How to pick a digital energy meter or panel meter: single vs three-phase, accuracy class, direct vs CT connection, and what to specify for sub-metering.
An energy meter looks like a simple box until you have to specify one: phase count, how it connects to the current, the accuracy you can defend to a billing authority, and whether it talks to anything. This guide covers the choices that actually change the part number, written for panel builders, building managers and distributors setting up sub-metering.
The first split is single-phase versus three-phase. A single-phase meter (for example an electromechanical DD862 or an electronic DDS228) measures one line against neutral and suits small shops, lighting sub-boards and residential tenants. A three-phase meter (DT862, DTS866) measures all three lines and is required for any load above roughly 5 kW, for balanced three-phase machinery, or wherever the supply authority or your tenant agreement demands a three-phase record.
| Feature | Single-phase | Three-phase |
|---|---|---|
| Typical load | under 5 kW, 1 line + N | above 5 kW, 3 lines (+N) |
| Measuring elements | 1 voltage, 1 current | 3 voltage, 3 current |
| Common models | DD862, DDS228, DDSY866 | DT862, DTS866, DTSY866 |
| Where used | Shops, flats, lighting boards | Factories, plant rooms, mains |
Direct-connect meters carry the load current straight through the meter and are rated for a maximum current, typically 40, 60 or 100 A. Above that, or where the cable is too large to terminate in the meter, you use a CT-connected (current-transformer) meter: the meter reads the secondary of a 5 A or 1 A CT and is programmed with the CT ratio. The same meter then covers a wide current range by changing only the CT.
| Parameter | Direct connect | CT-connected |
|---|---|---|
| Max current | up to 100 A | unlimited (CT ratio set) |
| Installation | Wire straight through | CT around cable, secondary to meter |
| Cost | Lower per point | CT + meter; cheaper at high current |
| Best for | Tenant sub-meters, small boards | Mains, large drives, busbars |
The accuracy class states the maximum permitted error at rated load: Class 1 means plus or minus 1 %, Class 2 means plus or minus 2 %. For billing or tenant sub-metering, Class 1 is the usual minimum and many authorities require it. For internal plant monitoring where you only need to see trends, Class 2 is acceptable and cheaper. Note that the class is a percentage of the reading, not of full scale, so a Class 1 meter at 10 % load is still within plus or minus 1 % of that 10 % reading — a point that surprises people used to analog panels.
Electronic meters (DDS228, DTS866) replace the electromechanical disc with a digital register and usually add a pulse output for remote reading. Prepayment versions (DDSY866, DTSY866) carry a built-in or remote token or RFID interface so power cuts off at zero credit — useful for dormitories, rental units and temporary sites. If the meter must feed a BMS or a landlord dashboard, confirm the output: pulse (S0), Modbus or RS-485, or M-Bus. Specify the communication at order time, because the electronics differ, not just the firmware.
DIN-rail electronic meters drop onto the 35 mm rail next to the breaker they monitor. The wiring that causes callbacks is CT polarity and the neutral: reversing a CT flips the sign, and a missing or shared neutral gives a reading that is wrong by a fixed offset. Keep CT secondaries on their own terminals, label them, and never leave a CT secondary open while live — an open CT develops a dangerous voltage that can injure or destroy the meter.
ELEHPD supplies single- and three-phase electromechanical and electronic watt-hour meters, prepayment meters, digital multimeters and meter sockets, with the CTs and DIN-rail accessories needed for a complete sub-metering point.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.