Cost-Saving PV Source-Circuit Wiring Method
Inside this Article
Ideally, project design documents explicitly describe the expected module-to-module wire management practices crews should use in the array field. When this is not the case, field technicians are left to develop and implement their own wire management solutions. These ad hoc solutions are not always Code compliant and may compromise project reliability.
For example, lead lengths on landscape-oriented modules are often too short to allow for proper wire management. When performing due diligence on PV systems with landscape-oriented modules prior to system commissioning, I have come across module leads that did not comply with the conductor bending radius requirements found in NEC Section 300.34. I have even discovered module leads pulled so taut that the conductor insulation split at the cable gland, effectively voiding the product warranty.
Portrait-oriented modules present a different wire management challenge. When adjacent modules are connected in series, the module leads are longer than necessary. Where this excess wire is not managed properly, it increases the likelihood of conductor or J-box damage, which could result in a ground fault or an arcing fault. The traditional approach to wiring a portrait-oriented PV array is to use some combination of labor and hardware to manage excess wire. For example, the installers might coil up the excess lead length and use module wire clips to hold everything in place.
But what if it were possible to pick up the slack in the PV array wiring and put it to good use? Here, I present a simple source-circuit wiring technique for portrait-oriented PV arrays that can turn the liability associated with excess module lead length into a cost-savings opportunity. Since there is no consensus industry term, I refer to this technique as leapfrog wiring.
Daisy Chain vs. Leapfrog Wiring
Standard practice for module-to-module wiring is to connect adjacent modules in a daisy chain, as shown in Figure 1a. Excess module lead length is often coiled up and organized using some type of PV cable clip. Where modules in the same string are mechanically mounted in the same row, the positive and negative homerun connections invariably wind up on opposite ends of the mechanical assembly.
Given adequate module lead length, leapfrog wiring can be used to connect portrait-oriented PV modules in series, as shown in Figure 1b. In this scenario, the excess module lead is used to leap over adjacent modules, so that every other module in the row is connected in series until the end of the row is reached. At that point, the source-circuit wiring circles back and picks up the skipped modules. Both the positive and negative home-run connections wind up on one end of the row of modules.
While the leapfrog method of stringing modules in series is not a new concept, it appears that installers are by and large unfamiliar with this option. Only one of the integrators I spoke with in Massachusetts was aware of this wiring strategy. In my experience, nearly every 60- or 72-cell module with a lead length of 1,100 mm or longer can accommodate leapfrog wiring. However, very few 60-cell PV modules meet this lead length requirement. Also, note that lead length is not the only determining factor, since the mounting system is often used to facilitate wire management. Before specifying this wiring method, the system designer must verify that the lead length is adequate after accounting for conductor routing as it relates to the racking system.
Guaranteed Cost Savings
Traditional daisy chain wiring results in excess module lead length that installers must manage using module wire clips or wire ties, as well as one long homerun wire that they need to manage. With the leapfrog wiring method, there is no excess lead length apart from one module-to-module connection in the middle of the string (at the end of the row); further, the total length of the homerun wiring is reduced by roughly the width of the row of modules. As a result, each source circuit requires fewer PV cable clips.
Using leapfrog wiring with 72-cell modules, you can expect to save $10–$15 per string in sub–600 V PV systems and $17–$23 per string in 1,000 V PV systems. Since each 72-cell PV module is roughly 3 feet wide, leapfrog wiring will reduce the length of your homerun conductor by roughly 33–36 feet per string in 600 V designs and 51–60 feet per string in 1,000 V designs. If you are paying $0.25 per foot for PV Wire, then you stand to save $8.25–$9 per 600 V string and $12.75–$15 per 1,000 V string. In addition, you can expect to save one or two cable clips per module at perhaps $0.20 each. You can easily modify these assumptions to account for your company’s preferred hardware solutions and the associated costs, which will vary based on purchase volume.