Cable sizing is the process of selecting the correct conductor type and cross-sectional area for an electrical circuit so it can safely deliver the required current and power under real installation conditions. In EV charging, proper cable sizing is essential because chargers are high, often continuous loads, and incorrect sizing can cause overheating, voltage drop, nuisance trips, and reduced charging reliability.
What Is Cable Sizing?
Cable sizing determines the cable specification needed to meet:
– Current-carrying capacity (ampacity) for the expected load
– Thermal performance under installation conditions (derating)
– Acceptable voltage drop over the cable run
– Short-circuit withstand capability until protection clears the fault
– Mechanical protection and environmental suitability (indoor/outdoor, buried, conduit)
– Compliance with local electrical standards and earthing system requirements
Cable sizing applies to:
– Individual charger branch circuits
– Feeders supplying multiple chargers
– Main distribution routes (including busbar trunking or sub-panels)
Why Cable Sizing Matters in EV Charging
EV chargers can run at high current for hours, which makes thermal design critical. Correct cable sizing helps:
– Prevent overheating and fire risk under continuous charging
– Avoid charger derating caused by low voltage at the terminals
– Reduce nuisance trips by keeping conductor temperature within limits
– Improve charger uptime and site availability rate
– Enable scalable expansion with predictable electrical performance
– Support compliance and easier approvals/inspections
For multi-charger sites, sizing errors can scale into major operational problems.
How Cable Sizing Is Done
A typical cable sizing workflow includes:
– Define the charger load
– Power rating, phase configuration, and maximum continuous current
– Diversity assumptions if feeding multiple chargers with load management
– Choose installation method
– In conduit, tray, trunking, underground, or free-air routes
– Ambient temperature and grouping conditions
– Apply ampacity and derating
– Base cable ampacity from standard tables
– Apply cable derating factors for temperature, grouping, insulation, soil, etc.
– Check voltage drop
– Confirm voltage drop is within acceptable limits for stable charger operation
– Increase cable size if run length is long or load is high
– Check protection and fault performance
– Ensure breaker/fuse sizing matches cable capacity
– Confirm short-circuit protection and coordination
– Verify earthing and RCD requirements for EV charging
– Select cable type and construction
– Copper vs aluminum, insulation type (e.g., XLPE), sheath, UV rating, burial rating
– Mechanical protection and gland/sealing suitability
Key Inputs That Affect Cable Size
– Charger current and duty cycle (continuous load)
– Cable run length and routing complexity
– Ambient temperature and ventilation
– Grouping with other loaded cables
– Installation in insulation or underground soil conditions
– Voltage drop limits and phase balance (three-phase systems)
– Short-circuit levels and protection clearing times
– Future expansion plans (spare capacity vs right-sized for today)
Typical Use Cases
– Sizing a single AC wallbox circuit in a workplace car park
– Designing feeder cables for a row of bollard chargers with long trench runs
– Multi-storey parking garages with long distribution routes and grouping
– Fleet depots where many chargers operate simultaneously under an import cap
– Fast charging hubs where feeder sizing and voltage drop are critical at high power
Key Benefits of Correct Cable Sizing
– Safe, compliant operation under continuous charging
– Stable charging performance and fewer session interruptions
– Lower maintenance costs and reduced cable/terminal failures
– Better scalability and cleaner electrical architecture
– Improved financial outcomes by avoiding rework and downtime
Limitations to Consider
– Cable sizing rules vary by country and standard; local compliance is mandatory
– Real-world site conditions can differ from design assumptions (added circuits later)
– Over-sizing increases CAPEX, while under-sizing increases risk and downtime
– Load management helps, but worst-case scenarios still must be safe
– Poor terminations and installation workmanship can defeat correct sizing
Related Glossary Terms
Cable Derating Factors
Voltage Drop
Branch Circuit
Circuit Breaker
Overcurrent Protection
RCD / GFCI
Protection Coordination
Available Import Capacity
Load Management
Charging Station Installation