Energy Meter Wiring Diagram: Single-Phase, Three-Phase and CT-Operated Meters
An energy meter wiring diagram is read from the terminal block, not from the display. A single-phase meter has four power terminals: line in, line out, neutral in and neutral out. A three-phase four-wire meter has seven: three line in/out pairs plus neutral in and out. A CT-operated meter replaces the heavy current terminals with small secondary terminals, two per current transformer. Everything else on the meter, meaning RS485, pulse output and tariff control, is low-voltage signalling and belongs to a separate terminal group.
Getting the power terminals right decides whether the meter works; getting the small terminals right decides whether it keeps working. Most meters that come back as failed in the first year were not faulty in the factory, they were miswired on site.
Before any wire goes in
Three checks save most of the callbacks.
- Read the terminal diagram printed on the side of the meter. Terminal numbering is not standardised across manufacturers. Some meters number power terminals 1 to 4 and signal terminals 5 to 8; others number line and load in the opposite order. The printed diagram is the authority for that unit.
- Confirm the supply and the meter agree. A three-phase meter with a 380 V marking does not belong on a system it was not built for, and a direct-connected meter does not belong on a feeder above its maximum current.
- Isolate and prove dead. Every terminal in this article is live at some point in the procedure. Isolate upstream, lock, and test before touching a conductor.
Single-phase two-wire: four terminals
The most common DIN-rail meter in the world, and the one most often wired backwards.
| Terminal | Connection | What goes wrong |
|---|---|---|
| L in (often 1) | Incoming live from the supply side | Swapped with L out, so the meter still counts but the installation's polarity downstream is reversed |
| L out (often 2) | Live to the load | Load taken from the incoming side, bypassing the meter entirely |
| N in (often 3) | Incoming neutral | Neutral borrowed from another circuit, which puts unmeasured current through the meter's reference |
| N out (often 4) | Neutral to the load | Neutral and earth confused, which is both a metering error and a safety fault |
The single-phase electronic meter in the YOMIN range, such as the mini DIN-rail single-phase kWh meter, follows this four-terminal arrangement in a two-module body. Where RS485 is included, as on the 80 A RS485 DIN-rail meter, the communication terminals sit apart from the power terminals and are clearly marked A and B.
Three-phase four-wire: seven terminals
Three line conductors and one neutral, each measured on its own element. The neutral is not optional on a four-wire meter: without it the voltage reference for each element is wrong and the readings are meaningless rather than merely inaccurate.
| Group | Terminals | Notes |
|---|---|---|
| Line, phase 1 | L1 in, L1 out | Phase sequence should follow the printed diagram; the meter will run with any sequence but the per-phase registers will not correspond to your labelling. |
| Line, phase 2 | L2 in, L2 out | Same element arrangement as phase 1 |
| Line, phase 3 | L3 in, L3 out | Same element arrangement as phase 1 |
| Neutral | N in, N out | Must be the same neutral that serves the load, not a borrowed one |
A current range such as 5–80 A on the 380 V 50 Hz three-phase four-wire DIN-rail meter describes a direct-connected device: the load current passes through the meter's own terminals. Above that current, the meter must be CT-operated instead. Our guide to three-phase meters covers the 3-wire and 4-wire distinction and the direct versus CT decision in more detail.
CT-operated meters: small terminals, large consequences
On a CT-operated meter the heavy conductors pass through the current transformers and never touch the meter. The meter receives the secondary of each CT, normally 5 A, on a pair of small terminals marked S1 and S2.
- Polarity decides direction. S1 must correspond to P1 on the primary side. Reverse one CT and that phase registers negative while the other two register positive, which produces a plausible but wrong total.
- The ratio must be programmed. The meter reads secondary current; the CT ratio is what converts it back to primary current. A 200/5 CT with a meter still set to 100/5 halves every reading, and nothing on the display will look wrong.
- Never open a CT secondary under load. An open secondary drives the core into saturation and produces a dangerous voltage at the terminals. Short the secondary through the test block before breaking the circuit.
- One CT per phase, same ratio. Mixing ratios across phases produces a total that cannot be corrected afterwards without knowing which phase was different.
The three-phase DIN-rail meter with CT input and M-Bus is built for this arrangement, and our notes on CT-operated metering explain how the ratio interacts with the register.
RS485 and pulse outputs
These terminals carry no load current, but they are the ones that fail quietly when they are wired carelessly.
| Signal | Terminals | Practice |
|---|---|---|
| RS485 | A and B, sometimes with a common | Shielded twisted pair, daisy-chained rather than star-wired, shield earthed at one end only, A to A and B to B throughout the bus |
| Pulse output | Pulse and common | Two-core screened cable kept away from power conductors; the pulse constant printed on the meter defines what each pulse is worth |
| Tariff control | Switched input | Dry contact from the tariff clock or controller, never a stolen live |
Keep signal and power conductors in separate ducts wherever the route allows it. Most intermittent communication faults on a metering installation are induced noise from a signal cable run alongside a feeder, not a failed meter. The Modbus and RS485 buyer's guide covers addressing and termination.
Meter family at a glance
| Family | Power terminals | Signal terminals | Typical model |
|---|---|---|---|
| Single-phase two-wire, direct | 4 | Pulse, optional RS485 | Mini DIN-rail single-phase kWh meter |
| Single-phase, direct, communicating | 4 | RS485, pulse | 80 A RS485 DIN-rail meter |
| Three-phase four-wire, direct | 7 | RS485, pulse | 380 V 50 Hz three-phase four-wire meter |
| Three-phase, CT-operated | 3 pairs S1/S2 | M-Bus, infrared | Three-phase CT-input meter with M-Bus |
Mistakes that account for most failures
| Mistake | Result |
|---|---|
| Load tapped from the incoming side of the meter | The meter is bypassed and registers nothing for that load, while still appearing to work |
| Neutral taken from a different circuit | Part of the current returns outside the meter; readings drift and become irreconcilable |
| Two phases transposed on a three-phase meter | Total may look acceptable while per-phase data is wrong, so fault-finding on the site is misled |
| CT secondary reversed | One phase registers negative; the total understates consumption |
| CT ratio not programmed | Every reading scaled by the wrong factor, with no visible symptom |
| RS485 star-wired instead of daisy-chained | Reflections and intermittent dropouts on the bus |
| Signal cable run in the power duct | Noise-induced errors that appear and disappear with load |
Commissioning checklist
- Terminal diagram on the meter matches the drawing you are working to.
- Supply voltage, phase count and maximum current are within the meter's rating.
- Line in and line out are not swapped; the load is downstream of the meter.
- Neutral is the load's own neutral, not a borrowed one.
- CT polarity P1/S1 respected on every phase, and the ratio entered in the meter.
- CT secondaries shorted before any loop was opened during the work.
- RS485 A/B consistent end to end, daisy-chained, shield earthed at one end.
- All terminals torqued to the value printed on the meter, and the cover refitted.
Frequently asked questions
Can a single-phase meter be connected with line and neutral reversed?
Many electronic meters will still count, because they measure the potential difference across the element rather than assuming which side is live. That does not make it correct: the downstream polarity of the installation becomes reversed and the neutral becomes switched. Wire it to the diagram.
Why does my three-phase meter show one phase negative?
Almost always a reversed CT secondary on that phase, or a CT installed with P1 facing the load instead of the supply. Check polarity at S1/S2 before suspecting the meter.
Do I need to program the CT ratio, or does the meter detect it?
You must program it. The meter sees only secondary current and has no way to know what primary current that represents. An unprogrammed ratio is the most common cause of a meter that reads a plausible but wrong figure.
How far can an RS485 run be on a metering installation?
It depends on baud rate, cable capacitance and how many devices share the bus, which is why the practical answer comes from the meter's datasheet rather than a rule of thumb. What matters at any length is daisy-chain topology, consistent A/B polarity and a shield earthed at one end only.
Wiring meters on a project?
Send the meter type, supply arrangement and CT ratio for each point, and we will confirm the terminal configuration before the panel is built.
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