Short answer: a kWh meter is wired either directly — the load current passes through the meter’s own terminals — or through current transformers, where the meter sees a scaled-down 5 A or 1 A signal. The break point is current: direct connection stops somewhere around 80–100 A per phase depending on the meter, and above that the CT route is the only practical one. The two are not just different products; they are different installation jobs, with different wiring, different accuracy behaviour and a different set of things that can be got wrong.
We supply DIN-rail meters and current transformers as an independent trading company — not a manufacturer, and not an appointed distributor of any brand. This is the practical difference between the two options when a buyer sends a load list and asks which meter to fit.
The decision in one table
| Attribute | Direct connection | CT connection |
|---|---|---|
| Typical current range | Up to about 80–100 A per phase | Any current, limited by the CT and the meter’s input |
| Wiring into the meter | Full load current through the terminals; cable sized for the load | Secondary wiring only, 5 A or 1 A, 2.5 mm² typically |
| Cable and terminal size | Often the limiting factor; 35 mm² and above is awkward | Small conductors, easy routing |
| Accuracy | Meter accuracy plus contact and cable losses | Meter accuracy plus CT accuracy and burden |
| Safety during service | Meter terminals are live at load current; isolation required | Secondary circuit; CT must be shorted before opening |
| Space | Meter must sit in the main current path | CTs go on the main bars or cables; meter can be remote |
| Failure mode to avoid | Loose terminal heating at full load | Open secondary, wrong polarity, wrong ratio |
Direct connection: simple, and unforgiving at the terminals
A direct meter is a series device. Every amp the installation uses passes through its terminal blocks, so the meter’s terminal rating and the installer’s torque discipline become part of the circuit’s thermal design. A 63 A direct meter with a 50 A load is fine; the same meter with a poorly torqued 25 mm² conductor is a hot spot that shows up as a discoloured terminal in six months and as a burnt meter in two years.
The practical rules are: use the cable the load needs, not the smallest that fits the terminal; torque to the figure in the manual with a torque screwdriver rather than by feel; and leave room for the bend radius, because a 35 mm² conductor forced into a 90° bend at the terminal puts a permanent load on the connection. Direct meters are also usually the ones that get installed in a hurry at the end of a job, which is exactly when the torque check is skipped.
CT connection: the secondary circuit is the delicate part
With CTs, the current path stays in the main bars and only a scaled signal reaches the meter. That solves the terminal problem and creates a different one: the secondary circuit. A current transformer with an open secondary develops a very high voltage across its terminals, which is dangerous to people and destructive to insulation. The rule is absolute — short the secondary before opening the circuit, and use a test terminal block with a shorting facility so that testing does not require improvisation.
The second issue is burden. Each CT has a rated burden in VA, and the secondary wiring, the meter’s input and any test terminals all add to it. A long run of thin secondary cable can exceed the CT’s rating, and the result is not a dramatic failure: the CT saturates slightly, the ratio error grows, and the meter under-reads at high current. For billing, that is a commercial problem; for internal monitoring, it is a number nobody can explain.
| CT secondary detail | Why it matters | Practical setting |
|---|---|---|
| 5 A vs 1 A secondary | 1 A reduces burden and cable loss over long runs | Use 1 A where the meter is more than a few metres away |
| Class (0.5, 1, 3) | Sets the ratio and phase error at rated current | Class 0.5 for billing, class 1 for monitoring |
| Burden in VA | Must cover the meter plus the wiring | Check the sum; do not assume the CT is oversized |
| Polarity (S1/S2) | A reversed CT subtracts instead of adding | Mark and check each phase before energising |
| Ratio | The meter’s programmed ratio must match the installed CT | Programme the meter, then seal it |
The ratio is programmed, not wired
A CT meter does not know what CT it is connected to. The ratio is entered into the meter as a setting — 200/5, 400/5, 600/5 — and if that setting does not match the installed transformer, every reading is wrong by the same factor and the error is invisible until someone compares the bill against a second meter. Two habits prevent it: programme the ratio before commissioning and seal the meter afterwards, and write the ratio on the panel label next to the meter so the next person does not have to open it to find out.
Where the CT is shared with protection or another instrument, the meter’s burden adds to a circuit that was sized without it. That is the case for a separate metering CT core, which costs a little more and removes the argument.
Choosing between them on a real project
For a shop or a small workshop with a 63 A supply, a direct three-phase meter is the correct answer: fewer components, less to program, less to go wrong. For a factory with a 400 A incoming, the CT route is the only sensible one, and the meter becomes a small DIN-rail device fed by three secondary circuits. In between — around 100 to 160 A — the decision usually follows the cable: if the conductors are too large to land in a meter, use CTs.
Accuracy follows the same logic. A direct meter’s accuracy claim is only meaningful if the terminals are tight and the ambient inside the enclosure is within the meter’s specification; a CT meter’s claim depends on the CT class and the burden. Both are affected by temperature, and a meter rated for 55 °C inside a closed box in a hot climate is a different proposition from the same meter in a ventilated cabinet.
What to put on the purchase order
System type (single-phase or three-phase, three- or four-wire); the load current per phase and the supply voltage and frequency; direct or CT connection; if CT, the ratio, class, burden and secondary current; the meter’s accuracy class and whether it must be MID-approved for billing; the communication interface if readings are to be collected; the terminal size the meter accepts; and the destination country’s requirements. Add the panel ambient and whether the meter will be sealed, because both change what is acceptable.
FAQ
What is the maximum current for a direct-connected meter?
It depends on the meter, but the practical ceiling is around 80–100 A per phase. Above that, the cable and terminal sizes become unmanageable and the CT route is the standard solution.
Can I add CTs later to a direct meter?
No. A direct meter measures the current through its terminals; a CT meter is a different product with a different input. Plan the connection type at the design stage, because it also decides the panel layout.
Why does my CT meter read exactly ten times too high?
The programmed ratio does not match the installed CTs — for example a 100/5 CT with the meter still set to direct or to 10/5. Check the ratio setting and the CT label before suspecting the meter.
What happens if the CT secondary is left open?
The CT develops a very high voltage across the open terminals, which can damage insulation and is dangerous to anyone nearby. Always short the secondary through a test terminal block before opening the circuit, and never leave a CT energised with its secondary disconnected.
Related: CT ratio selection for energy meters · DIN rail energy meter selection · Breaker and cable coordination · Copper busbar current rating · Planning an enclosure in DIN modules · MCB range overview · Cross-reference an existing device · Request a quotation