14 AWG.
- Step 3: Determine whether 10 AWG exceeds the minimum. 10 AWG is larger (lower gauge number = larger wire) than 14 AWG, so it meets and exceeds the minimum requirement.
- Step 4: Confirm there is no maximum restriction that would prohibit upsizing the EGC — NEC does not prohibit using a conductor larger than the minimum required.
- Step 5: Conclude that 10 AWG copper is acceptable and compliant; it provides a lower-impedance fault path than the minimum required.
- Step 1: Place one probe on the EGC termination lug (confirmed good) and the other on Section 1. Record result — expect continuity.
- Step 2: Move the second probe to Section 2 (the next splice point). If continuity is present, the break is downstream of Section 2.
- Step 3: Move the second probe to Section 3. An open reading here while Section 2 showed continuity pinpoints the break at the splice between Section 2 and Section 3.
- Step 4: Visually inspect and physically check the bonding hardware at the Section 2–to–Section 3 splice: verify the bonding clip is fully seated, the bolt is torqued to spec, and no oxide or debris is blocking contact.
- Step 5: Correct the connection (re-seat or replace the bonding clip, torque properly), then re-test the full assembly from the EGC lug to Section 4 to confirm continuity before signing off.
- Step 1: Recall the conversion factor: 1 ft-lb = 12 in-lb.
- Step 2: Multiply the specified torque by the conversion factor: 5 ft-lb × 12 in-lb/ft-lb = 60 in-lb.
- Step 3: Set the torque wrench to 60 in-lb before tightening the bonding bolt.
- Step 1: Count rail-splice bonding connections. There are 3 splices between 4 rail sections, each requiring 1 listed bonding connection = 3 bonding connections.
- Step 2: Count module-frame bonding connections. Each of 2 frames uses 2 bonding clips = 2 × 2 = 4 bonding connections.
- Step 3: Add the totals: 3 (rail splices) + 4 (module frames) = 7 individual bonding hardware connections.
- Step 4: Additionally, one EGC termination lug connects the assembly to the grounding electrode conductor path — note this separately in the lab log as it is a grounding connection, not a bonding connection between array metal parts.
Materials: NEC codebook, bench-top aluminum railing sections, mock module frames, listed bonding hardware kit (lugs, washers, bonding clips), 10 AWG and 6 AWG bare copper conductors, wire strippers, torque wrench set, two pre-wired inverter mock-ups, lab log forms, PPE (safety glasses, gloves)
Check: Instructor verifies each bonded assembly with a continuity tester; all four rail sections and both frames must show continuity to ground (pass/fail); lab log must include correct wire size, hardware listing number, and section citations for full credit—passing score 80 percent on log.
Practice check8
Answer each question, then check it. Nothing is recorded; this is your own practice.
