What is a Junction Box?
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
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 ? to 3 ? 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 box
es, 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
A junction box is an electrical enclosure that protects electrical conductors at the points where
they are spliced, tapped and pulled. These enclosures allow electricians to easily access the
conductors to perform work when needed while simultaneously keeping the conductors safe from damage
and unauthorized access.
Sometimes, you’ll hear the term “junction box” used to refer to an electrical fixture box. In
the true technical definition of a distribution box,
wires should connect only to other wires and raceways. However, in practice, the term is also
frequently used to refer to many other electrical box types in which wires connect to a fixture such
as a ceiling fan, light switch or wall socket.
Sizing a junction box is critical for making sure the junction box functions correctly. In a
junction box that’s too small, an electrician may have to bend conductors excessively to fit them
fully inside the box, which risks damaging the insulation on the wires. Conductors and other
components spaced too closely may also create interference or hazardous conditions.
Article 314 of the National Electrical Code contains most of the key regulations that you’ll need
to follow when choosing and installing a junction box. These regulations describe in detail the
required junction box size, depending on the number and size of conductors and fittings enclosed
within the box.
You’ll need to calculate both box volume and box fill when determining the appropriate size for a
power distribution box. What
do these concepts mean?
Box volume is the total enclosed volume of the junction box, including any extended space provided
by attachments such as domed covers or extension rings.
Box fill is the total space occupied by the components that will be contained within the junction
box with electric breaker.
The box volume should always be greater than or equal to the box fill. To calculate box volume and
box fill, you’ll need the tables from NEC 314.16 and knowledge of how to use them. Next, we’ll look
at how to use these important tools.
- Creado: 01-09-21
- Última sesión: 01-09-21