Spare duct capacity is the amount of unused ducting space intentionally built into an EV charging installation so additional power cables and data/communication cables can be installed later without new trenching or major civil works. In many projects, “duct” and “conduit” are used interchangeably, but ducting often refers to underground or multi-way duct banks used to serve multiple charge points.
Spare duct capacity can mean:
– Installing extra empty ducts during the first phase
– Installing a larger duct than currently needed
– Installing a multi-duct system with unused ways reserved for expansion
Why Spare Duct Capacity Matters in EV Charging Projects
EV charging sites frequently scale over time—especially in workplaces, apartment buildings, and public car parks. Spare duct capacity helps avoid the most expensive part of future expansion: groundworks.
It supports:
– Adding more AC EV chargers as demand grows
– Extending infrastructure to additional parking rows or bays
– Future upgrades to metering, monitoring, and access control
– Expanding connectivity for OCPP networks or site monitoring
– Phased fleet electrification where the number of charging bays increases over time
Where Spare Duct Capacity Is Commonly Used
Spare duct capacity is most valuable where retrofits are disruptive or costly:
– Underground car parks and multi-storey residential developments
– Public realm and municipal charging corridors
– Retail and hospitality sites with finished paving and landscaping
– Fleet depots with staged build-outs
– Charging hubs that may add additional pedestals or islands
How Spare Duct Capacity Is Planned
Planning typically starts from an expansion assumption (for example, “double the number of bays in phase 2”) and then reserves ducting accordingly:
– Provide a main duct route (“spine”) sized for future feeder additions
– Provide spare branches to future charger positions
– Include pull points or pull pits to keep future pulls practical
– Separate power and comms ducting where appropriate
– Keep ducts accessible at key interfaces (LV room, risers, foundations)
Key Engineering Considerations
Spare duct capacity must remain usable across the site lifecycle:
– Duct fill and pulling limits must support future cable sizes
– Bend radius and route geometry should avoid pull failures
– Ducts must be sealed to prevent water, debris, and silt build-up
– Mechanical protection is essential in trafficked areas
– Clear identification prevents future crews from cutting or blocking spare ways
Benefits for Property Owners, Installers, and CPOs
– Lower total cost of ownership by reducing future civil works
– Faster addition of new charge points with minimal disruption
– Improved flexibility for load management upgrades and monitoring
– Better ROI from phased deployment rather than overbuilding chargers upfront
– Reduced downtime in operational sites (tenants, customers, fleets)
Common Pitfalls
– Installing spare ducts but not bringing them to accessible termination points
– Routes that are too long or too bend-heavy without pull pits
– Unlabeled ducts that become “lost” in later phases
– Poor sealing leading to blocked ducts and unusable spare capacity
Relationship to Load Management and Site Scaling
Spare duct capacity supports scaling strategies where electrical capacity is managed rather than immediately upgraded:
– Add chargers while controlling demand using load balancing
– Expand metering and reporting as billing needs grow (including MID metering)
– Improve networking for remote management, uptime, and diagnostics via OCPP
Even if the grid connection is initially limited, spare ducting keeps expansion physically feasible when capacity or budgets increase.
Related Glossary Terms
Spare Conduit Capacity
Spare Conduit Routing
EV-ready Parking
Load Management
Load Balancing
Pull Pits
Main LV Panels
OCPP
MID Metering