Solar Mounting Aluminium Rail 45 x 45 mm for PV Array Frames

01How does a wind load become a rail span?

In three steps, and the middle one is where projects go wrong. Wind pressure becomes a pressure on the array, the pressure becomes a load per metre on each rail, and the load per metre becomes a span through a deflection limit. A 45 x 45 mm solar mounting aluminium rail at about 2.55 kg/m is comfortably strong for a residential array; what limits it is deflection, and deflection grows with the fourth power of the span.

Step one, for orientation only: wind pressure is roughly q = 0.5 x density x velocity squared. At 30 m/s with air at 1.25 kg/m3 that is about 562 Pa, or 0.56 kN/m2. Step two is where the real numbers live — the net pressure coefficient for an array on a roof is not 1.0, it depends on the array geometry, the roof zone and the exposure, and it can be well above the dynamic pressure in uplift. That calculation belongs to the system designer, and it is not something a profile supplier can do for you.

02What span does this rail actually give?

Take a worked case: a design load of 1.5 kN/m2 combining wind, snow and a safety factor, on a 1.0 x 2.0 m module of 2 m2. That is 3.0 kN per module, shared between two rails, so 0.75 kN/m on each rail. The section weighs 2.55 kg/m, which gives a cross-section of about 944 mm2 and, over a 180 mm perimeter, an average wall of about 5.25 mm.

Design load (assumed)

1.5 kN/m2

Module

1.0 x 2.0 m, two rails per module

Load per rail

0.75 kN/m

I (estimated)

about 2.24 x 10^5 mm4, ignoring the slot grooves

E, aluminium

70 000 N/mm2

Formula

delta = 5 w L^4 / (384 E I)

Two things are worth taking from that table. First, the stress is nowhere near the limit: at a 1.2 m span the bending moment gives roughly 14 N/mm2 in a 6063-T5 section that yields at well over 100 N/mm2. The rail is not close to breaking, it is only close to bending. Second, the jump from 1.6 m to 2.4 m is a factor of five, which is the fourth-power relationship doing its work. Reducing the span is always cheaper than increasing the section.

03Why does clamp position matter more than rail size?

Because there are two spans on a PV array and people only design one of them. The rail span is the distance between roof attachments and it governs the rail. The module span is the distance between the two rails under one module and it governs the module — and the module is the expensive, fragile part.

• Rails go at the module maker’s clamp zones, typically the quarter points, not wherever the roof purlins happen to be. The clamp zone drawing is in the module datasheet and it is not negotiable.
• The cantilever beyond the outer clamp is limited by the module maker, often to a quarter of the module length. A module clamped too far inboard cracks at the frame corner, and it cracks in the first windy season.
• The rail span is then whatever the purlin spacing gives, and if that is 2.4 m the rail has to be deeper — not the clamp position changed.
• Mid-clamps and end-clamps are different parts with different load paths. An end-clamp carries half the module it carries a mid-clamp, and it is the one that gets substituted wrongly.

The practical consequence: fix the module clamp positions first, then design the rail span around the purlins, then choose the section. Doing it in the other order produces a rail that is stiff enough and a module that cracks.

04What finish survives outdoors?

Not mill finish. Bare aluminium outdoors forms an oxide layer that protects it in clean air and stops protecting it near the coast, where chloride breaks the layer down. On a roof that cannot be repainted, the finish is a durability decision rather than a cosmetic one.

One detail matters more than the class: every cut end and every drilled hole is bare aluminium. On a rail that is cut to length on site, that is two bare ends per rail and they are exactly where water sits. End caps, or a touch-up with a suitable coating, are worth more than moving from AA20 to AA25.

05How do you earth an anodised rail?

Deliberately, because anodising is an insulator. The module frame is usually anodised aluminium as well, so a clamp pressing an anodised module frame onto an anodised rail gives you two insulators in series and no bond at all. This is the most common electrical defect on a PV array, and it is invisible.

1. Use bonding clips or toothed washers that bite through the anodising at each module to rail interface. A plain clamp does not bond, however tight it is.
2. Bond the rail to the structure at defined points, not by hoping the roof fixings do it.
3. Provide a continuous earth conductor along the array and connect it to the equipotential bonding of the building, per the electrical design.
4. Do not rely on the mid-clamp as the bond. Some are listed as bonding devices and most are not, and the difference is a listing, not a shape.
5. Check continuity after assembly, not before. A bond that reads correctly on the bench can be lost when the clamp is torqued onto a coated surface.

06What about the joint between the module and the rail?

Two dissimilar metals in a wet place, which is the definition of a galvanic couple — except that on a modern array both are usually anodised aluminium, so the couple is small and the coating is doing the work. The problems appear when the hardware is not aluminium.

The practical rule is the same as everywhere else on aluminium: keep water out of the interface and keep dissimilar metals small in area. A stainless clamp with a bonded washer is fine. A large bare steel bracket against an aluminium rail is not.

07What is our scope?

A mounting system is a structural product with a load calculation behind it, and that calculation depends on the site. We supply the extrusion that goes into it, to the section tolerance and the alloy specified, and we will cut and machine it accurately. The system, and its compliance, is the system designer’s.

08Questions from installers and system designers

Related sections and categories

Browse 45 Series Aluminium Profiles, Strut Profiles, or see the 45 x 45 mm four slot section for the full section data, and the 45 x 90 mm six slot heavy section when the purlin spacing forces a longer rail span.