Using Y-Connectors in String Inverter Systems
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At Solar Power International last year, a sales representative for one of our distribution partners inquired: “Why do so many of my customers order 30 A fuses in their source-circuit combiner boxes?” This is a good question. After all, most crystalline silicon (c-Si) PV modules have a short-circuit current (Isc) rating in the 8–9 A range and carry a 15 A–series fuse rating. This is so common that source-circuit combiners typically come standard with 15 A series fuses. Occasionally, an engineer might specify 20 A fuses to account for thermal derating. However, 30 A fusing assumes an Isc of roughly 18 A, which is an unprecedented series fuse rating for today’s PV modules.
So why do integrators request combiners with 30 A fuses? The answer is not a function of module ratings per se, but rather of how system integrators deploy these modules. Specifically, more and more installation companies use special Y-connector assemblies to parallel PV source circuits in the array field as a way to optimize electrical balance of system (eBOS) costs.
Most industry veterans have seen parallel branch connectors or Y-connector assemblies at conferences or pictured in trade publications or product catalogues. For example, both Amphenol and Multi-Contact offer male and female branch connectors rated for 30 A, as well as overmolded Y-connector assemblies with optional inline fuses. Many eBOS companies also offer customizable Y-connector assemblies. What these connectors and assemblies all have in common is that they have two inputs and one output, allowing installers to make plug-and-play parallel connections within the array.
Until recently, paralleling source circuits within an array was most common in thin-film applications. Compared to c-Si PV modules, thin-film technologies tend to have a higher Voc and a lower Isc. As a result, it behooves integrators to use wire harnesses with inline fuses to parallel thin-film PV source circuits prior to landing them in a combiner box. This practice is cost-effective because it improves conductor utilization within the array and limits the number of combiner box inputs.
Designers can apply these same principles to c-Si PV arrays. After all, touch-safe fuseholders in combiner or inverter wiring boxes are generally 30 A rated, whereas most PV modules have a 15 A series fuse–rating. Therefore, integrators may be able to improve project economics by using Y-connectors to parallel a pair of source circuits ahead of these fuseholders. Before evaluating the potential cost savings associated with this approach, let us review some practical considerations.
Code implications. NEC Section 690.9 requires overcurrent protection for PV modules or source circuits, except when there are no external sources of fault current, or when the short-circuit currents from these sources do not exceed the ampacity of the conductors and the maximum series fuse rating. To make a parallel connection ahead of a combiner box, designers need to account for potential sources of fault currents as well as the module manufacturer’s series fuse ratings. Generally speaking, parallel connections within the array require Y-connector assemblies with inline fuses. In effect, designers need to relocate 15 A series fuses from the combiner box out into the array wiring.
Since parallel connections increase current, designers also need to evaluate conductor ampacity between the Y-connector and the dc combiner or inverter-input wiring box. To achieve the desired cost savings, integrators need to be able to parallel source circuits within the array without unnecessarily incurring the expense of larger-diameter conductors. To avoid having to step from 10 AWG to 8 AWG copper conductors, for example, designers should avoid or minimize situations that require conductor ampacity adjustments according to Article 310. The two most common ampacity adjustment scenarios relate to the number of current-carrying conductors (see Table 310.15[B][a]) and distance above the roof (see Table 310.15[B][c]). When paralleling source circuits within the array, therefore, it generally makes sense to limit the number of conductors bundled or grouped together to no more than three and to maintain a distance above the roof of at least 12 inches.
Manufacturer limitations. While most of the finger-safe fuseholders for 10 mm by 38 mm fuses found in combiner boxes are manufacturer rated for 30 A, the busbars connected to the fuseholders are not always capable of carrying 30 A of current. Integrators should check with the combiner or inverter manufacturer to ensure that the product is compatible with the use of 30 A fuses.
In some cases, equipment manufacturers require an allowance for heat dissipation where fuseholders are fused at 30 A. The concern is that a lack of space between fuseholders can cause a fuseholder to overheat, potentially melting the plastic and causing a fault. This is not an issue when inputs are fused at 15 or 20 A, as is typical of most string inverter or combiner box applications. However, it may become an issue under continuous loading at full power with 30 A fuses. Landing input conductors on alternating fuseholders, as shown in Figure 1, and removing the unused fuses is one way to improve heat dissipation.