Dynamic Load Management for Multiple EV Chargers with KNX: Demand Monitoring and Current Distribution
Installing multiple EV chargers in a building without dynamic load management risks tripping the main supply breaker during simultaneous charging peaks. KNX with an ABB B24 Modbus energy meter continuously monitors total building demand and distributes available current proportionally across chargers — preventing overload while maximising charging throughput within the contractual grid connection limit.
The multi-charger overload challenge
A typical residential or small commercial building with a 3-phase 80A main supply and a 20 kW base building load leaves approximately 35 kW (50A 3-phase) for EV charging. Three 22 kW chargers running simultaneously at maximum current would demand 66 kW — nearly double the available capacity. Without dynamic load management, simultaneous charging triggers the main breaker, potentially causing a building-wide supply interruption.
| Scenario | Building load | Available for EV | Per charger (3 active) |
|---|---|---|---|
| Main limit: 80A (55 kW) | Base load 20 kW (29A) | 35 kW (50A 3-phase) | ~16A per charger |
| Main limit: 80A (55 kW) | Base load 30 kW (43A) | 25 kW (36A 3-phase) | ~12A per charger |
| Main limit: 80A (55 kW) | Base load 40 kW (58A) | 15 kW (22A 3-phase) | ~7A per charger |
| Main limit: 80A (55 kW) | Base load 50 kW (72A) | 5 kW (7A 3-phase) | Below 6A minimum → shed Charger 3 |
ABB B24 Modbus TCP energy meter as demand sensor
The ABB B24 is a compact DIN-rail 3-phase energy meter with Modbus TCP (via ABB B24 Ethernet communication module) providing real-time per-phase current, total power, and energy data. It installs on the main supply incomer inside the distribution panel and gives KNX the total building current consumption on all three phases via Modbus registers polled by the Intesis IN701KNX gateway.
ABB B24 Modbus TCP register map for DLM
ABB B24 Modbus TCP (port 502, unit ID 1): Register 0x0009 → Total active power (int32, 0.01W resolution) Register 0x000B → L1 current (uint16, 0.01A resolution) Register 0x000C → L2 current (uint16, 0.01A resolution) Register 0x000D → L3 current (uint16, 0.01A resolution) Register 0x0003 → L1 voltage (uint16, 0.1V) Register 0x0005 → L2 voltage (uint16, 0.1V) Register 0x0007 → L3 voltage (uint16, 0.1V) Key DLM registers: Total power → GA 11/0/0 DPT 13.010 (W, signed) L1 current → GA 11/0/1 DPT 9.024 (A, 2-byte float) L2 current → GA 11/0/2 DPT 9.024 L3 current → GA 11/0/3 DPT 9.024 Intesis IN701KNX MAPS setup: Protocol: Modbus TCP Master Remote IP: <ABB B24 Ethernet module static IP> Remote port: 502 Poll interval: 5 seconds (faster poll for DLM responsiveness) Byte order: Big-endian
KNX current allocation algorithm
The current allocation algorithm calculates available amperes for EV charging by subtracting total building current from the contractual grid limit, then divides the remainder across the number of actively charging vehicles. The result is written to each charger's OCPP current setpoint via the KNX-to-OCPP bridge.
Current allocation algorithm (KNX Logic Module or HA script)
Variables:
main_limit_A = 80 # Contractual grid limit (3-phase A)
building_current_A = GA 11/0/0 (total power) ÷ (3 × 230V)
num_charging_vehicles = count of GA 10/x/2 (plug connected = 1)
safety_margin_A = 5 # Buffer below hard limit
Algorithm (runs every 10 seconds):
available_A = main_limit_A − building_current_A − safety_margin_A
IF available_A > 0 AND num_charging_vehicles > 0:
per_charger_A = available_A ÷ num_charging_vehicles
per_charger_A = max(6, min(32, round(per_charger_A))
→ Write per_charger_A to each active charger GA (10/0/0, 10/2/0, 10/3/0)
IF available_A ≤ 0:
→ All charger current → 6A (minimum: keep session alive)
IF building_current_A > (main_limit_A − safety_margin_A − 6A × num_vehicles):
→ Priority shedding (see priority table below)Three-charger priority scheme
In a three-charger installation with mixed priority users (resident, guest, visitor), the KNX logic assigns current priority before proportional sharing, ensuring the highest priority charger always receives at least minimum current before the remaining capacity is distributed to lower-priority chargers.
Three-charger priority current distribution
Charger priority assignment: Charger 1 (GA 10/0/0): Resident 1 — Priority HIGH (always gets min 6A) Charger 2 (GA 10/2/0): Resident 2 — Priority MEDIUM Charger 3 (GA 10/3/0): Visitor — Priority LOW (shed first) Available capacity: 50A total (main 80A − 30A building base) Scenario A — All 3 charging, 50A available: Charger 1: 50A ÷ 3 = 16A (capped to Easee Home max 16A) Charger 2: 16A Charger 3: 50A − 16A − 16A = 18A (capped to charger max) Scenario B — 30A available (building load increased to 50A): Charger 1: 30A ÷ 3 = 10A Charger 2: 10A Charger 3: 10A Scenario C — 15A available (high building load): Charger 1: 6A (guaranteed minimum for priority HIGH) Charger 2: 6A (minimum for priority MEDIUM) Charger 3: 15A − 12A = 3A → below minimum → suspend Charger 3 Charger 3 GA 10/3/1 (charger enable) → write 0 (suspend) Scenario D — 10A available (overload approaching): Charger 1: 6A (guaranteed minimum) Charger 2: 4A → below minimum → suspend Charger 2 Charger 3: already suspended Alert: KNX GA 11/0/10 (overload warning) → write 1 → alarm
Overload prevention: shed EV current before HVAC
EV charging is the first load to shed during a demand spike because HVAC setback degrades comfort immediately, while an EV charge session pause is transparent to the vehicle occupant (the session resumes automatically when demand drops). The shedding sequence defines the exact order and thresholds.
| Building current | Action | Hysteresis |
|---|---|---|
| > 65A (81%) | Reduce all EV chargers proportionally toward minimum | 30s |
| > 72A (90%) | Suspend Charger 3 (lowest priority visitor charger) | 60s |
| > 76A (95%) | Reduce Charger 2 to 6A minimum | 60s |
| > 79A (99%) | Suspend Charger 2 — only Charger 1 at 6A minimum | 30s |
| > 80A (100%) | Emergency: HVAC setback −2°C + suspend all EV chargers | Immediate |
HVAC setback as last resort: the HVAC setback at 100% supply is an emergency measure only. Under normal DLM operation, EV charging reduction should prevent building current from ever reaching 80A. If the 80A threshold is consistently hit despite DLM, the contractual supply limit should be reviewed with the DNO — a higher tariff connection tier is typically more economical than ongoing HVAC comfort degradation.
Easee Equalizer: hardware DLM as complement
The Easee Equalizer is a DIN-rail current sensing module that clips onto the main supply cables inside the distribution board. It communicates directly with Easee chargers over the Easee network (Wi-Fi or Zigbee) and automatically limits combined charger current to prevent supply overload — without requiring any KNX integration.
Easee Equalizer advantages
- Self-contained hardware DLM — no KNX or server required
- Measures all 3 phases with internal CTs (clip-on, no wiring)
- Response time: <1 second (faster than KNX logic polling)
- Automatic Easee charger discovery and current distribution
- Easee app shows live current allocation per charger
Easee Equalizer limitations
- Only works with Easee chargers (not Wallbox, ABB)
- No awareness of PV surplus — supply protection only
- No KNX group address for DLM state monitoring
- Cloud-dependent for configuration (local LAN operation only)
- Cannot implement priority-based shedding between users
Combined KNX DLM + Easee Equalizer deployment
Recommended architecture for Easee-only installations: Layer 1 — Easee Equalizer (hardware, <1s response): Prevents hard overload at main breaker in real-time Automatic current sharing between all Easee chargers No configuration required beyond Easee app setup Layer 2 — KNX DLM via ABB B24 + EVCC (software, 10s poll): Reads ABB B24 total building power GA 11/0/0 Prioritises PV surplus allocation to EV chargers Implements user priority scheme (resident vs visitor) Provides KNX monitoring GAs for building management Interaction: Equalizer handles fast hardware overload protection KNX DLM handles slower strategic current optimisation If KNX server goes offline, Equalizer continues protecting supply If Equalizer cable falls off, KNX DLM still limits via OCPP
Commissioning and testing with simulated overload
DLM commissioning requires verifying that KNX logic correctly reduces charger current when building demand increases, without waiting for a real peak demand event. Simulated overload testing uses large resistive loads (electric heaters) to drive building current toward the main limit and verify the KNX logic response at each threshold.
Simulated overload commissioning test
Prerequisites: All 3 EV chargers connected with test EVs or resistive load plugs All chargers at maximum current (16A or 32A per charger) ETS6 Group Monitor open, monitoring GA 11/0/0–11/0/3 Test sequence: Step 1 — Baseline recording: Record GA 11/0/0 (total power) with only EV chargers active Confirm per-charger GA values match expected current allocation Step 2 — Simulate 65A total current: Switch on portable electric heaters (3kW each) across all phases until GA 11/0/0 indicates ~65A total building current Expected: KNX DLM reduces all charger GAs proportionally Record new charger current values from KNX GAs Step 3 — Simulate 72A total current: Add additional heaters until 72A reached Expected: Charger 3 (visitor, lowest priority) suspended GA 10/3/1 (charger 3 enable) → KNX writes 0 Verify Charger 3 app shows "Suspended by DLM" Step 4 — Simulate 76A total current: Add more load until 76A reached Expected: Charger 2 reduced to 6A minimum GA 10/2/0 (charger 2 current) → KNX writes 6A Step 5 — Recover test: Remove heaters one by one Verify: as building current drops below each threshold, suspended chargers resume and current increases back to proportional allocation within 30–60s (hysteresis) Step 6 — Easee Equalizer parallel verification: With KNX DLM active, add load until 80A threshold Verify Equalizer also responds (Easee app shows equalizer active) Confirm both layers protect independently
Documentation for handover: record all threshold crossing times, building current values, and charger current responses during the simulated overload test. This test record serves as the DLM commissioning certificate and is required evidence for PAS 1899:2022 compliance and BREEAM Tra 03 active DLM credit.
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