Maximum site demand limit is a configured cap on the total electrical demand a site is allowed to draw (in kW, kVA, or A) to prevent overloading the supply, tripping protection devices, or exceeding contracted grid capacity. In EV charging, it is commonly used as a site-wide control target for dynamic load management and managed charging, ensuring the combined load of chargers and other building consumers stays within a safe, agreed boundary.
What Is a Maximum Site Demand Limit?
It is a “do not exceed” threshold applied to the whole site (or a defined supply section). The limit may be defined by:
– Contracted connection capacity with the DSO/utility
– The site’s main fuse rating or incomer rating in the main distribution board
– Thermal limits of incoming cables, busbars, or switchgear
– Tariff-driven targets to reduce demand charges
Unlike a single feeder limit (one charger circuit), a maximum site demand limit controls the sum of all loads connected to that supply.
Why Maximum Site Demand Limit Matters for EV Charging
EV charging adds large, simultaneous loads and can create new peaks. A site demand limit helps:
– Prevent main fuse operation and avoid whole-site outages
– Maintain safe operation of main LV panels and upstream equipment
– Enable more chargers on an existing connection by intelligently sharing capacity
– Keep demand within contractual limits and avoid penalties or expensive upgrades
– Stabilize operations for fleets where charging must be reliable every day
It is a core parameter in multi-charger deployments where “everyone charging at full power” is not feasible.
How It Works in Practice
A typical workflow for EV charging control:
– Measure total site load in real time using an energy meter or CT clamps at the incoming supply
– Compare measured demand against the maximum site demand limit
– Calculate available headroom (limit minus current building load)
– Allocate that headroom across chargers by adjusting their current setpoints
– Apply a safety buffer to avoid limit exceedance during fast load changes
If building demand rises (HVAC, production loads), EV charging power is reduced automatically. If building demand falls, chargers can ramp up again.
Ways to Define the Limit
The limit can be set as:
– Current-based: e.g., 3 × 63 A site limit
– Power-based: e.g., 120 kW maximum import demand
– Contract-based: aligned to utility capacity agreement (often with a margin)
– Cost-based: tuned to keep the site below a demand-charge threshold
For three-phase sites, the limit may also consider phase imbalance constraints.
Site Demand Limit vs Charger Power Limit
Maximum site demand limit (site-wide cap)
– Controls total demand of the whole site supply
– Dynamic: responds to changing building load
Charger power/current limit (per charger cap)
– Controls an individual charger or connector
– May be static (fixed) or dynamic (managed charging rule)
Both are commonly used together: a site-wide cap plus per-charger maximums for fairness and equipment protection.
Key Design and Commissioning Considerations
– Place metering at the correct point (total site import, or the supply section feeding chargers)
– Set an appropriate buffer to account for measurement delay and load spikes
– Confirm communication reliability between meter, controller, and chargers
– Define priority logic (equal sharing, first come first served, fleet priority)
– Ensure fail-safe behavior if communication is lost (e.g., fall back to safe fixed limits)
– Document the configured limit in commissioning and O&M documentation
Common Issues
– Limit set too high, causing nuisance trips at the main incomer or fuses
– Limit set too low, resulting in unnecessarily slow charging and user complaints
– Meter installed in the wrong location, missing part of the building load
– Control lag leading to short exceedances during rapid load changes
– Phase constraints not considered, causing one phase to overload even if total kW looks acceptable
Related Glossary Terms
Maximum demand
Peak demand
Demand charges
Connection capacity
Main fuse rating
Main LV panels
Dynamic load management
Load balancing
Managed charging
Energy meter