What Is a Main Distribution Board (MDB)? Differences From Sub-Distribution Panels

What Is a Main Distribution Board (MDB)? Differences From Sub-Distribution Panels

A main distribution board (MDB), also called a main switchboard, is the low voltage assembly that receives incoming power from the transformer or utility supply and distributes it to every downstream point in a facility. It is the electrical backbone of the plant: the incoming circuit breaker, the main busbar system, metering devices and the protection for every outgoing feeder are gathered in a single enclosure. This guide explains what a main distribution board does, how it differs from a sub-distribution panel, which components it contains and how it should be sized.

What does a main distribution board do?

Energy enters a facility at one point but is consumed at hundreds of points. The main distribution board manages that transition. It takes the supply through a single incoming breaker, carries it to the main busbar and from there provides individually protected outgoing feeders to sub-distribution panels, motor control centres, the power factor correction system and large single loads.

This centralised architecture delivers three concrete benefits. The first is discrimination: a fault on one feeder trips only that feeder, instead of blacking out the whole plant. The second is measurability, because current transformers and the energy analyser sit in the main board, so total consumption, power factor and harmonic levels can be monitored from one place. The third is operational safety, since switching, locking and tagging procedures are carried out at one defined location.

Main distribution board vs sub-distribution panel

Both assemblies distribute low voltage power, but they sit at different levels of the hierarchy and are designed to different criteria. The main board is engineered for high short-circuit currents and large rated currents, while the sub-distribution panel is a smaller unit installed close to the final circuits and produced in much larger quantities.

Criterion	Main distribution board	Sub-distribution panel
Hierarchy	First board after the transformer or utility intake	Second level, fed from the main board
Typical rated current	630 A – 6300 A	63 A – 630 A
Short-circuit withstand	High, often 50–100 kA	Lower, typically 10–35 kA
Busbar system	Engineered multi-layer main busbar	Limited busbar or direct terminals
Internal separation	Usually Form 3b / Form 4b	Form 1 or Form 2 may be sufficient
Location	Substation or main electrical room	Floor, workshop or near machinery
Quantity	One or a few per facility	Can be dozens

In short, the main board collects and shares energy, while the sub-distribution panel delivers it to the point of use. If the coordination between the two is not designed correctly, a minor fault can stop the entire plant.

Components of a main distribution board

• Incoming circuit breaker: usually a withdrawable air circuit breaker (ACB) that can isolate the whole installation in one operation.
• Main busbar system: copper or aluminium bars sized for rated current, short-circuit forces and temperature rise.
• Outgoing breakers: moulded case circuit breakers (MCCB) or fused switch disconnectors feeding sub-boards, MCCs and large loads.
• Metering and monitoring: current transformers, energy analyser, voltage and current indication, communication module.
• Protection accessories: surge protective devices (SPD), earth leakage protection, arc protection and the neutral earthing arrangement where required.
• Control and signalling: phase presence lamps, fault signalling and remote open or close commands.
• Thermal management: fans, filters, heat exchangers or a panel cooling unit, plus temperature sensors where needed.
• Enclosure and separation: sheet steel body, internal barriers, cable routing space and the earthing bar.

Sizing: which data is required?

A main distribution board is not selected by rule of thumb. The following data shortens the design and quotation process and prevents surprises on site.

• Transformer rating and voltage levels, for example 1600 kVA, 34.5/0.4 kV.
• Calculated prospective short-circuit current, which determines the Icw and Icc ratings of the assembly.
• Total installed load and diversity factor, needed for a realistic rated current.
• Feeder schedule, with the rating, cable size, protection type and spare capacity of every outgoing way.
• Ambient conditions: ambient temperature, altitude, humidity, dust and the presence of corrosive atmospheres.
• Layout constraints: electrical room dimensions, door width, service corridor, front access only or front and rear access.
• Operational expectations: maintenance without shutdown, redundancy such as a double busbar arrangement, remote monitoring requirements.

If you want to see how the short-circuit calculation translates into panel design, we covered it in detail in our guide to short-circuit withstand (Icw).

Standards and design verification

Main distribution boards are assessed under IEC 61439-1 and IEC 61439-2. The standard requires the assembly to be verified as a whole, not merely through the brand of the devices inside it. Temperature rise, short-circuit withstand strength, dielectric properties, degree of protection and mechanical operation are all proven through design verification.

When purchasing a board, ask for the design verification reports, the routine test records and the technical file. The scope of the standard and the permitted verification methods are explained in our IEC 61439 guide.

Why does the form of internal separation matter?

Internal separation determines whether maintenance can be carried out while the installation is energised. In a Form 1 assembly there are no internal barriers, so a single feeder usually cannot be worked on without shutting the whole board down. In Form 3b and Form 4b assemblies the busbars, the devices and the outgoing terminals are separated from each other, so other feeders can remain live while work is done on one of them.

In production plants, data centres and hospitals where downtime is expensive, higher form levels are effectively mandatory.

Thermal management and layout

Because currents are high in a main board, losses are high as well. Losses in busbars and breakers turn into heat, and the internal temperature derates the devices. For this reason the internal power loss is calculated and the ventilation or cooling solution is selected accordingly.

A few practical layout rules also apply: leave at least one metre of service corridor in front of the board, decide the cable entry direction early, do not push the enclosure against the wall if rear access is needed, and ensure adequate ventilation in the electrical room. For the choice of enclosure protection level, see our guide to IP protection classes.

Operation and maintenance

When a main distribution board fails the whole facility is affected, so its maintenance cannot be postponed. At least once a year, busbar connection torques should be checked, thermal imaging should be used to scan for hot spots, filters should be cleaned, breaker mechanisms should be exercised and protection settings should be reviewed. Our periodic inspection guide sets out a practical programme.

Key takeaways: main distribution board

• What does a main distribution board do — Energy enters a facility at one point but is consumed at hundreds of points.
• Main distribution board vs sub-distribution panel — Both assemblies distribute low voltage power, but they sit at different levels of the hierarchy and are designed to different criteria.
• Sizing: which data is required — A main distribution board is not selected by rule of thumb.
• Standards and design verification — Main distribution boards are assessed under IEC 61439-1 and IEC 61439-2.
• Standard source — IEC publication catalogue: iec.ch.

Frequently asked questions

Is a main distribution board the same as a transformer panel?

No. A transformer panel refers to the medium voltage cubicle or the transformer connection compartment on the MV side. A main distribution board is the first distribution assembly connected to the low voltage output of the transformer.

Can a facility have more than one main board?

Yes. In installations with several transformers, multiple buildings or high redundancy requirements, a separate main board is built for each transformer, and a bus coupler can be used to transfer load between them.

Should power factor correction be built into the main board?

At small ratings the compensation equipment can be housed in a compartment of the main board, but at medium and large ratings a separate compensation panel is preferred because of capacitor heating and maintenance needs. See our article on reactive power compensation.

Is it necessary to leave spare ways?

It is strongly recommended. Facilities grow and new machines are added. Allowing 20 to 30 per cent spare feeder positions and busbar capacity from the start is far cheaper than replacing the board later.

The Devpan approach

Related articles

• Choosing the Incoming Device: MCCB or ACB?
• Cable Sizing and Voltage Drop Calculation
• Sizing the Neutral and Earth Bars in a Switchboard
• All technical articles

Devpan designs main distribution boards around your project data, verifies them under IEC 61439 and ships them after factory acceptance testing. Rated current, short-circuit withstand, form of separation and the thermal solution are all determined by the real requirements of your installation. Share your project and we will define the right configuration and delivery schedule together.