Most buildings distribute power in tiers. Power arrives at a main board, is split to sub boards, and finally reaches the final distribution boards that feed lights, sockets and equipment. The three names you will see on drawings are MDB, SMDB and DB.
The three tiers at a glance
| Board | Full name | Role in the system | Typically feeds |
|---|---|---|---|
| MDB | Main Distribution Board | Receives the incoming supply and distributes it to the rest of the installation | SMDBs, large loads, motor control centres |
| SMDB | Sub Main Distribution Board | Takes a feeder from the MDB and splits it across a floor, zone or section | DBs and medium loads |
| DB | Distribution Board | Final board that feeds the end circuits | Lighting, sockets, small equipment |
MDB: the entry point
The MDB is where the incoming supply is received and protected. It normally holds the main incoming breaker and outgoing breakers for each sub board and each large load. Because everything depends on it, it is specified for the full connected load, the expected fault level and room for future outgoing ways.
SMDB: the middle tier
An SMDB is fed from the MDB and distributes to a defined area, for example one floor of a building or one production zone. Splitting the supply this way keeps cable runs shorter and means a fault in one zone does not have to take down the whole site.
DB: the final circuits
A DB feeds the end circuits. It is where miniature circuit breakers (MCBs) and residual-current protection typically protect individual lighting and socket circuits. For the difference between breaker types, see our guide MCB vs MCCB.
What to check when specifying a panel board
- Load and diversity: total connected load and the demand you actually expect.
- Fault level: the prospective short-circuit current at the board, so breakers and busbars are rated for it.
- Breaker coordination: upstream and downstream devices should discriminate so only the nearest device trips.
- Spare ways: leave capacity for growth.
- Enclosure and environment: indoor or outdoor, dust and moisture exposure, and the enclosure rating that follows.
- Standards and documentation: ask which standard the assembly is built and tested to and what documentation comes with it. Confirm the requirement for your project with your consultant or the authority.
- Motor loads: large motors are usually fed from a dedicated motor control centre rather than an ordinary DB.
Where Arab Arc Trading fits
Arab Arc Trading supplies and works on electrical panel assemblies and low-voltage protection and distribution equipment in Jeddah. See Electrical Panel Assemblies and LV Protection and Distribution, or contact us with your load list and drawings.
FAQ
Is an SMDB always needed? No. Small installations may go from the MDB straight to DBs. SMDBs are used where the building or site is large enough that a middle tier makes sense.
Is "panel board" the same as "distribution board"? In everyday use the terms overlap; "panel board" is more common in North American usage and "distribution board" elsewhere.
