Junction boxes are metal or plastic enclosures used as housings for wiring connections. The connections within are called branch circuits and usually represent the end of a conduit run. Junction boxes make wire access easy, since all one must do is remove the covering to make alterations, repairs, or additions to a conduit. Junction boxes also protect wiring from the elements or environment, which can sometimes be corrosive or otherwise harmful to wiring material. Finally, junction boxes protect wiring from unwanted tampering, whether malicious or unintentional.
Junction Box Wiring Basics
Essentially, a junction box houses wire connections in order to split off power from a single source to multiple outlets. For instance, a junction box might contain one wire power source that is connected through multiple wires to power several different lights.
Electrical junction boxes are usually between 2 ½ to 3 ½ inches long and made of metal or hard plastic. The functional difference between plastic and metal depends whether or not the junction box is supposed to support any weight. Some metal junction boxes can support light fixtures; plastic junction boxes cannot withstand this weight. Additional differences include installation, in that plastic IP65 junction boxes are typically quicker and easier to install than metal ones. However, a standard junction box designed to simple cover wire splices can be either metal or plastic.
Wire Splices in Junction Boxes
All wire splices must be contained within a junction box for a building to meet electric code, although sometimes splices are missed and may present hazards as a result. Any exposed wiring can be dangerous, but exposed wire splices are especially prone to accident because they can be tripped over, expel sparks or misrepresent themselves be misperceived as playthings by children or pets. Junction boxes are helpful for wire splices because they also allow one to easily locate the wire splice area.
Waterproof junction boxes provide an important function of providing a safe transition of power produced from the photovoltaic (PV) cells in the laminate to the external wiring system for interconnecting modules and loads. Electric distribution boxes, also referred to as “jboxes,” are used in a wide variety of electrical applications and serve to protect the connections between wires and cables. In a PV module, however, the jbox also houses the bypass diode that provides a PV cell-protective function and it is prewired and terminated for rapid interconnection of modules in the field. The junction box enclosure consists of a body enclosure and lid both of which are almost exclusively made of injection-molded polymeric construction and are adhered to the substrate of the module with an adhesive compatible with both the junction box polymer and PV module substrate.
The combination of the enclosure, bypass diode, transitioning wiring, connectors, and adhesive makes the jbox a complex component that appears deceptively simple and is not designed to be maintained or serviced. This combination of system behavior and lack of maintainability poses a reliability challenge for the PV module manufacturer, where a service lifetime of 25-years is expected. The importance of design, characterization of the failure and degradation modes, sufficient qualification and durability testing, and manufacturing quality control for the Jbox system (box and connectors) cannot be overestimated since failures of this system account for approximately 18% of 180 fire incidents studied from 1995 to 2012. This study indicates that the jbox with its associated PV connector was the number one cause for a fire, with the second being inverters at 16%. The International Energy Agency’s 2014 report suggests that PV connectors alone may be responsible for up to one-third of PV module fire incidents suggesting that special attention to this component is required.