A distribution board (DB) is an enclosure that receives an incoming electrical supply and splits it into smaller, individually protected circuits for lighting, sockets, equipment and other loads. You choose the right one by matching its current rating, short-circuit rating, protection coordination, construction and environment to the area it serves, and by leaving room to grow. This guide from Paklink LLC explains the types of distribution board, what goes inside them, how to size and specify one, and what commonly goes wrong. If you are planning a new facility or an upgrade, our team works as industrial electrical contractors in Dubai, covering the work from design to handover.
In short:
- Distribution boards sit between the main switchgear and the loads, in a chain of main, sub-main and final boards.
- The incomer rating comes from the design current, not from the sum of all connected loads.
- Short-circuit rating and discrimination decide whether a fault stays local or takes out the whole facility.
- Heat, dust and poor labeling cause most day-to-day DB problems, so specify the environment and the documentation up front.
What is a distribution board, and what does it do?
A distribution board takes power from a larger source, such as the main low voltage switchboard, and divides it among many circuits, each with its own protective device. It does three jobs at once: it distributes the supply, protects each circuit against overload and short circuits, and gives a safe point to isolate a circuit for maintenance.
Two terms cause confusion, so it is worth separating them. A distribution board distributes power to general loads. A motor control center (MCC) starts, controls and protects motors. They often share an electrical room, but they do different jobs.
Distribution boards are covered by the IEC 61439 series for low voltage switchgear and controlgear assemblies. Part 3 of the series deals specifically with distribution boards operated by ordinary persons, while Part 2 covers power switchgear assemblies. You can browse the standards on the IEC website.
What types of distribution board are there?
In a typical industrial or commercial building, distribution boards form a hierarchy. Power flows down from the main board, which is fed from the building’s low voltage switchgear (see how LV and MV switchgear differ), through sub-main boards to final boards, with a few special-purpose boards alongside.
| Board | Fed from | Feeds | Typical location |
|---|---|---|---|
| Main distribution board (MDB) | Transformer or main incomer | Sub-main boards and large loads | Main electrical room |
| Sub-main distribution board (SMDB) | MDB | Floors, zones, process areas | Risers and plant areas |
| Final distribution board (DB) | SMDB | Lighting, sockets, small equipment | Floors and workshops |
| Essential or emergency board | Standby generator or UPS | Life-safety and critical loads | Dedicated service areas |
| Special-purpose boards | MDB or SMDB | HVAC, lifts, kitchens, data rooms | Near the load |
Boards also differ by how they are built. Wall-mounted and recessed boards suit small final boards, while floor-standing boards suit larger MDBs and SMDBs. Three-phase boards (TPN) serve three-phase equipment and mixed loads, and single-phase boards serve small single-phase circuits.
What is inside a distribution board?
A distribution board contains the devices that protect and control each circuit, plus the busbars that connect them. The typical contents are:
| Component | What it does |
|---|---|
| Incomer (isolator, MCCB or ACB) | Receives the supply and isolates the whole board |
| Busbars | Carry current to the outgoing devices |
| Miniature circuit breakers (MCBs) | Protect smaller circuits against overload and short circuit |
| Moulded case circuit breakers (MCCBs) | Protect larger circuits and feeders |
| Residual current devices (RCDs) or RCBOs | Detect earth leakage and cut supply to protect people from shock |
| Surge protective devices (SPDs) | Limit voltage spikes that could damage equipment |
| Metering and indication | Show voltage, current and energy use |
| Neutral and earth bars | Provide the return and protective earth connections |
An MCB and an MCCB protect in the same basic way, but they suit different current ranges and applications. MCBs are compact and used for final circuits, while MCCBs handle higher currents and often have adjustable settings.
How do you size a distribution board?
Size the board from the design current, which allows for diversity, rather than from the sum of every connected load. Not every load runs at full power at the same time, so applying a diversity factor gives a realistic figure. The steps are:
- List the loads on the board, with their power and type.
- Add them up to get the total connected load.
- Apply diversity according to the design standard and the nature of the loads.
- Calculate the design current for the three-phase supply.
- Choose the incomer rating at the next standard size above that current, with a margin for growth.
A worked example makes this concrete. Suppose a floor has 120 kW of connected load, a diversity factor of 0.8 and a power factor of 0.9 on a 415 V three-phase supply.
| Step | Calculation | Result |
|---|---|---|
| Design load | 120 kW multiplied by 0.8 | 96 kW |
| Design current | 96,000 divided by (1.732 × 415 × 0.9) | About 148 A |
| Incomer choice | Next standard rating above 148 A, with growth margin | 160 A at minimum, and many designers choose 200 A |
The diversity factor here is only an illustration. The right figure depends on your design standard, the consultant’s specification and how the loads actually behave.
Leave spare ways in the board for future circuits. A fully populated board has nowhere to add a circuit without replacing the enclosure, so follow the spare capacity your specification requires.
What is the short-circuit rating, and why does it matter?
The short-circuit rating is the fault current a board can safely withstand, and it must be at least as high as the fault current available where the board is installed. If the real fault current is higher than the board’s rating, the devices may fail to clear the fault safely, and the damage can be severe.
Two ratings appear on datasheets, and they are not the same thing:
- The assembly rating: the short-circuit withstand of the busbars and the board as a whole.
- The device rating: the breaking capacity of each breaker, which must also meet or exceed the fault level.
The fault level is highest close to the transformer and falls with distance and cable impedance, so boards nearer the source need higher ratings than final boards at the end of long cable runs. Ask the designer for the calculated fault level at each board.
What is discrimination, and how does it affect board selection?
Discrimination, also called selectivity, means that only the protective device closest to a fault trips, so the rest of the installation keeps running. Without it, a fault on a single socket circuit could trip the main breaker and shut down a whole floor.
Good discrimination needs coordination between the breakers at each level, based on manufacturer tables or time-current curves. Mixing devices from different brands without checking can break that coordination. When you review a design, ask whether discrimination has been verified between the incomer, the sub-main and the final breakers.
Which distribution board is right for your environment?
The environment sets the enclosure type, the ingress protection (IP) rating and the materials. A board that works well in an air-conditioned office will not survive in a dusty plant room.
| Environment | Main risks | Specification response |
|---|---|---|
| Air-conditioned electrical room | Few, mainly heat from dense loads | Standard enclosure, adequate ventilation |
| Industrial hall or plant room | Dust, vibration, mechanical knocks | Higher IP rating, robust enclosure, filtered ventilation |
| Outdoor, pump rooms and washdown areas | Water, dust, sun | High IP rating, corrosion-resistant material, shade where possible |
| Coastal or chemical sites | Corrosion | Stainless steel, GRP or other corrosion-resistant finishes |
Heat deserves special mention in Dubai. Breakers and busbars carry their rated current only up to a certain ambient temperature, and above it they must be derated. If a board sits in a hot room, ask the supplier how the ambient temperature affects the ratings, and size accordingly.
How should neutrals and earth protection be handled?
Treat the neutral conductor with care wherever the loads are electronic. Computers, LED lighting and variable speed drives create harmonic currents that add up in the neutral instead of canceling out, so the neutral can carry more current than the phases. Designers often specify a full-size or oversized neutral for boards serving these loads. The same harmonics also stress capacitor banks on the same supply, which we cover in how harmonics affect capacitor banks.
For earth protection, RCDs and RCBOs reduce shock risk, and the earthing arrangement must follow the design standard and local requirements. Confirm with the consultant which devices are needed on which circuits.
What should you check in a distribution board specification?
A clear specification removes guesswork and makes quotes comparable. Check these points:
- Standard compliance, including the relevant parts of IEC 61439 and the device standards.
- Incomer type and rating, busbar rating and short-circuit rating.
- Outgoing devices, with their ratings, breaking capacity and type.
- Spare ways and spare space for future circuits.
- Form of separation, if the project needs it.
- Enclosure material, IP rating and finish, matched to the environment.
- Metering, SPDs and indication, where required.
- Labeling, including circuit identification and a circuit schedule inside the door.
- Testing and documentation, including test certificates and as-built drawings.
If the project connects to the utility network, the incoming arrangement and metering are agreed with the utility, so confirm those requirements early.
How are distribution boards tested and commissioned?
A sound process tests the board at the workshop and again after installation. At the workshop, expect visual inspection, mechanical checks, wiring verification, insulation and dielectric tests, and functional checks. On site, the usual tests are:
- Insulation resistance of the circuits
- Earth continuity and earth fault loop checks
- Polarity and phase sequence checks
- RCD trip tests
- Functional checks of any metering, indication or control devices
At handover, the contractor should provide a circuit schedule, test records, as-built drawings and manufacturer documents. The circuit schedule is the item most often missing, and the one maintenance teams miss most.
How do you maintain a distribution board?
Preventive maintenance finds loose connections and ageing devices before they fail. A sensible routine includes thermal imaging of connections and breakers, checking terminal tightness, cleaning dust from the enclosure, testing RCDs, and updating the circuit schedule when circuits change.
The warning signs of trouble are warm or discolored terminals, buzzing noises, breakers that trip repeatedly, and burning smells. Any of these should trigger an inspection by a qualified person.
What mistakes do distribution board projects commonly make?
Most problems come from decisions made before the board is built. Six mistakes appear again and again:
- Sizing from connected load, which leads to oversized boards, or from guesses, which leads to undersized ones
- No spare ways, so the first extra circuit means a new board
- Short-circuit rating below the real fault level
- Unverified discrimination, which causes nuisance trips of main breakers
- Weak enclosures in harsh locations, such as a low IP rating in a dusty plant room
- Missing or wrong labeling, which slows every fault investigation
Planning a distribution system for your facility?
If you are specifying new boards or reviewing an existing installation, talk to the team at Paklink LLC about your load, fault level and environment. You can see how we deliver electrical contracting projects on our services page.
Frequently asked questions
What is the difference between an MDB, an SMDB and a DB?
An MDB receives the main supply and feeds sub-boards. An SMDB feeds a floor, zone or process area. A final DB feeds the end circuits, such as lighting and sockets.
What is the difference between an MCB and an MCCB?
Both protect a circuit against overload and short circuit. MCBs are smaller and used for final circuits, while MCCBs handle higher currents and often have adjustable trip settings.
What does IP rating mean on a distribution board?
The IP (ingress protection) rating shows how well the enclosure keeps out dust and water. A higher number means better protection, so dusty or wet locations need a higher rating than clean indoor rooms.
How many spare ways should a distribution board have?
It depends on the project specification. Many designs leave a margin for future circuits, so follow what your consultant specifies rather than a general rule.
Do I need an RCD on every circuit?
Not necessarily. The requirement depends on the circuit type, the location and the applicable regulations, so confirm with your consultant and the local requirements.
What standard covers distribution boards?
The IEC 61439 series covers low voltage switchgear and controlgear assemblies, and Part 3 covers distribution boards operated by ordinary persons.
