Photovoltaic Bolts: Types, Materials & Selection Guide

Photovoltaic bolts are the fasteners that secure solar panels, mounting rails, racking frames, and ground anchors into a complete, structurally sound PV system. They are not generic hardware-store bolts. The outdoor environment a solar installation operates in — UV radiation, thermal cycling from −40°C to +85°C, coastal salt spray, or industrial pollution — places demands on fasteners that ordinary carbon steel cannot meet over a 25–30 year system lifespan.

Choosing the wrong bolt grade or material can lead to galvanic corrosion, loosening under wind load, or structural failure that voids equipment warranties and creates safety hazards. This guide covers the types, materials, standards, torque requirements, and selection criteria that matter most in real PV installations.

Where Photovoltaic Bolts Are Used in a Solar System

A typical rooftop or ground-mount PV system uses fasteners at multiple structural joints, each with different load requirements and exposure conditions:

• Panel clamp bolts: Secure mid-clamps and end-clamps that hold the panel frame to the mounting rail. These see the most frequent thermal cycling as panel temperatures fluctuate daily.
• Rail splice and rail-to-bracket bolts: Connect rail sections and attach rails to roof mounting brackets or ground posts. Critical for transferring wind uplift and snow loads through the structure.
• Roof penetration lag bolts: Anchor flashing mounts or ballast feet to roof rafters. Require stainless steel to prevent rust staining and corrosion inside the roof structure.
• Ground mount anchor bolts: Secure driven piles, helical anchors, or concrete footings to the racking frame. Often the largest fasteners in the system — M16 to M24 in large utility-scale installations.
• Inverter and combiner box mounting bolts: Attach electrical enclosures to walls or frames. Require electrical isolation in some configurations to avoid stray currents.
• Grounding bonding hardware: Serrated flange bolts or grounding clips that pierce the anodized surface of aluminum rails to establish electrical continuity.

Bolt Types Commonly Used in PV Racking Systems

PV racking manufacturers design their systems around specific bolt geometries that allow fast installation and reliable clamping without over-torquing thin aluminum profiles. The most common types are:

Material Selection: Why Stainless Steel Dominates PV Fasteners

The single most important material decision for photovoltaic bolts is corrosion resistance. Solar systems are designed for 25–30 year operational lifespans, and the fasteners must survive that entire period without rust-induced loosening, seizing, or structural degradation.

Stainless Steel A2 (304) vs. A4 (316)

Most PV racking systems specify either A2 or A4 stainless steel bolts. The practical difference is molybdenum content: A4 (316) stainless contains 2–3% molybdenum, which dramatically improves resistance to chloride-induced pitting corrosion — the specific failure mode in coastal and marine environments. A2 (304) is suitable for inland installations; A4 (316) should be used within 1–5 km of a coastline or in industrial zones with airborne chlorides.

Hot-Dip Galvanized (HDG) Steel for Ground Mount

Large ground-mount installations — particularly utility-scale solar farms — often use hot-dip galvanized (HDG) structural bolts for anchor bolts and primary frame connections. HDG coating thickness of 85 µm or greater (per ASTM A153 or ISO 1461) provides 30–50 years of corrosion protection in most soil environments at significantly lower cost than all-stainless hardware at large scale. HDG is not recommended where direct contact with aluminum racking occurs, due to galvanic corrosion risk.

Galvanic Corrosion: The Hidden Risk at Aluminum-Steel Interfaces

Virtually all PV racking rails are extruded aluminum (6005-T5 or 6061-T6). When stainless steel bolts contact aluminum in a wet environment, a galvanic cell forms. Stainless steel and aluminum are 0.25–0.50 V apart on the galvanic series — close enough that A2/A4 bolts are generally considered acceptable in this pairing. However, carbon steel or HDG bolts in direct contact with aluminum are problematic — the potential difference accelerates aluminum corrosion at the joint. Always use stainless or aluminum-compatible hardware at aluminum-to-fastener interfaces.

Bolt Grade and Strength Requirements for PV Applications

PV bolts must withstand sustained wind uplift loads, snow loads, and seismic forces over decades. Bolt strength is defined by property class (metric) or grade (inch):

For most residential and commercial rooftop systems, A2-70 stainless is the standard specification. Upgrading to A4-80 is advisable for coastal sites, high-wind areas (ASCE 7 wind speed ≥ 130 mph), or any installation where the racking engineer's structural calculations call for higher bolt preload.

Torque Specifications for PV Bolts: Why Over-Tightening Is as Dangerous as Under-Tightening

Torque specifications for photovoltaic bolts are tighter than most installers expect. The aluminum extrusions used in solar racking are relatively soft — 6005-T5 aluminum has a yield strength of only 240 MPa compared to 700+ MPa for the stainless bolts threading into it. Over-torquing strips threads in the aluminum nut or rail channel, requiring replacement of the entire rail section.

Under-torquing leaves the joint insufficiently preloaded, allowing movement under wind vibration that causes fretting fatigue and gradual loosening. Typical manufacturer-specified torque values for common PV bolt sizes:

Always use a calibrated torque wrench for final tightening. Impact drivers set to maximum torque are not acceptable for PV panel clamp bolts — they routinely exceed manufacturer limits. Many racking manufacturers specify torque values dry (no lubricant); if anti-seize or thread lubricant is applied, reduce torque by approximately 20–25% to achieve the same clamp force.

Anti-Loosening Features: Preventing Bolt Loosening Over 25 Years

Solar installations experience continuous low-amplitude vibration from wind and the daily thermal expansion and contraction of aluminum rail (aluminum expands at 23.6 µm/m·°C — a 6-meter rail section grows and shrinks by approximately 12mm between a −10°C winter night and a 60°C summer panel surface temperature). This movement can gradually back out bolts that lack anti-loosening provisions.

Common anti-loosening solutions used in PV systems include:

• Nylon-insert lock nuts (Nyloc): The most widely used solution in rooftop PV. The nylon collar grips the bolt thread and resists rotation. Effective up to approximately 100°C — verify suitability if junction box enclosures create localized heat.
• Serrated flange nuts and bolts: Serrations bite into the mating surface and resist rotation mechanically. Used for grounding bonds and structural joints where the surface hardness allows the serrations to engage properly.
• Spring lock washers: Generally considered less reliable than nyloc nuts under dynamic loading per DIN 65151 vibration testing — use only where specified by the racking manufacturer.
• Thread-locking adhesive (Loctite 243 or equivalent): Effective for small-diameter bolts in low-vibration locations. Not practical for field connections that may require future re-torquing or panel repositioning.
• Two-nut (jam nut) configuration: Used on ground-mount adjustment legs where vibration resistance and field adjustability are both required.

Standards and Certifications Relevant to Photovoltaic Bolts

PV system certifications and structural permits increasingly require that fasteners meet documented material and dimensional standards. The most referenced standards are:

• ISO 3506: The primary international standard governing mechanical properties of stainless steel fasteners. Specifies A2-70, A2-80, A4-70, and A4-80 property classes. Certificates of conformity referencing ISO 3506 are required by many utility-scale EPC contractors.
• ASTM F593 / F594: US equivalent for stainless steel bolts and nuts. Required for projects under US building codes where metric ISO hardware is not specified.
• ASTM A307 / A325 / A490: Govern carbon and alloy steel structural bolts used in HDG ground-mount steel structures.
• ISO 1461 / ASTM A153: Specify minimum coating thickness for hot-dip galvanized fasteners — critical for verifying corrosion life of HDG anchor bolts.
• IEC 61215 / IEC 61730: While these are module standards, they indirectly govern mounting hardware by specifying the mechanical load conditions (wind, snow, hail) that racking and fasteners must be designed to withstand.

For permit-required installations, request material test reports (MTRs) or certificates of conformity from bolt suppliers that reference these standards. Counterfeit or substandard fasteners labeled with false property class markings are a documented problem in low-cost supply chains — third-party chemical composition and hardness verification is advisable for large projects sourcing hardware outside established distribution channels.

Practical Checklist for Selecting Photovoltaic Bolts

Use the following checklist when specifying or procuring fasteners for a PV project:

1. Confirm the bolt type required by the racking manufacturer's installation manual — T-head, hex head, carriage bolt, or lag bolt — and do not substitute different head styles without engineering approval.
2. Select A4-316 stainless for coastal, marine, or high-humidity environments; A2-304 stainless is acceptable for inland dry climates.
3. Specify A2-70 or A4-70 minimum property class for standard rooftop systems; upgrade to A4-80 for high-wind or high-snow load sites per structural calculations.
4. Ensure all nuts and washers are the same stainless grade as the bolt — mixing A2 bolts with carbon steel nuts creates a galvanic couple that accelerates nut corrosion.
5. Verify anti-loosening provisions: nyloc nuts for panel clamps and most rail connections; serrated flange hardware where grounding continuity is required.
6. Obtain and file certificates of conformity (ISO 3506 or ASTM equivalent) with project documentation for permit and warranty purposes.
7. Use a torque wrench — not an impact driver — for final tightening, and record torque values in the commissioning report for any future O&M inspection reference.