A sub-distribution board (SDB) is an electrical distribution panel that receives power from a main distribution point (such as a main LV panel or main switchboard) and then distributes that power to downstream circuits. In EV charging projects, an SDB is often installed closer to the chargers (for example in a car park, basement, or service area) to supply multiple charge points efficiently and to simplify protection, metering, and maintenance.
An SDB is sometimes called a sub-board, secondary distribution board, or local distribution panel, depending on local terminology.
Why SDBs Matter in EV Charging Installations
SDBs are a common building block in scalable EV charging infrastructure because they:
– Reduce cable run lengths by placing distribution nearer to charger groups
– Provide a structured way to add more circuits as the site expands
– Allow better protection coordination for multiple chargers
– Support staged rollouts and phased commissioning
– Simplify maintenance access and fault isolation
For larger sites, using an SDB can be more practical than running every charger circuit back to the main LV panel.
How an SDB Is Used in a Typical EV Charging Architecture
A common arrangement looks like:
– The main LV panel supplies a feeder circuit to the SDB
– The SDB contains protective devices for each charger circuit
– Each charger is supplied via its own outgoing circuit (or a grouped feeder where applicable)
– Optional metering and monitoring are integrated at feeder or circuit level
– The SDB may include a local controller for load management
This architecture is widely used in workplaces, depots, multi-tenant parking, and public destination sites.
What an SDB Typically Contains
Depending on site requirements and local standards, an SDB may include:
– Main isolator and incoming protection
– Overcurrent protection devices (OCPD) for outgoing circuits
– RCDs or EV-specific fault protection where required
– Surge protection devices (SPD), especially for outdoor or long cable runs
– Circuit labeling and lockable isolation for maintenance safety
– Optional energy metering (including MID metering where relevant)
– Space for future breakers to support expansion
Benefits for Scalability and Cost
SDBs are often chosen to improve future readiness:
– Easier expansion by adding breakers rather than reworking the main switchboard
– Better support for spare conduit capacity and phased charger rollout
– Lower installation cost when long home-run cabling is avoided
– Cleaner troubleshooting because faults can be isolated per charger circuit
Design Considerations and Common Pitfalls
Key design points for EV charging SDBs include:
– Correct feeder sizing based on diversity and maximum demand assumptions
– Protection coordination so a single fault doesn’t trip the whole SDB unnecessarily
– Thermal management and enclosure rating (especially in outdoor car parks)
– IP/IK requirements for public or semi-public environments
– Space for communication gateways, CT clamps, and control wiring if using load balancing
– Clear documentation and labeling to support commissioning and service
Relationship to Load Management
SDBs often act as the physical hub for site-level power control:
– Measure site load using CT clamps on the feeder
– Enforce a maximum site demand limit
– Allocate power dynamically across multiple chargers via load balancing
– Support staged electrification without immediate grid upgrades
Related Glossary Terms
Main LV Panels
Distribution Board
Feeder Circuit
Overcurrent Protection Device (OCPD)
Residual Current Device (RCD)
Load Management
Load Balancing
Maximum Site Demand Limit
MID Metering