Feeder capacity is the maximum electrical load (kW / kVA and current in A) that an electrical feeder—the cable or busbar system supplying a distribution board, charger cluster, or sub-panel—can safely carry under defined installation conditions. In EV charging projects, feeder capacity is a key constraint that determines how many chargers can be installed, how much power they can deliver, and whether load management or upgrades are required.
What Is a Feeder?
A feeder is a circuit that carries power from one part of the electrical system to another, typically:
– From the main switchboard to a sub-distribution board
– From a distribution board to a group of EV chargers
– From a transformer or main incomer to a building’s LV distribution
Feeders are usually designed for higher currents than final circuits and often serve multiple downstream loads.
Why Feeder Capacity Matters for EV Charging
– Sets the practical limit for total charging power available to a charger group
– Prevents overheating and insulation damage in cables and terminations
– Influences voltage drop, charging stability, and fault performance
– Determines whether a site can scale charging without civil/electrical rework
– Helps manage peak demand and avoid demand charge spikes
– Enables proper sizing of switchboards, protective devices, and cable routes
What Determines Feeder Capacity
Feeder capacity is not one fixed number; it depends on design and installation factors.
– Cable size (cross-sectional area), conductor material (copper/aluminum)
– Installation method (in conduit, tray, buried, in insulation, grouped with other cables)
– Ambient temperature and thermal environment
– Cable length and allowable voltage drop
– Protective device rating and coordination (breaker/fuse settings)
– Duty cycle and diversity factor (continuous vs intermittent load behavior)
– Connection and termination quality (hot spots often occur at terminations)
– Power factor and harmonic content (relevant for some load types)
Feeder Capacity vs Site Import Capacity
Feeder capacity is internal to the site and can be more limiting than the grid connection.
– Import capacity: what the grid connection allows at the point of connection
– Feeder capacity: what the site’s internal distribution can carry to a charger area
A site may have enough import capacity but still need feeder upgrades to expand charging bays.
How Feeder Capacity Is Used in EV Charging Design
– Decide how many chargers a feeder can serve at full power
– Decide whether chargers must be grouped by sub-panels and multiple feeders
– Define the maximum site or zone power limit used by dynamic load management
– Evaluate whether the feeder needs upsizing during expansion planning
– Validate future phases for EV-ready parking and scalable architecture
Managing Limited Feeder Capacity
If feeder capacity is limited, common strategies include:
– Dynamic load balancing to cap total power within feeder limits
– Staged expansion: add chargers but limit per-charger power until feeder upgrades occur
– Add additional feeders or sub-distribution boards to split load
– Improve diversity assumptions using measured utilization data (not all chargers draw peak simultaneously)
– Use scheduling and “energy by departure” logic for fleets to reduce coincident peaks
– Upgrade cable size, installation method, or routing to increase ampacity (site-dependent)
Common Mistakes to Avoid
– Assuming feeder capacity equals breaker rating without considering installation conditions
– Ignoring voltage drop on long runs, leading to unstable charging or nuisance trips
– Underestimating continuous-load behavior (EV charging can be long-duration)
– Poor termination practices causing hot spots and derating the feeder in reality
– Not reserving spare capacity or conduit size for future expansion
– Failing to align feeder limits with load management settings in the CPMS/EMS
Limitations to Consider
– Feeder capacity can change with temperature, grouping, and future cable additions in the same pathway
– Real-world usable capacity may be lower due to aging, poor ventilation, or installation constraints
– Increasing feeder capacity may require permits, shutdown windows, and major civil work
– Harmonics and power quality issues can influence thermal loading in some scenarios
– Accurate assessment requires proper design calculations and, for existing sites, a site survey and as-built verification
Related Glossary Terms
Distribution Board (DB)
Switchboard
Cable Sizing
Voltage Drop
Load Management
Dynamic Load Balancing
Diversity Factor
Demand Charges