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 ? 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 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.
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.