A three-phase energy meter with Modbus measures a circuit and publishes its readings in registers that a gateway, a BMS or a SCADA system can read. Hundreds of models fit that description, from DIN-rail submeters to panel-mounted power analysers. Five questions narrow the choice: the supply, the current, the accuracy, the interface and the register map. This guide takes them in order, then covers commissioning.
What a three-phase Modbus meter measures
A typical three-phase meter publishes:
- voltage on each phase, line to neutral and line to line;
- current on each phase, and on some models the neutral current;
- active, reactive and apparent power, per phase and in total;
- power factor and frequency;
- energy counters: import and export active energy (kWh), and often reactive energy (kvarh).
Many meters also publish demand values, harmonic distortion and counters for several tariffs. A long list on a datasheet does not mean every value is in the register map. Check the map for each value you need.
Choose for the supply
Wiring system. A four-wire supply, three phases and neutral (for example 230/400 V), needs three measuring elements. A three-wire supply without neutral needs two. Most meters support both, and some also a single-phase mode, selected by a setting. The meter must match the voltages: check the line-to-neutral and line-to-line ranges on the datasheet, and the supply for the meter's own electronics. The three-phase power guide explains the measuring elements.
Current. A direct-connected meter carries the load current through its terminals, so it suits circuits up to its rating. The three meters in the direct-connected meter comparison are rated 63 A, 65 A and 100 A. Above that, or where the circuit cannot be disconnected long enough to wire through the meter, choose a CT-operated meter. Check which CT output it accepts: 1 A, 5 A, 333 mV or a Rogowski input. The CT meter guide explains the ratio setting, and the split-core CT guide covers fitting CTs to a connected cable.
Generation and storage. On a circuit with PV, a battery or a generator, choose a meter with separate import and export energy registers, and record its sign convention.
Accuracy and approval
IEC 62053-21 sets accuracy classes 0.5, 1 and 2 for active energy meters. IEC 62053-22 sets the tighter classes 0.1S, 0.2S and 0.5S for transformer-operated meters. Class 1 suits most energy monitoring. Class 0.5S suits large feeders, reconciliation against a utility meter and measurement and verification.
For a CT-operated meter, the class applies to the meter alone. The CTs add their own error, which is largest at low current. The CT selection guide shows how to size and class them.
A meter used to bill tenants must also be of an approved design. In the EU, Annex V of the Measuring Instruments Directive (MI-003) defines classes A, B and C and covers the meter only. An accuracy class on a datasheet is not an approval. Ask for the certificate for the exact order code, as the submetering guide explains.
Modbus RTU or Modbus TCP
| Modbus RTU meter | Modbus TCP meter | |
|---|---|---|
| Interface | RS-485 terminals | Ethernet port |
| Wiring | One twisted-pair bus, daisy-chained between meters, terminated at both ends | A network cable to a switch for each meter |
| Settings | Address 1 to 247, baud rate, parity, stop bits | IP address, port 502, unit identifier |
| Who can read it | One master on each bus | Several clients, up to the meter's connection limit |
| Suits | Meters in one panel, or an existing bus | Panels that a managed network reaches |
The register map and function codes are the same on both. The Modbus TCP vs RTU guide compares the frames, and the RS-485 wiring guide covers termination, bias and cable. To let several systems read RTU meters over the network, a Modbus RTU to TCP gateway sits in front of the bus. Some meter families come in both versions: the Eastron SDM630-Modbus has RS-485 and the SDM630-TCP has an Ethernet port.
Read the register map
The register map for the exact model and firmware gives, for each value:
- the function code: 03 for holding registers or 04 for input registers;
- the address, in the manual's notation (30001, 40001 or a zero-based number);
- the data type: 16-bit or 32-bit integer, or 32-bit IEEE 754 float;
- the word order of 32-bit values: which of the two registers comes first;
- the scaling and unit: for example kWh, or Wh with a multiplier.
On the Eastron SDM630, for example, import active energy is a 32-bit float in input registers 30073 and 30074, read with function code 04. The 30001-style reference converts to address 72 in the request:
If the two registers return 4640 E666, with the high word first, the value is 12,345.6 kWh:
A float32 counter loses resolution as it grows: above 100,000 kWh it resolves about 0.01 kWh, and above 10 million kWh about 1 kWh. Where a meter also offers a 32-bit or 64-bit integer energy counter, prefer it for billing and reconciliation. The register map guide covers word order, scaling and the checks for each data type.
Commission the meter
- Set the wiring mode (three-phase four-wire, three-phase three-wire or single-phase) to match the circuit.
- On a CT-operated meter, set the CT primary and secondary, and record them.
- Set the Modbus address, baud rate and parity, or the IP address and unit identifier, from the site's address plan.
- Read one value that the meter displays, such as the L1 voltage, over Modbus. Compare the value and its scaling with the display.
- Check each phase under load: positive kW for an importing load, a plausible power factor, and current within tolerance of a clamp meter.
- Load the rest of the register map, then read the energy counters twice, a known time apart, and check that the difference matches the average power.
The Modbus RS-485 commissioning guide gives the complete sequence and the records to keep, and the Modbus troubleshooting guide covers timeouts and exception codes.
Reading three-phase Modbus meters with Edge
Edge on the ZGW-20 Gateway reads Modbus TCP meters on the site network and Modbus RTU meters on a serial port, or through a ZMB Modbus interface mounted next to the meter, which sends the registers over the Zigbee mesh so no data cable runs back to the Gateway. The device pages for the SDM630-Modbus, SDM630MCT, Carlo Gavazzi EM24 and Siemens SENTRON PAC2200 each carry a read-only starter template with the register addresses, data types and word order already set.
To try this with one meter before you buy a Gateway, a Raspberry Pi set up with System Manager runs Edge with one Modbus device of up to 32 registers licensed free, with every dashboard, analysis and alerting tool. The Pi runs from a microSD card, so use it to evaluate, not for a business-critical site. See evaluate Edge on a Raspberry Pi.
Common questions
What is a Modbus energy meter?
An electricity meter with a Modbus interface, usually RS-485 for Modbus RTU or Ethernet for Modbus TCP. It measures voltage, current, power and energy, and publishes each value in registers that a gateway, a BMS or a SCADA system can read.
Do I need a three-phase meter on a three-phase supply?
To measure the whole circuit, yes. A three-phase meter measures the current and voltage of every phase and adds the three phase powers, so it stays correct when the load is unbalanced. A single-phase meter on one phase of a three-phase circuit measures that phase only.
Should I choose a Modbus RTU or a Modbus TCP meter?
Modbus RTU suits meters in one or two panels, or meters added to an existing RS-485 bus. Modbus TCP suits panels that a managed network already reaches, and sites where several systems must read the same meter. The register maps and function codes are the same; only the transport differs.
Which register holds total kWh?
It depends on the model, so use the register map for the exact meter and firmware. On the Eastron SDM630, for example, import active energy is a 32-bit float in input registers 30073 and 30074, which is address 72 in the request.