A low-voltage distribution board is an enclosed electrical assembly that receives power and distributes it to outgoing circuits. It brings switching and protective devices together so circuits can be controlled and faults can be isolated at the appropriate circuit. It is part of an electrical installation; it does not generate electricity.
How a distribution board works
Think of the board as a controlled junction between the incoming supply and the circuits serving a building. Internal distribution components feed power to outgoing circuits, while switches and protective devices provide ways to control those circuits and interrupt current under specified fault or overload conditions. Protection is not infallible, and the board alone cannot make an electrical installation safe.
The exact arrangement depends on the board design, the installation and its earthing system. A board is only one element of a wider low-voltage distribution system, which also involves equipment and wiring choices, earthing arrangements and environmental conditions. Schneider Electric’s Electrical Installation Guide discusses these considerations.
What is inside a low-voltage distribution board?
Components vary by type and application; not every board has every item in the same form. Typical features include:
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- An enclosure and mounting system.
- Incoming terminals and, often, a main switch or isolator.
- Busbars or another internal arrangement for distributing incoming power.
- Outgoing circuit breakers, fuses or other protective devices.
- Neutral and protective-earth bars or terminals, as appropriate to the electrical system.
- Labels and a circuit schedule identifying the circuits served.
Hager’s Guide to IEC 61439 illustrates how assembly configurations differ, from incoming switching and busbars to modular or higher-current devices.
How main and sub-distribution boards differ
The terms describe where a board sits in the installation, rather than two interchangeable names for the same position.
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| Board type | Position and role |
|---|---|
| Main switchboard | Typically receives the supply near its origin, such as in a transformer area, and feeds the wider installation. |
| Main distribution board | Receives power from the main switchboard and distributes it onward, often from a technical area. |
| Sub-distribution board | Receives a feeder from an upstream board and distributes power locally to a smaller group of circuits. |
| Consumer unit or domestic distribution board | Common terms for household-scale boards in some markets; terminology and configuration vary by country. |
Hager uses the distinctions between low-voltage main switchboards, main distribution boards and sub-distribution boards in its IEC 61439 guide. A home consumer unit is not a template for every commercial or industrial board.
What “low voltage” and the standards mean
Standards define particular equipment categories and their requirements; a scope limit for one category should not be treated as a universal limit for all low-voltage boards.
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- There are 3 connectors for the input of voltage and 2 x 10 connectors for the outputs on the distributor.
- 4 holes for mounting with wide spacers.
- Maximum voltage: 30V AC or DC voltage.
- Total supply maximum current: 8A (2-pole Power supply to terminal).
- Current carrying capacity per connection: 6A (max).
Boards intended for operation by ordinary persons
IEC 61439-3:2024, published on 26 March 2024, covers a defined category of distribution boards intended to be operated by ordinary persons (DBOs), including domestic applications. For that category, its scope includes nominal voltage to earth not exceeding 300 V AC, outgoing-circuit rated current not exceeding 125 A, and assembly rated current not exceeding 250 A. These are boundaries of the standard’s DBO scope, not general ratings for every low-voltage distribution board. IEC lists the edition’s stability date as 2028.
General low-voltage switchgear rules
IEC 60947-1:2020 provides general rules for low-voltage switchgear and controlgear when called for by the relevant product standard. Its stated circuit-rating scope does not exceed 1,000 V AC or 1,500 V DC. Those figures concern that broader switchgear and controlgear scope; they do not replace the narrower requirements for ordinary-person boards or local installation rules.
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What matters when identifying or specifying a board
The right assembly depends on the supply, loads, building, environment and applicable jurisdiction. Useful points to establish with the responsible electrical designer or contractor include:
- Whether the board is a main board or a downstream board, and where it will be installed.
- Its rated voltage and current, and the supply and earthing arrangement it must suit.
- The number and type of outgoing circuits and compatible protective devices.
- The enclosure’s suitability for the installation environment and any planned expansion.
- Whether the assembly is designed and verified for the applicable standard and installation requirements.
No single board specification can be selected responsibly from the label “low voltage” alone. The design must account for the actual installation and local electrical rules.
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- Input terminals: pitch - 7.62mm/0.3", current 30A for per position, wire size 26~10 AWG, stripping length 8mm, Metric M3 slotted screw.
- P1 output terminals: pitch - 5.08mm/0.2", maximum current 10A for per position, wire size 26~12 AWG, stripping length 7mm, Metric M2.5 slotted screw.
- P2-1 to P2-10 output terminals: pitch - 7.5mm/0.295", maximum current 10A for per position, wire size 26~12 AWG, stripping length 7mm, Metric M2.5 slotted screw.
Safety and responsibility
Distribution boards can expose people to hazardous electrical energy. Installation, modification, replacement and fault diagnosis should be performed by appropriately qualified people under the rules that apply where the building is located. A board’s devices do not replace upstream supply protection, and the board itself does not establish the installation’s earthing arrangement.
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