By Terance J Harper March 6, 2020
Solar is the new “cash crop” for landowners who have had land for agricultural uses for generations, but now cannot yield traditional crops due to topsoil degradation. This type of site often consists of a few inches of topsoil below grade and afterwards a mix of shale rock, bedrock and other rocky materials that make large ground mount installations more challenging and difficult.
For many years, racking installers have had solutions for rocky terrains, but trenching in these areas has been cumbersome and costly toward the Levelized Cost of Energy (LCOE) for projects, resulting in projects not moving past due diligence stages to construction. However, solutions in racking and cable management have improved significantly to allow for above-grade cable management for large ground-mount systems.
With racking manufacturers having the solution to drive posts into the ground for rocky terrains, one can now create an above grade cable route for PV array sources and output circuits to combiner boxes and inverters. Currently I-beams and C-channels are the most common post types utilized for racking installs, with I-beams being the preferred post type for equipment posts.
Notice I say equipment posts because racking manufacturers will not allow you to attach above grade cable management to the racking post without prior engineering and consent. A few racking manufacturers have partnered with above grade cable management providers for pre-approved use of racking posts as support without additional piers. When designing and engineering, always consult a structural engineer for wire capacity loading on the posts and sizing the correct post and embedment depth. Depending on the geographic nature of the site, some posts may have to be reinforced in the ground.
Think about routes
For the cable route, the equipment posts must be installed with two end posts and (x) number of mid posts required for the route, with spans no more than 25 feet. It is best to route equipment posts in a linear path with minimal 90 degree turns. There is no code reference on maximum turns within an above grade cable system, however best practice is to minimize the number of bends for all cables and abide by NEC Article 392 for cable tray systems.
When determining the best route, ensure there are no row-to-row string jumpers and that ease of access to inter-row space is maintained. Having no row-to-row string jumpers will allow for one continuous path for PV source circuit homeruns to combiner boxes, along with PV output circuits to inverters. Having only one linear path per array will allow O&M and vegetative maintenance personnel to be able to navigate the site with ease and not have obstructions to access aisles to maintain vegetation.
For fixed-tilt ground-mount arrays and single-axis trackers without drivelines, stick to the perimeter of the array near an access road for ease of maintenance and navigation. For single-axis tracker systems with drivelines, it’s ideal to route the cable management system parallel and within feet of the driveline for ease of access to both sides of inter-row spacing. Don’t be the rocky solar site that mistakenly blocked sections of an array due to multiple linear paths of above grade cable management. Also, when needing to cross access roads and maintenance roads, it’s best to transition to a small trench just to get under the road via an under-ground sweep.
Free air
Before getting to the providers of such systems, we must pay respect to the NEC. The 2017 edition of NEC, Article 392.80(A)(2)(c) allows wire sizing in “Free Air” to be sized per Table 310.15(B)(17). All OCPD sizing and correction factors should still be calculated per NEC 690 for corrected ampacity per site. Once ampacity is known for source and output circuits, there are only two types of cable allowed for outdoor use, sunlight resistance, and wet or dry locations. Only single conductor cable USE-2 or PV-wire should be used for all above grade PV source circuits and output circuits. When routing cable that is 1/0 AWG and smaller, NEC 690.31(C)(2) needs to be followed. This article states that conductors must be supported every 12 in. and secured no more than every 4.5 ft.
Though USE-2 and PV-wire have thicker insulation than THWN-2 and other conductor types, extra measures should be taken to protect the wires in the cable system from abrasion and direct UV Light for 20+years. When doing this, keep in mind that certain protection negates the Free Air assumption unless there is visible and ample ventilation. For example, split wire loom could be used to create bundles to sit in the cable system, however, when sizing the conductor, NEC Table 310.15(B)(17) cannot be used — NEC Table 310.15(B)(16) has to be used to verify ampacity and temperature ratings.
Another protection that will allow designers and engineers to use the Free Air table are braided cable sleeves that are manufactured from PET material that’s abrasion and UV resistant. These braided cable sleeves are available as wraps to allow for ease of install and protection of individual output circuits of 1/0 or larger to bundles of source circuits. When bundling three or more conductors with braided cable sleeves, abide by raceway correction factors due to the center conductors of a bundle not being in free air any longer.
Where can I get these?
You’re probably curious by now about whom you can reach out to for such systems. For ground-mount systems there are two solid providers that are great for different applications in the field. SnakeTray and CAB Solar both offer their own unique above grade cable management solutions for sites where traditional trenching is cost prohibitive.
SnakeTray offers three different above grade cable systems at the moment, two of which allow for true Free Air conditions for conductors. For conductor 1/0 and larger, designers can individually separate the conductors in slotted clips of five per row, stacked four to five high per NEC separation codes, and routed on a messenger cable system. The messenger cable system is attached mechanically to the I-beam equipment posts that were routed during the design phase of construction. Again, be sure to only have one linear path per array for cost savings, ease of install and ease of maintenance. For very long runs and more than 5 PV output circuits of 1/0 or larger conductors, SnakeTray’s Solar Snake Max is ideal to manage correction factors when sizing conductors.
CAB Solar’s system offers a range of hangers that are hung on a messenger cable at intervals of 12 in. for wires for conductors that are smaller than 1/0. The CAB Solar hangers are offered with one, two or three compartments to separate and bundle source and output circuits, along with a hook for data circuits with clearances per NEC. Again, the messenger cable the hangers will attach to should be in a linear path attached mechanically to the equipment posts. CAB Solar systems have an added advantage for their system with an integrated grounding messenger cable that can be utilized as the functional grounding connection for the EGC along with GEC for mid-post grounding. The messenger cable solution is readily available and UL listed to further reduce installation time, equipment and labor costs. When sizing conductors for CAB solar hangers, be mindful of the number of conductors in the bundle, correction factors may be needed to correctly size for the ampacity of the center conductor(s).
So, as land becomes rockier and challenging to trench, it may become more attractive to install solar cash crops with above grade cable management that leads to lower overall LCOE, ease and efficiency of installation and ease of maintenance.
Published at Solar Builder Magazine
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