Sub-metering with Modbus power meters: choosing the meter and CTs

How to choose single-phase, three-phase or multi-circuit Modbus power meters, size the current transformers and pick Modbus RTU or TCP.

Publicerad 28 oktober 2026

Sub-metering with Modbus power meters: choosing the meter and CTs — Foto: taner ardalı · CC0 · Wikimedia Commons

Sub-metering means putting a meter on each circuit you want to account for: a production line, a tenant, a chiller, a floor. Most problems come from current transformers (CTs) that do not suit the current or the conductor, and from the link that brings the readings to a controller or SCADA system. This article covers the choices for sub-metering with Modbus power meters: meter type, CT rating and inner diameter, accuracy, wiring, Modbus RTU or Modbus TCP, and which ICP DAS PM series fit.

Single-phase, three-phase or multi-circuit

A list of circuits, their supply type and maximum current decides the meter type.

Single-phase: the PM-3112 measures two single-phase loops and the PM-3114 four, in 1P2W wiring with voltage inputs up to 300 V and CTs up to 200 A.
Three-phase: the PM-3133 series handles 3P4W, 3P3W, 1P3W and 1P2W wiring at 10 to 500 V. In 1P2W-1CT mode one meter can also measure three single-phase circuits.
Many circuits in one board: the PM-4324 series measures up to 8 three-phase or 24 single-phase circuits, or a mix, through 24 CT inputs. It is intended for multi-tenant buildings, offices and shopping centres.

Sizing current transformers: rating and inner diameter

The CT rating should cover the highest normal operating current of the circuit. The inner diameter must fit around the conductor including its insulation, or around the busbar. Place one CT around each phase conductor, never around a complete multi-core cable: the currents in the cores cancel out and the meter reads close to zero.

The PM-3133, PM-2133D, PM-5133 and PM-4324 series are supplied with matched CTs, and the order code suffix shows which. Not every suffix exists in every series.

Suffix	CT rating	Inner diameter	Measuring range
-005P	5 A	10 mm	0.05 to 5 A
-100P	60 A	10 mm	0.05 to 60 A
-160P	100 A	16 mm	0.1 to 100 A
-240P	200 A	24 mm	0.15 to 200 A
-360P	300 A	36 mm	0.3 to 300 A
-400P	400 A	36 mm	0.3 to 400 A

These CTs have 4 m leads. For large conductors, the PM-3133-RCT series uses flexible Rogowski coils: 500 A (55 mm), 1000 A (80 mm), 2000 A (105 mm) and 4000 A (185 mm), each measuring from 5 A.

Reusing existing CTs

Where the switchboard already has CTs with a 1 A or 5 A secondary, the PM-3033 accepts them directly; note their ratio, for example 400/5 A, as it is needed to scale the readings. For 333 mV CTs, use the PM-3133P, PM-2133DP or PM-4324P, which come without CTs and do not support Rogowski coils. The matching CA-SCTxxP split-core CTs cover 5 to 400 A with inner diameters of 10 to 36 mm; the CT ratio must be set correctly in the meter.

Accuracy: reading the figures

ICP DAS states active power accuracy at power factor 1:

Better than 0.5% for the PM-3112, PM-3114, PM-3133, PM-4324 and PM-5133 with their supplied CTs, and for the PM-3033 (above 0.5 A input current for the PM-3033 and PM-5133).
Better than 2% for the Rogowski versions, above 50 A for the 500 A and 1000 A coils and above 200 A for the larger ones.
Approximately 2 to 5% for the 333 mV models, depending on the CT.

Choose the smallest CT rating that covers the maximum current; an oversized CT spends most of its time at the bottom of its range. Rogowski coils suit large incomers, not lightly loaded feeders. If readings are used to invoice tenants, check which meter approval national rules require.

Wiring and installation

CT direction: mount each CT in the direction marked on it. The three-phase models measure bi-directional energy, so a reversed CT shows up as exported energy rather than as an obvious fault.
Phase assignment: each CT must sit on the same phase as its voltage input, or power and power factor will be wrong.
Auxiliary supply: the PM-3112, PM-3114 and PM-3133 need 12 to 48 VDC, with Power over Ethernet as an alternative on the PM-3133-MTCP. The PM-4324 and PM-2133D take 85 to 264 VAC.
Outputs: many models have two Form A power relays (5 A at 250 VAC) for alarms.

Voltage connections and CTs in a distribution board must be installed by a qualified electrician.

Modbus RTU or Modbus TCP

The standard versions use Modbus RTU on RS-485. One twisted-pair bus runs from meter to meter, each meter has its own address, and the bus is terminated at both ends. Baud rates from 9600 to 115200 bit/s are supported.

Versions with the suffix -MTCP have a 10/100 Mbit/s Ethernet port with Modbus TCP, which avoids a separate bus where meters are spread out and Ethernet is already in place. Several series also come with CANopen (suffix -CPS).

Choosing a Modbus power meter from the PM series

Series	Circuits	Current input	Interfaces
PM-3112, PM-3114	2 or 4 single-phase	CTs, 60 to 200 A	RTU, TCP, CANopen
PM-3133	1 three-phase or 3 single-phase	CTs, 5 to 400 A	RTU, TCP, CANopen
PM-3133-RCT	1 three-phase	Rogowski, 500 to 4000 A	RTU, TCP, CANopen
PM-3133i	1 three-phase, isolated inputs up to 600 V	CTs, 60 to 400 A	RTU, TCP, CANopen
PM-3033	1 three-phase	Existing 1 A or 5 A CTs	RTU, TCP, CANopen
PM-3133P, PM-4324P, PM-2133DP	1 three-phase; up to 24 circuits on PM-4324P	333 mV CTs	RTU; TCP on PM-3133P, PM-4324P
PM-4324, PM-4324A	8 three-phase or 24 single-phase	CTs, 60 to 400 A	RTU, TCP
PM-2133D	1 three-phase, panel meter with LED display	CTs, 5 to 400 A	RTU
PM-5133	1 three-phase, 3.5" touch screen and data logger	CTs, 5 to 400 A	RTU

All of them measure voltage, current, power, energy, power factor and frequency, updated every second. Where no PLC or SCADA system reads the meters, a PMC or PMD power meter concentrator can poll them and log the data.

Send technical sales a list of circuits with voltages, maximum currents and cable sizes, and we will propose suitable meters and CTs.

Produkter för denna tillämpning

CA-SCT16I-100A-L080 CA-SCT16I-100A-L080 100A Split-Core Current Transformer, Φ16mm, 8 m Wire

The CA-SCT16I-100A-L080 is a split-core current transformer for measuring AC current on existing conductors. Its split design lets it be clamped around a cable without disconnecting the circuit, which suits retrofit energy monitoring. It provides one channel with a 100A rating and a Φ16mm aperture.This variant is intended for use with the iWSN and iXN series. The 8 m wire gives room to route the sensor from the conductor to the measuring device. The source data states a maximum input voltage of 500 VAC at 50/60 Hz.Accuracy is specified as 1% for the CT alone. When used with iWSN-1110X, iWSN-1131P, iWSN-750P or iWSN-757P, the stated accuracy is ±3% or ±0.3 A. The unit is RoHS and WEEE certified.ApplicationsCurrent measurement for power metering with iWSN and iXN series systemsRetrofit energy monitoring on existing conductors

609,37 kr per st. · mängdrabatt från 10 st.

Detaljer

CA-SCT24I-200A-L080 CA-SCT24I-200A-L080 Split-Core Current Transformer, 200A Φ24mm, 8 m Wire

The CA-SCT24I-200A-L080 is a single-channel split-core current transformer rated at 200A, with a Φ24mm aperture. Its split-core construction lets it be fitted around an existing conductor, which suits retrofit metering work where the circuit cannot easily be disconnected.Supplied with an 8 m lead, this CT is intended for use with the iWSN and iXN series. The maximum input voltage is 500VAC at 50/60Hz. Accuracy is rated at 1% for the CT alone, and at ±3% or ±0.3A when used with the iWSN-1110X, iWSN-1131P, iWSN-750P and iWSN-757P.Choose this variant when you need a 200A-rated CT with a Φ24mm aperture and an 8 m lead for iWSN or iXN metering installations.ApplicationsEnergy managementPower meteringCurrent monitoring on iWSN and iXN series installations

830,96 kr per st. · mängdrabatt från 10 st.

Detaljer

CA-SCT36I-400A-L080 CA-SCT36I-400A-L080 Split-Core Current Transformer, 400A Φ36mm, 8 m Wire

The CA-SCT36I-400A-L080 is a single-channel split-core current transformer for energy management and power metering. Its Φ36mm opening takes conductors up to a 400A rating, and the split core fits around an existing cable without disconnecting the circuit.This variant is specified for use with the iWSN and iXN series. The 8 m wire gives room to route the sensor from the conductor to the measuring device. Accuracy is 0.5% for the CT alone, and the source lists a combined accuracy of +-3% or +-0.3A when used with iWSN-1110X, iWSN-1131P, iWSN-750P and iWSN-757P.Single-channel split-core CT for retrofit installation400A rating through a Φ36mm apertureMaximum input voltage 500VAC, 50/60HzApplicationsEnergy managementPower metering with iWSN and iXN series devices

919,60 kr per st. · mängdrabatt från 10 st.

Detaljer

iWSN-110X-160-ME CR iWSN-110X-160-ME Self-Powered Current Sensing Module, 1-ch CT Input

The iWSN-110X-160-ME CR is a wireless current sensing module for AC current monitoring where running mains cabling to the sensor is impractical. It runs from a built-in Li-ion battery that is charged by the current induced in its split-core CT, so no separate power supply line is required.Readings are sent over a sub-1 GHz 433 MHz RF link. Channels, group ID and node ID are set with DIP and rotary switches, which keeps configuration simple on site. The ordering code includes one 100A split-core CT with an 8 m wire, and the module provides an expansion interface for future flexibility.Mounting by wall-mount bracket or magnet adsorptionDimensions 85 x 152 x 25 mm (W x L x H)Operating temperature 0 °C to +45 °CApplicationsEnergy saving and power monitoringBig data analysis of electrical consumptionPredictive maintenance

1 828,12 kr per st. · mängdrabatt från 10 st.

Detaljer