A distribution board (also known as panelboard, breaker panel, electric panel, DB board or DB box) is a component of an
electricity supply system that divides an electrical power feed into subsidiary
circuits while providing a protective
fuse or
circuit breaker
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the ris ...
for each circuit in a common
enclosure
Enclosure or Inclosure is a term, used in English landownership, that refers to the appropriation of "waste" or " common land" enclosing it and by doing so depriving commoners of their rights of access and privilege. Agreements to enclose land ...
. Normally, a main
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
, and in recent boards, one or more
residual-current device
A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device that quickly breaks an electrical circuit with leakage current to ground. It is to protect equi ...
s (RCDs) or
residual current breakers with overcurrent protection (RCBOs) are also incorporated.
In the United Kingdom, a distribution board designed for domestic installations is known as a consumer unit.
North America
![ElectricalPanel](https://upload.wikimedia.org/wikipedia/commons/1/1e/ElectricalPanel.jpg)
North American distribution boards are generally housed in
sheet metal
Sheet metal is metal formed into thin, flat pieces, usually by an industrial process. Sheet metal is one of the fundamental forms used in metalworking, and it can be cut and bent into a variety of shapes.
Thicknesses can vary significantly; ex ...
enclosures, with the
circuit breaker
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the ris ...
s positioned in two columns operable from the front. Some panelboards are provided with a door covering the breaker switch handles, but all are constructed with a ''dead front''; that is to say the front of the enclosure (whether it has a door or not) prevents the operator of the circuit breakers from contacting live electrical parts within.
Busbar
In electric power distribution, a busbar (also bus bar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high volt ...
s carry the current from incoming line (''hot'') conductors to the breakers, which are secured to the bus with either a ''bolt-on'' connection (using a threaded screw) or a ''plug-in'' connection using a retaining clip. ''Panelboards'' are more common in commercial and industrial applications and employ bolt-on breakers. Residential and light commercial panels are generally referred to as ''load centers'' and employ plug-in breakers. The neutral conductors are secured to a neutral bus using
screw terminal
A screw terminal is a type of electrical connection where a wire is held by the tightening of a screw.
Description
The wire may be wrapped directly under the head of a screw, may be held by a metal plate forced against the wire by a screw, o ...
s. The branch circuit
bonding conductors are secured to a terminal block attached directly to the panelboard enclosure, which is itself
grounded.
During servicing of the distribution board, when the cover has been removed and the cables are visible, American panelboards commonly have some live parts exposed. In Canadian service entrance panelboards the main switch or circuit breaker is located in a ''service box'', a section of the enclosure separated from the rest of the panelboard, so that when the main switch or breaker is switched off no live parts are exposed when servicing the branch circuits.
Breaker arrangement
![OpenClosedPanelboardEEUU(GE)CloseupTop](https://upload.wikimedia.org/wikipedia/commons/e/ef/OpenClosedPanelboardEEUU%28GE%29CloseupTop.jpg)
Breakers are usually arranged in two columns. In a U.S.-style board, breaker positions are numbered left-to-right, along each row from top to bottom. This numbering system is universal with numerous competitive manufacturers of breaker panels.
Each row is fed from a different phase (''A'', ''B'', and ''C'' below), to allow 2- or 3-pole common-trip breakers to have one pole on each phase. In North America, it is common to wire large permanently installed equipment line-to-line. This takes two slots in the panel (two-pole) and gives a voltage of 240 V for
split-phase electric power
A split-phase or single-phase three-wire system is a type of single-phase electric power distribution. It is the alternating current (AC) equivalent of the original Edison Machine Works three-wire direct-current system. Its primary advantage is t ...
, or 208 V for
three-phase power
Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, transmission, and distribution. It is a type of polyphase system employing three wires (or four including an optional neutral ...
.
Interior
![EEUU(GE)BreakerpanelInnards](https://upload.wikimedia.org/wikipedia/commons/e/ea/EEUU%28GE%29BreakerpanelInnards.jpg)
The photograph to the right shows the interior of a residential service panelboard manufactured by
General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable en ...
. The three service conductors—two 'hot' lines and one neutral—can be seen coming in at the top. The neutral wire is connected to the neutral
busbar
In electric power distribution, a busbar (also bus bar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high volt ...
to the left with all the white wires, and the two hot wires are attached to the main breaker. Below the main breaker are the two bus bars carrying the current between the main breaker and the two columns of branch circuit breakers, with each respective circuit's red and black hot wires leading off. Three wires (hot black, neutral white, and bare ground) can be seen exiting the left side of the enclosure running directly to a
NEMA 5-15 electrical receptacle
AC power plugs and sockets connect electric equipment to the alternating current (AC) mains electricity power supply in buildings and at other sites. Electrical plugs and sockets differ from one another in voltage and current rating, shape, si ...
with a
power cord
A power cord, line cord, or mains cable is an electrical cable that temporarily connects an appliance to the mains electricity supply via a wall socket or extension cord. The terms are generally used for cables using a power plug to connect ...
plugged into it. The incoming bare, stranded ground wire can be seen near the bottom of the neutral bus bar.
The photograph on the left shows a dual panel configuration: a main panel on the right (with front cover in place) and a subpanel on the left (with cover removed). The subpanel is fed by two large hot wires and a neutral wire running through the angled conduit near the top of the panels. This configuration appears to display two violations of the current U.S. National Electric Code: the main panel does not have a grounding conductor (here it is fed through the subpanel instead) and the subpanel neutral bar is bonded to the ground bar (these should be separate bars after the first service disconnect, which in this case is the main panel).
Fuse boxes
![Murray-fuse-box](https://upload.wikimedia.org/wikipedia/commons/b/b0/Murray-fuse-box.jpg)
A common design of fuse box that was featured in homes built from 1940 through 1965 was the 60-amp fuse box that included four plug fuses (i.e. Edison base) for branch circuits and one or more fuse blocks containing cartridge fuses for purposes such as major appliance circuits. After 1965, the more substantial 100 A panel with three-wire (230 V) service became common; a fuse box could have fuse blocks for the main shut-off and an electric range circuit plus a number of plug fuses (Edison base or Type S) for individual circuits.
United Kingdom
![UKDistributionBoard](https://upload.wikimedia.org/wikipedia/commons/c/ca/UKDistributionBoard.JPG)
This picture shows the interior of a typical distribution panel in the United Kingdom. The three incoming phase wires connect to the busbars via a main switch in the centre of the panel. On each side of the panel are two
busbar
In electric power distribution, a busbar (also bus bar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high volt ...
s, for neutral and earth. The incoming neutral connects to the lower busbar on the right side of the panel, which is in turn connected to the neutral busbar at the top left. The incoming earth wire connects to the lower busbar on the left side of the panel, which is in turn connected to the earth busbar at the top right. The cover has been removed from the lower-right neutral bar; the neutral bar on the left side has its cover in place.
Down the left side of the phase busbars are two two-pole
RCBOs and two single-pole breakers, one unused. The two-pole RCBOs in the picture are not connected across two phases, but have supply-side neutral connections exiting behind the phase busbars. Down the right side of the busbars are a single-pole breaker, a two-pole RCBO and a three-pole breaker.
The illustrated panel includes a great deal of unused space; it is likely that the manufacturer produces 18- and 24-position versions of this panel using the same chassis.
Larger commercial, public, and industrial installations generally use three-phase supplies, with distribution boards which have twin vertical rows of breakers. Larger installations will often use subsidiary distribution boards.
In both cases, modern boards handling supplies up to around 100 A (CUs) or 200 A (distribution boards) use circuit breakers and RCDs on
DIN rail
A DIN rail is a metal rail of a standard type widely used for mounting circuit breakers and industrial control equipment inside equipment racks. These products are typically made from cold rolled carbon steel sheet with a zinc-plated or chroma ...
mountings. The main distribution board in an installation will also normally provide a main switch (known as an ''incomer'') which switches the phase and neutral lines for the whole supply. (n.b., an incomer may be referred to, or sold as, an ''
isolator'', but this is problematic, as it will not necessarily be used as an isolator in the strict sense.)
For each phase, power is fed along a
busbar
In electric power distribution, a busbar (also bus bar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high volt ...
. In split-phase panels, separate busbars are fed directly from the incomer, which allows RCDs to be used to protect groups of circuits. Alternatively RCBOs may be used to provide both overcurrent and residual-current protection to single circuits.
Other devices, such as
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s (e.g. for bell circuits) and
contactors
:''In semiconductor testing, contactors can also be referred to as the specialized socket that connects the device under test.''
:''In process industries, a contactor is a vessel where two streams interact, for example, air and liquid. See Gas ...
(relays; e.g. for large motor or heating loads) may also be used.
New British distribution boards generally have the live parts enclosed to
IP2X, even when the cover has been removed for servicing.
Consumer units
In the United Kingdom,
BS 7671
British Standard BS 7671 "Requirements for Electrical Installations. IET Wiring Regulations", informally called in the UK electrical community "The Regs", is the national standard in the United Kingdom for electrical installation and the safet ...
defines a
consumer unit
A consumer unit is a type of distribution board (a component of an electrical power system within which an electrical power feed provides supply to subsidiary circuits).
UK
The IET defines a consumer unit, also known as a consumer contro ...
as "A particular type of distribution board comprising a type tested coordinated assembly for the control and distribution of electrical energy, principally in domestic premises..."
[BS7671 17th Edition] These installations usually have single-phase supplies at 230 V (nominal standard); historically, they were known as
fuse boxes, as older consumer units used
fuses
Fuse or FUSE may refer to:
Devices
* Fuse (electrical), a device used in electrical systems to protect against excessive current
** Fuse (automotive), a class of fuses for vehicles
* Fuse (hydraulic), a device used in hydraulic systems to protec ...
until the advent of
mini-circuit breakers (MCBs).
A normal new domestic CU used as a main panel might have from 6 to 24 ways for devices (some of which might occupy two ways), and will be split into two or more sections (e.g. a non-RCD section for alarms etc., an RCD-protected section for socket outlets, and an RCD-protected section for lighting and other built-in appliances). Secondary CUs used for outbuildings usually have 1 to 4 ways plus an RCD.
Recent (pre-17th edition wiring regulations) CUs would not normally have RCD protected sections for anything other than socket outlets, though some older CUs featured RCD incomers. Before 1990, RCDs (and split busbars) were not standard in CUs.
File:Mem_1957_cu_closed.jpg, 1950s MEM rewirable fuse box (covered)
File:MEM_1957_cu_open.jpg, 1950s MEM rewirable fuse box (open)
File:Mem_1977_cu_closed.jpg, 1970s MEM rewirable fuse box (covered)
File:Mem_1977_cu_open.jpg, 1970s MEM rewirable fuse box (open)
Fuse boxes normally use cartridge or rewirable fuses with no other protective device, and basic 4-ways boxes are very common. Some older boxes are made of brown-black bakelite, sometimes with a wooden base. Although their design is historic, these were standard equipment for new installs as recently as the 1980s, so they are very common. Fuseholders in these boxes may not provide protection from accidental contact with live terminals.
The popular 4-way fusebox normally had two lighting and two socket circuits, with heavy or sustained loads such as immersion heater and oven on a socket circuit. This arrangement is not recommended practice today, but it is common for existing installations. Larger boxes with more ways will have separate fuses for large loads such as immersion heater, oven and shower.
Legacy fuseboxes
A small number of pre-1950 fuseboxes are still in service. These should be treated with caution because exposed live parts are common on these boxes. The installations they supply will not meet modern standards for
electrical safety Electrical safety testing is essential to make sure electrical products and electrical installations are safe. To meet this goal, governments and various technical bodies have developed electrical safety standards. All countries have their own elect ...
. Another characteristic of very old installations is that there may be two fuses for each circuit; one on the
live and one on the
neutral
Neutral or neutrality may refer to:
Mathematics and natural science Biology
* Neutral organisms, in ecology, those that obey the unified neutral theory of biodiversity
Chemistry and physics
* Neutralization (chemistry), a chemical reaction in ...
. In rare instances, old ring circuits may be encountered with no fewer than 4 15 A fuses per ring, one on each of L and N, and this duplicated for each of the two feeds for the ring.
Manufacturer differences
Most of the time, the panels and the breakers inserted inside them must be by the same manufacturer. Each manufacturer has one or more "systems", or kinds of breaker panels, that will only accept breakers of that type. In Europe, this is still the case, despite the adoption of a standard
DIN rail
A DIN rail is a metal rail of a standard type widely used for mounting circuit breakers and industrial control equipment inside equipment racks. These products are typically made from cold rolled carbon steel sheet with a zinc-plated or chroma ...
for mounting and a standard cut-out shape, as the positions of the busbar connections are not standardized.
Certain panels use seemingly interchangeable breakers. However, a given manufacturer will specifically mention exactly which devices may be installed in their equipment. These assemblies have been tested and approved for use by a recognized authority. Replacing or adding equipment which "just happens to fit" can result in unexpected or even dangerous conditions. Such installations should not be done without first consulting knowledgeable sources, including manufacturers.
Location and designation
![220v service panel](https://upload.wikimedia.org/wikipedia/commons/7/7a/220v_service_panel.jpg)
For reasons of
aesthetics
Aesthetics, or esthetics, is a branch of philosophy that deals with the nature of beauty and taste, as well as the philosophy of art (its own area of philosophy that comes out of aesthetics). It examines aesthetic values, often expressed t ...
and security, domestic circuit breaker panels and consumer units are normally located in out-of-the-way
closet
A closet (especially in North American usage) is an enclosed space, with a door, used for storage, particularly that of clothes. ''Fitted closets'' are built into the walls of the house so that they take up no apparent space in the room. Closet ...
s,
attics,
garages, or
basements, but sometimes they are also featured as part of the aesthetic elements of a building (as an art installation, for example) or where they can be easily accessible. However, current U.S. building codes prohibit installation of a panel in a bathroom (or similar room), in closets intended for clothing, or where there is insufficient space for an electrician to gain access to the panel. Specific situations, such as an installation outdoors, in a hazardous environment, or in other out-of-the-ordinary locations might require specialized equipment and more stringent installation practices.
Distribution boards may be designated for three phase or single phase and normal power or emergency power, or designated by use such as distribution panels for supplying other panels, lighting panels for lights, power panels for equipment and receptacles and special uses. Panels are located throughout the building in electric closets serving a section of the building.
Theatre lighting
In a theatre, a specialty panel known as a ''
dimmer rack'' is used to feed stage lighting instruments. A U.S. style dimmer rack has a 208Y/120 volt 3-phase feed. Instead of just circuit breakers, the rack has a solid state electronic dimmer with its own circuit breaker for each stage circuit. This is known as a ''dimmer-per-circuit'' arrangement. The dimmers are equally divided across the three incoming phases. In a 96 dimmer rack, there are 32 dimmers on phase A, 32 dimmers on phase B, and 32 on phase C to spread out the lighting load as equally as possible. In addition to the power feed from the supply transformer in the building, a control cable from the
lighting desk carries information to the dimmers in a control protocol such as
DMX-512
DMX512 is a standard for digital communication networks that are commonly used to control lighting and effects. It was originally intended as a standardized method for controlling stage lighting dimmers, which, prior to DMX512, had employe ...
. The information includes lighting level information for each channel, by which it controls which dimmer circuits come up and go out during the lighting changes of the show (light cues), and over what fade time.
Distribution boards may be surface-mounted or flush. The former arrangement provides easier alteration or addition to wiring at a later date, but the latter arrangement might be neater, particularly for a residential application. The other problem with recessing a distribution board into a wall is that if the wall is solid, a lot of brick or block might need to be removed—generally for this reason, recessed boards would only be installed on new-build projects when the required space can be built into the wall.
See also
*
Circuit total limitation
*
Consumer unit
A consumer unit is a type of distribution board (a component of an electrical power system within which an electrical power feed provides supply to subsidiary circuits).
UK
The IET defines a consumer unit, also known as a consumer contro ...
*
Electric switchboard
An electric switchboard is a device that distributes electricity from one or more sources of supply to several smaller load circuits. It is an assembly of one or more panels, each of which contains switching devices for the protection and contro ...
*
Electricity meter
*
Electrical busbar system
*
Switchgear
In an electric power system, a switchgear is composed of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be ...
References
{{Electricity delivery, state=collapsed
Electrical wiring
Electric power distribution