In short. A PCC takes power from the source and distributes it to large feeders; an MCC feeds and controls motors; distribution boards serve smaller loads. Their design sets fault withstand, safety during maintenance and how easily the system can grow.
Roles
| Assembly | Typical role |
|---|---|
| PCC | Incomers, bus coupler, large outgoing feeders |
| MCC | Motor starters, drives, control |
| PDB or sub-distribution | Branch circuits to areas |
| Generator panel | Generator breaker, metering, protection |
Key specification points
- Busbar rating: continuous current after derating for temperature and enclosure. See switchboard thermal engineering.
- Short-circuit withstand: the busbar and supports must withstand the prospective fault for the trip time.
- Form of separation: internal partitions that limit what is exposed during maintenance or fault. Higher forms cost more and improve safety and availability.
- Ingress protection and ventilation suited to the room.
- Withdrawable or fixed devices: withdrawable allows isolation without shutdown of the whole board.
- Spare capacity and space for future feeders.
Distribution layouts
- Radial: simple and cheaper, a single path to each load.
- Bus-section with coupler: two incomers and a normally open coupler for resilience.
- Ring or dual-fed: alternative paths for critical loads.
The choice follows the redundancy goal. See N+1.
Generator and ATS integration
Decide whether the generator feeds the whole PCC or an essential bus. Include interlocks to prevent paralleling of unsynchronised sources. See interlocking and transfer logic.
Selection steps
- Prepare the load schedule and the single-line diagram.
- Find the fault level at the board.
- Choose busbar and device ratings with margin.
- Choose the form of separation and layout.
- Specify type-test certificates and routine tests.
- Plan access and clearances in the room.
Illustrative example
A plant needs a 1,600 A PCC with two incomers (grid and generator), a coupler and eight outgoing feeders. With a fault level of 40 kA, the board is specified for a higher short-time withstand, with withdrawable breakers and a higher form of separation for the essential section, so that a feeder can be serviced without losing the whole board.
Reading the architecture of an LV system
Incomer (transformer or generator) → main busbar in the PCC → outgoing feeders → MCCs and distribution boards → final circuits. The PCC (power control centre) holds the main breakers and large feeders. MCCs (motor control centres) group starters. Distribution boards serve lighting and small power.
Form of separation
| Form | Level of internal separation | Effect on maintenance |
|---|---|---|
| Form 1 | No internal separation | Whole board usually isolated |
| Form 2 | Busbars separated from functional units | Some live parts remain during work |
| Form 3 | Units also separated from each other | Individual units can be worked on more safely |
| Form 4 | Terminals also separated | Highest practical safety for withdrawable units |
Higher forms cost more but allow maintenance without shutting the whole board, valuable in continuous-process plants.
Architecture choices
- Single bus with one incomer: simple, a single point of failure.
- Bus section with bus coupler: two incomers, either can feed both sides.
- Separate generator bus: critical loads on an emergency board.
- Dual-fed boards with automatic transfer for critical loads.
Ratings to verify
- Rated current of busbar, including ambient and enclosure derating.
- Short-circuit withstand and making capacity.
- Ingress protection suited to the room.
- Internal arc classification where specified.
Space and future growth
Allow spare feeders and space to extend the board. Retrofitting a fully packed switchboard is expensive and often needs a shutdown.
Illustrative example
A plant puts its fire pumps, lifts and controls on an emergency board fed from the generator through an ATS, while production loads on the main board are shed during generator operation. This keeps the generator within its capacity and the critical services protected.
Common mistakes
- Choosing a busbar rating from the incomer alone.
- No spare space for growth.
- Under-sized room for cooling and access.
What to send us
Send the load list and single-line diagram. Send your application details.
Questions engineers ask
What is the difference between PCC and MCC?
A PCC distributes power from sources to large feeders. An MCC houses motor starters and controls.
Is withdrawable always better?
It improves maintainability and safety but costs more. Use it where uptime matters.
How much spare capacity should I allow?
Set by growth plans. Typical practice leaves spare spaces and bus capacity for future feeders.
Does the board need a type test?
Assemblies should be supplied with evidence of design verification to the applicable standard.
This guide gives general principles. Specifications should follow the applicable standards.