The Shelly Pro 3EM-3CT63 shares its chassis and electronics with the Pro 3EM, but changes the way it measures entirely. Instead of three split clamps it uses the 3CT63: a single piece that takes all three phases and sits directly on the breaker.
It is the meter for boards where nothing else will fit.
Two numbers you settle before buying
This device has two hard limits, and both are checked with the board open and a tape measure, not with the order already placed:
- Conductor opening: 9 mm. The most important limitation of the device. If your phase cables will not pass through that diameter, this model is no use to you and there is no way to force it.
- Maximum current: 63 A. Above that, the 120 A Pro 3EM.
Measure them first. Checking a diameter is cheaper than returning a device.
And as always: switch off at the main breaker and verify the absence of voltage on all three phases before touching anything.
Power supply
Four line terminals: A, B, C and N. From 100 to 260 V, drawing under 3 W, protected by a type B or C breaker of 16 A maximum.
One detail its name hides: it accepts single-phase installation as well as three-phase. It is more versatile than it sounds, and on a refurbishment with a tight board it can handle a single-phase measurement without taking the space of three clamps.
The combined clamp
Here is the real difference. The 3CT63 is a single 14×54×26 mm piece that sits on the phase cables themselves, fitting over the breaker, with no rail fixing.
It connects to the meter with three two-pin connectors, and that is the delicate point: each phase must go to its own input. Watch the colour coding — red and black — as you connect them, because mixing up two connectors produces exactly the problem crossing clamps does on a Pro 3EM: the device starts, connects, and one phase reports nonsense.
Respect the direction marked K → L. If you see negatives where you expected positives, the piece is fitted the wrong way round.
There is no relay
Same as the Pro 3EM: no built-in relay. Driving a contactor needs the switch add-on, and its interface is at high voltage while the device is powered. Always connect it unpowered.
Network, and an advantage the others lack
Wi-Fi, Ethernet and Bluetooth. And something that matters in the environment it is aimed at: it supports Modbus TCP on port 502, alongside MQTT and RPC.
That is what makes it fit into building management and industrial systems, where Modbus is still the common language. On a commercial board it usually counts for more than any other feature.
Give it a static IP from the start. If it is going to speak Modbus to a management system, changing address after a router restart means the system stops reading it — and in a building that gets discovered late.
Where the board holds several devices, the SIT-IoT app puts them all on the network with their static addresses in one pass.
What to switch on
- Phase sequence detection: warns if the order of zero crossings is not A → B → C. With three connectors that can be mixed up, this is precisely the safety net you want.
- Channel-by-channel calibration: needs at least 500 W per channel to run.
- Minimum load threshold: below 30 VA per channel it shows power but does not accumulate energy, so noise does not pollute the statistics.
Commissioning
- Restore the supply. The power LED turns red.
- Hold the button for 5 seconds to enable the access point.
- Configure it from the app or by joining the network it broadcasts.
- Check all three phases before closing up. If one gives values that do not fit, check which input its connector went to.
What data you get
Four-quadrant measurement: active and apparent power, energy, power factor, and fundamental active and reactive energy. Accuracy of 1% between 2 and 63 A, 2% from 1 to 2 A and 5% below.
It stores up to 60 days at one-minute resolution, exportable to CSV and JSON — fifteen days more than the Pro 3EM, oddly enough. The internal clock keeps time even if the connection drops.
Where it belongs
Its natural home is boards with no space: commercial buildings, motor control centres, data centres, substations, and refurbishments where conventional clamps will not fit.
In a home it makes sense when the board is full and there is no room for three 120 A clamps. If there is space to spare and the supply exceeds 63 A, the Pro 3EM is the choice.
What gets watched afterwards
In the settings this device is aimed at, nobody drops by to look at the board every week. And a meter feeding a management system that stops reporting does not raise an alarm: it leaves a gap in the data that someone notices months later.
In the console, last seen turns that into something visible at a glance, and telemetry lets you follow the imbalance between phases — which on a commercial board moves as loads come and go — without returning to site.
The recurring mistakes
- Cables that will not pass 9 mm: discovered with the device already bought.
- K → L direction reversed: negative readings.
- Connectors mixed up between phases: the same symptom as crossed clamps.
- Expecting it to switch loads without the add-on: it has no relay.
If the numbers do not add up, the general causes are in why a meter reads higher than expected. And if you are choosing between this and the Pro 3EM, the difference is set out in its installation guide.
The app and the console work with Gen2, Gen3 and Gen4. The app is on Google Play: SIT-IoT Smart Installer Tool.