A main distribution board and a final distribution board occupy different levels of a low-voltage distribution system. The main board normally receives a larger incoming supply and feeds major loads or downstream boards. The final board distributes power to branch circuits close to lighting, socket outlets or local equipment.
These names are useful, but they are not enough to manufacture or price a board. EPC contractors and project buyers should define each board from the approved single-line diagram, circuit schedule, ratings, protection requirements and installation conditions.
Main distribution board vs final distribution board
| Project question | Main distribution board (MDB) | Final distribution board (FDB) |
|---|---|---|
| Position in the system | Near the transformer, generator, utility incomer or main LV distribution point | Downstream and closer to the branch-circuit loads |
| Typical outgoing circuits | Sub-main feeders, major equipment or downstream distribution boards | Lighting, small power and local equipment circuits |
| Important inputs | Incoming source, bus rating, fault level, feeder schedule, metering and room layout | Circuit schedule, incomer, outgoing protective devices, spare ways and mounting method |
| Common construction | Floor-standing multi-section assembly where the project ratings and feeder count require it | Wall-mounted or floor-standing assembly according to circuit count, rating and site space |
| Quotation risk | Missing fault-level, bus or cable-interface data can change the complete arrangement | Missing circuit details can change device quantities, board size and enclosure layout |
Browse the distribution board range for the product family, or review the individual main distribution board and final distribution board pages.
What defines a main distribution board?
An MDB is normally the principal low-voltage board for a building, process area or package. It may receive power from a transformer, generator, utility source or upstream switchgear and distribute it through outgoing feeders. Depending on the approved design, its scope can include a main breaker, bus coupler, metering, surge protection, earth-leakage functions and interfaces with standby power or a building-management system.
The label “MDB” does not establish the required rated current, short-circuit capability or form of separation. Those values must come from the project documents. The supplier also needs the cable-entry direction, conductor type and quantity, gland space, access requirements and available electrical-room dimensions before finalizing the layout.
What defines a final distribution board?
An FDB is commonly the last distribution stage before branch circuits. Its circuit schedule may include lighting, socket outlets, small mechanical loads or local equipment. The schedule should state the circuit reference, connected or design load, number of poles, protective-device rating and any residual-current protection requirement.
Final boards are often smaller than main boards, but they still require a defined incomer, bus rating, short-circuit requirement, neutral and earth arrangement, spare ways, enclosure protection and mounting method. A description such as “12-way DB” is incomplete if it does not state whether the 12 ways are single-pole circuits, three-pole circuits or a mixture.
Trace the board hierarchy on the SLD
The single-line diagram should show the upstream source, MDB, sub-main feeders and downstream boards. Each tag on the SLD should match the BOQ and the relevant circuit or feeder schedule. This prevents the same board from being described under different names and makes the supply boundary easier to verify.
- Identify the source. Record the transformer, generator or upstream-board reference and system voltage.
- Follow each feeder. Match the outgoing reference, load description, cable information and destination board.
- Confirm protection. State the incomer and outgoing protective-device requirements rather than assuming them from the board name.
- Check spare capacity. Separate fitted spare breakers from empty spare ways and future extension space.
- Align document tags. Use the same MDB and FDB references on the SLD, BOQ, schedules and equipment list.
The SLD and BOQ preparation guide provides a practical document structure for a comparable quotation.
Information required for an MDB quotation
| Input | What to provide | Why it matters |
|---|---|---|
| Electrical system | Voltage, frequency, phases, earthing arrangement and source | Defines the fundamental board and component selection basis |
| Ratings | Incoming and bus current plus required short-circuit rating | Affects busbar, protection and assembly construction |
| Feeder schedule | Outgoing references, duties, poles, ratings and quantities | Defines sections, devices and cable interfaces |
| Operation | Fixed or withdrawable requirement, interlocking and control functions | Changes construction, access and control wiring |
| Installation | Indoor or outdoor location, IP requirement, room size and cable entry | Guides enclosure and physical arrangement |
| Documents | Applicable project specification, drawings and required submittals | Defines review, testing and documentation scope |
Information required for an FDB quotation
- Board tag and location.
- Incoming device type, poles and rating.
- Complete outgoing circuit schedule, including spare devices and spare ways.
- Required protective-device characteristics and residual-current protection where specified.
- Metering, surge protection and indication requirements.
- Flush, surface or floor mounting; cable entry; enclosure material and IP rating.
- System voltage, short-circuit requirement and applicable project standard.
Common scope errors to resolve before RFQ
Using MDB, panel board and switchgear as interchangeable labels
Terminology varies by market and consultant. Define the electrical function and supplied components instead of relying only on the label. If the boundary between a board and a wider assembly is unclear, use the low voltage switchgear vs distribution board guide.
Counting ways without identifying poles
A schedule must distinguish single-pole, double-pole, three-pole and four-pole devices. It should also distinguish an installed spare protective device from an unused enclosure way.
Leaving incomers and source conditions undefined
The same outgoing schedule can require a different arrangement when the incoming source, fault level, neutral treatment, transfer requirement or cable entry changes.
Requesting a final layout before resolving project data
Preliminary dimensions can be discussed, but an approved arrangement depends on confirmed ratings, device schedule, segregation, access and cable space. Open items should remain visible in the clarification list.
Frequently asked questions
Can an MDB feed another MDB?
Project naming conventions vary. Electrically, one main board can feed another downstream main or sub-main board when the approved SLD and protection study define that hierarchy. Keep the tags consistent across all documents.
Is an FDB always wall-mounted?
No. Many final boards are wall-mounted, but the mounting arrangement depends on rating, circuit count, enclosure size, access and the project installation requirements.
Are spare ways and spare breakers the same?
No. A spare breaker is an installed protective device available for a future circuit. A spare way is reserved physical capacity and may not include the protective device. State each quantity separately.
Can MDB and FDB prices be compared from the number of ways alone?
No. Price and construction also depend on ratings, device type, fault level, metering, enclosure, cable space, installation conditions and documentation requirements.
Send the board schedules for review
Zejiefu can review the available SLD, BOQ, MDB feeder schedule and FDB circuit schedules before preparing a project quotation. Mark any unconfirmed values clearly so they can be handled as technical clarifications.