Distribution Hierarchy: MDB, SDB & FDB

Coordinated 3-tier low voltage power architecture from primary intake to final circuits

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The 3-Tier Power Distribution Hierarchy

A distribution hierarchy groups the primary intake, floor or zone feeders and final circuits into appropriate assemblies. Main distribution boards (MDBs), sub-distribution boards (SDBs) and final distribution boards (FDBs) perform different roles; their current ratings can overlap between projects.

Select each tier from the load schedule, cable routes, prospective fault current and required access. Verify protective-device selectivity with the manufacturer’s coordination data; neither a three-tier layout nor a reference photograph guarantees full discrimination.

The Three Distribution Tiers

Tier 1 · Primary intakePictured: ABB MNS Digital low-voltage switchgear and motor control lineup.

Main Distribution Switchboards (MDB)

ABB MNS Digital is pictured as a primary-intake and motor-control lineup reference. NPE project assemblies are specified from the load schedule, prospective fault current and operating conditions; the photograph does not establish a particular current, fault or separation rating.

RolePrimary intake

AssemblyIEC 61439-1 / -2 scope

RatingsProject-specific verification

Tier 2 · Floor or zonePictured: NHP Concept Plus wall-mounted distribution panelboard family.

Sub-Distribution Switchboards (SDB)

The NHP Concept Plus family shows a wall-mounted panelboard with its accessory enclosure, suitable as a sub-distribution reference. This Australian AS/NZS 61439-3 range has isolator incomers up to 250 A or MCCB incomers up to 200 A; it is not a Form 4 main-switchboard lineup.

Isolator incomerUp to 250 A

MCCB incomerUp to 200 A

ReferenceNHP Concept Plus

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Tier 3 · Final circuitsPictured: Schneider Electric Easy9 EZ9EFS12CMCU consumer unit.

Final Distribution Boards (FDB) & Consumer Units

The Schneider Electric Easy9 EZ9EFS12CMCU is a 12-way single-phase metal consumer-unit reference for final circuits. MCB, RCD and RCBO selection, loading and enclosure configuration must be confirmed for the actual board.

ConfigurationSingle phase + neutral

ReferenceEasy9 · 12-way

ProtectionSelect per circuit

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Hierarchy Tier Specifications

Selection item	Engineering requirement
MDB · primary intake	Main incomer and feeders; determine assembly current, fault withstand and separation from the project. ABB MNS is a lineup reference.
SDB · floor or zone	Distributes selected outgoing feeders. Pictured NHP panelboard family is not an 800 A / Form 4 assembly.
FDB · final circuits	Feeds lighting, sockets and equipment through selected MCBs / RCBOs. Pictured Easy9 is a single-phase consumer unit.

Tier 1 — Main Distribution Boards (MDB)

Selection item	Engineering requirement
Assembly standard	Determine the applicable IEC 61439 part and obtain design-verification and routine-verification records for the supplied assembly.
Current and voltage	From the approved load schedule, diversity, supply arrangement and service conditions.
Short-circuit performance	Prospective fault current, duration, protective-device coordination and assembly verification must agree.
Separation and access	Specify the required form and maintenance method. Internal separation does not authorise live work.
Enclosure and ambient	Specify IP rating, location, ventilation, temperature rise and cable access for the actual installation.
Photographs	Manufacturer references illustrate equipment classes; ratings are not inferred from appearance.

Tier 2 — Sub-Distribution Boards (SDB)

Selection item	Engineering requirement
Reference family	NHP Concept Plus wall-mounted panelboards, AS/NZS 61439-3
Reference incomers	Isolator up to 250 A; MCCB incomer up to 200 A, subject to exact configuration
Project SDB	Select pole count, outgoing ways, loading, metering and fault performance independently.

Tier 3 — Final Distribution Boards (FDB)

Selection item	Engineering requirement
Reference	Schneider Electric Easy9 EZ9EFS12CMCU, 12-way single-phase consumer unit
Final circuits	Choose device current, trip curve, pole count, residual-current protection and breaking capacity per circuit.
MCB vs RCD	An MCB provides overcurrent protection; use the specified RCD or RCBO for residual-current protection.

Advantages of Coordinated Hierarchy

Selectivity: Verify downstream and upstream device coordination against manufacturer tables and the fault study.
Safety & Segregation: High energy busbars stay isolated in primary plant rooms under Form 4 barriers, while final user boards are touch-safe.
Scalable Expansion: Add-on cableways, spare outgoing ways, and modular enclosures allow plant expansion without redesigning main switchboards.
Granular Metering: Energy sub-metering at SDB and FDB levels enables ISO 50001 energy accounting and departmental billing.

Typical Applications

Multi-storey commercial offices Hospitals and critical healthcare wings Data centres and server halls Manufacturing and packaging plants Shopping malls and retail chains Educational campuses Hotels and resorts Residential apartment towers

Intermediate and final boards can group loads, shorten final-circuit routes and support local isolation. Choose the number of tiers from the site layout, cable design, loading and maintenance requirements; three tiers are not necessary for every installation.

Use the actual prospective fault current, protective-device settings and manufacturer time-current and selectivity tables. Confirm the result for the proposed combination; topology alone does not guarantee discrimination or a universal clearing time.

Wall mounting is possible where the selected board construction, weight, fixings, cable access and location allow it. The pictured NHP Concept Plus family is a wall-mounted panelboard reference with its own published incomer limits; do not apply a generic main-switchboard rating to it.

Agree spare ways, space, busbar capacity and enclosure allowance in the project schedule. A spare device position alone does not establish capacity for a future load.

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Conceptual engineering scenes illustrate applications. Manufacturer reference photos are identified separately. Confirm exact models, dimensions and ratings before ordering.