Figure 1: Fronius Rapid Shutdown Box installation
Figure 2: Wall mounting of RSB
FRONIUS RAPID SHUTDOWN BOX MOUNTING OPTIONS
For Rail and Rail-less systems on L-feet, stand-offs, and other attachment types
Summary
The Fronius Rapid Shutdown Boxes (RSBs), Multi- and
Single string versions, are designed for wall and roof-top
mounting with 600V maximum rated PV systems. Any
mounting design must consider PV source circuit wire runs
and PV output circuit conduit runs. Both RSBs include
water-tight glands/connectors for PV source circuit inputs
(PV Wire, USE-2, etc.) and a pre-drilled hole for the PV
output circuit using home-run wiring (THHN, THWN-2, etc.)
inside appropriate conduit. To maintain the box’s NEMA 4X
rating, only water-tight fittings should be used and unused
gland fittings should be plugged with included black plugs.
The wall mounting is achieved with wood, metal, or masonry
fasteners through fabricated holes in the base of the RSB.
Roof-top mounting can vary depending on the mounting
system and roofing material. In general, the included bracket
is attached to the rail of a rail-based system but a different
attachment method might be required if using a rail-less
system. For ideal aesthetics, the RSB may be installed
under the array unless there is insufficient vertical clearance;
it should fit between the roof surface and the rear of the
module while providing an air gap for each, as determined
by the local AHJ. Please review Appendix A for the NEC
code regarding installation of an electrical box under a PV
module, and access to the provided DC disconnect.
Wall Mounting
If a portion of the array is within 10 feet of a vertical surface (wall, side of
dormer, parapet, etc.), it can be mounted onto that surface. In this case,
mount the RSB without the included bracket, being mindful of providing room
for the conduit run back to the inverter. Four anchors are needed (as
illustrated)
Roof Mounting
Since the RSBs are NEMA 4x rated, they can be placed horizontally,
vertically, or tilted on a roof. When mounted to racking via the bracket, the
un-affixed end is providing additional stability by the conduit attachment.
Rail-based System
The larger mounting bracket is designed for bolting to the top or side of a rail. Most rail systems provide top and
side channels for mounting the bracket with bolts also supplied with the mounting system. The most common bolt is
a T-bolt and this often is the same bolt for top and side channels.
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Figure 4: SnapN’Rack’s snap in bolting device works in both its
top and side rails.
The top-mounting channel design of the multi-string mounting bracket mimics commonly used microinverter or
optimizer (aka, MLPE device) brackets. Thus this bracket will also fasten to most rail top channels just like MLPEs
with similar bracketing. However, the RSB is generally taller than MLPEs which may preclude attachment under a
module.
Figure 3: A generic rail system demonstrates typical top and side channels for mounting bolts
Even proprietary rail systems should work if their bolting
method also works for MLPE devices, as seen at right in this
SnapN’Rack rail system.
Height Matters
The ability to fasten to the top or side of a rail is
inconsequential if the RSB doesn’t fit under the module or
along the side of the array. Most rail-based systems are
height-adjustable but that’s no guarantee of a fit under the
array. Some L-feet are not that tall or adjustable, leaving
insufficient room height-wise. This results in the top bracket
sitting above the top rail or at least the RSB touching the roof.
To avoid this, make sure the dimension from roof face to top of
the rail is equal to or greater than the following:
RSB Model & Rail Height
Single-string: 4.0” (10.2 cm)
Multi-string: 4.5” (11.4 cm)
If the height adjustment means provided by the racking system is insufficient, consider third party products: (1)
substituting with a third-party L-foot with greater height (e.g., EcoFasten Solar® Compression Brackets) (2) or
mounting the footing atop a third-party stanchion (e.g., ProSolar FastJack®, Quick Mount PV QBase®) will provide
greater clearance.
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Figure 5: the bracket is in contact with the rail top, but the RSB is touching the roof. Therefore, the RSB
needs to be raised by either adjustable attachment to the side rail or by raising the rail itself.
Rail-less System
While rail-based systems are commonly used, rail-less systems are gaining in popularity. The stanchions used to
support and connect adjoining modules often provide a top channel. However, an additional stanchion would likely
be required to provide complete support. This necessitates an additional roof penetration. Some manufacturers
include hardware for attaching boxes (e.g. stanchion bracket from Equilibrium Solar) but they will likely not fit the
RSB footprint or attachment points. So you may need to devise a solid method or just use an attachment method
that is independent of the racking system.
Figure 6: for rail-less systems without hardware for attaching junction or combiner boxes, consider adding
another stanchion; if this is not acceptable, consider mounting the RSB directly to the roof (see below).
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