Energy Metering with KNX: Eastron SDM630, Carlo Gavazzi EM340 and MID Certification
Sub-metering individual circuits and end-use categories via RS-485 Modbus RTU gives the KNX system granular energy data for BREEAM reporting, tenant billing and EN ISO 50001 energy management — provided the meters are MID-certified where legal billing is required.
Why sub-metering is mandatory
A whole-building meter at the supply incomer tells you total consumption but cannot identify which end-use drives energy cost or carbon. Regulatory drivers and certification schemes increasingly mandate circuit-level sub-metering, making it a design requirement rather than an optional extra.
Regulatory requirements
- EU Energy Efficiency Directive EED 2023: sub-metering obligations for multi-tenant commercial buildings — heating, cooling and domestic hot water per tenant unit
- EN ISO 50001: energy management system standard requires metering of significant energy uses (SEUs) — typically HVAC, lighting and process loads
- UK MEES 2023: landlord compliance evidence for non-domestic EPCs increasingly relies on sub-meter data to demonstrate improvement
BREEAM sub-metering credits
- Hea 05 (formerly Ene 02): mandatory sub-metering credit — separate metering of each end-use category (lighting, small power, HVAC, catering, IT) for buildings >1,000 m²
- Ene 09 (renewable energy): PV yield sub-metering required to demonstrate renewable contribution to total demand
- Man 04 (commissioning): M&V plan requiring sub-meter data trending for BREEAM Outstanding
MID certification: when it is legally required
The Measuring Instruments Directive 2014/32/EU (MID) establishes mandatory conformity requirements for meters used for legal-for-trade billing between parties. If sub-meter data is used to apportion energy costs between tenants or for invoicing purposes, the meters must carry MID approval — non-MID meters are valid only for monitoring.
MID accuracy classes for electricity meters
MID Annex V — Active electrical energy meters:
Class A: ±2% accuracy (basic residential, rarely used for sub-metering)
Class B: ±1% accuracy (minimum for commercial billing sub-metering)
Class C: ±0.5% accuracy (high-precision billing, revenue metering)
Eastron SDM630 MCT: MID Class B approved
→ suitable for commercial tenant billing sub-metering
Carlo Gavazzi EM340 MID: MID Class 1 (= Class B equivalent)
→ DIN-rail, European certification, strong local support
Non-MID meters (e.g. Eastron SDM120, SDM72D standard):
→ monitoring only — cannot be used for legal billing
→ acceptable for BREEAM energy monitoring (not billing)
→ acceptable for KNX self-consumption trackingBilling without MID approval is a legal liability: using a non-MID meter to apportion energy costs between tenants exposes the building owner to challenge under consumer protection law. Confirm MID approval on the meter nameplate — look for the CE mark followed by M and a two-digit year (e.g. M22 for 2022 approval). Both SDM630 MCT and EM340 carry this marking.
Eastron SDM630 Modbus RTU: wiring and registers
The Eastron SDM630 is a 3-phase, 4-wire DIN-rail energy meter with a dedicated RS-485 Modbus RTU port. The MCT variant (current transformer input) supports circuits above the meter's 100A direct connection limit. Communications parameters are fixed at 9600 baud, 8 data bits, no parity, 1 stop bit (8N1).
| Register (hex) | Description | Type | Unit |
|---|---|---|---|
| 0x0000 | L1 Voltage | float32 big-endian | V |
| 0x0006 | L1 Current | float32 big-endian | A |
| 0x000C | L1 Active Power | float32 big-endian | W |
| 0x0034 | Total Active Power (3-phase) | float32 big-endian | W |
| 0x0046 | Total kWh import (accumulated) | float32 big-endian | kWh |
| 0x004E | Total kWh export (accumulated) | float32 big-endian | kWh |
| 0x0156 | L1 Power Factor | float32 big-endian | — |
| 0x0160 | Total Power Factor | float32 big-endian | — |
RS-485 bus wiring and addressing
Wiring: RS-485 A (D+) and B (D−) from SDM630 Modbus port → daisy-chain to next meter (not star topology) → terminate with 120Ω resistor at far end of bus → maximum 31 SDM630 per RS-485 bus segment Modbus device address: set via front-panel DIP switches (S3) → Addresses 1–31 (avoid address 0 = broadcast) → Each meter on the same bus must have a unique address Connect bus to gateway: → MDT KNX/Modbus (SCN-MODBUS.01): RS-485 port A/B terminals → WAGO 750-362: RS-485 fieldbus coupler with KNX gateway → Maximum bus cable length: 1200m at 9600 baud (use shielded twisted pair)
Carlo Gavazzi EM340: wiring and registers
The Carlo Gavazzi EM340 is an alternative 3-phase MID meter with identical RS-485 Modbus RTU interface and DIN-rail mounting. The EM340-DIN.AV23MID5A3X variant supports up to 63A direct connection without CTs, simplifying installation for sub-circuits below 63A.
| Register (hex) | Description | Unit |
|---|---|---|
| 0x0001 | Total kWh import (accumulated) | kWh |
| 0x0003 | Total kWh export (accumulated) | kWh |
| 0x0013 | L1 Active Power | W |
| 0x001B | Total Active Power (3-phase) | W |
| 0x001D | Total Apparent Power | VA |
| 0x001F | Total Reactive Power | VAr |
| 0x002B | L1 Voltage | V |
| 0x0033 | L1 Current | A |
EM340 vs SDM630 selection: both meters are MID-approved and use identical RS-485 Modbus RTU integration. Choose SDM630 for cost-sensitive monitoring installations. Choose EM340 where Carlo Gavazzi local distributor support is preferred, or where MID Class 1 accuracy is specified in the contract. The EM340 63A direct-connection variant eliminates CT installation cost for sub-circuits up to 63A.
CT selection for high-current circuits
For circuits above 63A (main incomer, large HVAC, distribution boards), current transformers (CTs) step down current to a level the meter can measure. Accuracy class of the CT determines whether the combined meter + CT system meets MID billing requirements.
CT specification and wiring
CT accuracy class: Class 0.5: required for MID billing sub-metering Class 1.0: acceptable for non-billing energy monitoring Class 3.0: basic monitoring only, insufficient for MID Split-core CTs (Eastron SCT-013-100, 100A / 33mV output): → clip around existing conductors — no disconnection required → suitable for retrofits where shutdown is not possible → mV output connects to SDM72D-M (voltage input CT meter) Solid-core CTs (Chauvin Arnoux C series, 100A/5A output): → higher accuracy, better for MID Class 0.5 applications → installed at initial wiring stage (conductor must pass through) → 5A output → SDM630 MCT (CT/5A input version) CT wiring: Secondary leads S1/S2 in pairs (twisted) Polarity: P1 mark (K) facing supply side of conductor Short-circuit CT secondary before disconnecting from meter — open-circuit CT secondary at load produces dangerous HV
KNX group address mapping via MDT gateway
The MDT KNX/Modbus gateway (SCN-MODBUS.01) connects the RS-485 Modbus RTU bus to the KNX TP bus, publishing meter register values as KNX telegrams on group addresses. The following signal table covers a typical floor sub-metering installation.
MDT SCN-MODBUS.01 signal table — floor sub-metering
SDM630 address 1 (Floor 1 total):
Register 0x0034 (Total Power, W)
→ DPT 13.010 (4-byte signed W)
→ GA 9/3/0 — Floor 1 Total Power W
→ Poll interval: 60 seconds
Register 0x0046 (kWh import, accumulated)
→ DPT 12.001 (4-byte unsigned, ×1000 = Wh)
→ GA 9/3/1 — Floor 1 kWh Import
→ Poll interval: 60 seconds
SDM630 address 2 (Floor 1 Lighting circuit):
Register 0x0034 → GA 9/3/2 — Lighting Power W
Register 0x0046 → GA 9/3/3 — Lighting kWh
SDM630 address 3 (Floor 1 HVAC):
Register 0x0034 → GA 9/3/4 — HVAC Power W
Register 0x0046 → GA 9/3/5 — HVAC kWh
Note: DPT 12.001 is uint32 (0–4,294,967,295 Wh = ~4.3 GWh)
— sufficient for any building-level accumulation without rolloverPoll interval selection: 60 seconds is appropriate for energy accumulation registers (kWh). Power registers (W) may be polled at 10–30 seconds for demand trending. Polling too fast on a large RS-485 bus (30+ meters) can cause response timeouts — calculate bus cycle time as number of signals × poll interval to ensure each meter is polled within the required interval.
BREEAM Hea 05 multi-circuit sub-metering architecture
BREEAM requires separate metering of each end-use category across each floor. For a multi-floor commercial building, this means multiple RS-485 bus segments — one per floor — each carrying meters for the five BREEAM end-use categories.
BREEAM Hea 05 metering architecture
Five mandatory end-use categories (buildings > 1,000m²): 1. Lighting → separate MCB group, separate SDM630 2. Small power → office equipment, desks, sockets 3. HVAC → fan coils, pumps, AHU, refrigeration 4. IT equipment → server room, comms rooms 5. Catering → kitchen, dishwasher, vending Per floor: → 1× RS-485 bus segment → 5× SDM630 (one per end-use, addresses 1–5) → 1× MDT SCN-MODBUS.01 gateway per floor → Gateway connects to KNX TP line for that floor Building total: KNX IP-Symcon or ARISTO BewO: → Sum GA values per end-use across all floors → Daily trending at 1-minute resolution → CSV export for BREEAM assessor at project completion BREEAM documentation required: → Meter schedule showing meter ID, end-use, floor, MID approval → Wiring diagram showing CT position and polarity → 30-day trending data export from commissioning period
KNX energy visualisation
Sub-meter data published to KNX group addresses is available to any KNX-capable visualisation platform. The choice of platform determines the quality of trending, reporting and export capability.
Gira X1 / Homeserver
- Daily kWh bar chart per circuit
- Self-consumption pie (PV vs grid)
- Configurable KPI widgets on dashboards
- Historical data to internal storage
IP-Symcon
- KNX energy plugin with 1-minute resolution
- Unlimited trend logging to SQL database
- Automated CSV export by schedule
- Multi-site aggregation via WebHook
ARISTO BewO
- Daily script: read GA at 23:59, log to database
- Weekly energy report generation (PDF)
- ISO 50001 energy register compatible
- Historical comparison (year-on-year)
Commissioning and verification
Verifying sub-meter accuracy during commissioning is essential — especially before the installation is used for billing. The three-step verification confirms Modbus communication, CT direction and meter accuracy against a reference.
Sub-meter commissioning verification sequence
Step 1 — Modbus communications check: Use Modbus Poll PC software or MDT gateway diagnostic Read holding register 0x0034 (Total Power) from each meter Confirm response received within 500ms timeout Confirm device address matches meter DIP switch setting Step 2 — CT direction (polarity) check: Apply a known resistive load on the metered circuit (electric panel heater, known wattage, power factor = 1) Read GA for power in ETS6 Group Monitor Positive value = correct CT polarity (consumption direction) Negative value = swap CT secondary leads S1 and S2 Step 3 — Accuracy verification (MID meters): Connect calibrated reference power meter in parallel Apply load for 30-minute test period Compare accumulated kWh: SDM630 vs reference Acceptable tolerance: ± 1% for Class B MID meter Record results in commissioning documentation for BREEAM
Float32 register verification: compare KNX-logged energy totals against the meter front-panel accumulated kWh display. Values should agree within 1% — any larger discrepancy indicates a byte-order mismatch (swap between big-endian and little-endian in the gateway signal configuration) or incorrect register address. SDM630 uses float32 big-endian for all registers; configure the gateway Modbus data type accordingly.
Need MID-certified sub-metering panels designed for BREEAM compliance?
We design and build sub-metering panels with Eastron SDM630 MCT, Carlo Gavazzi EM340, RS-485 Modbus bus, MDT KNX gateway and full BREEAM Hea 05 meter schedule documentation — delivered commissioned to your site.
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