The Pro 3EM-3CT63 is the compact sibling of the Pro 3EM: instead of three loose clamps it has one combined clamp that closes around all three conductors. That solves the space problem in small boards and creates a new one — diameter.
The fitting is in how to install the Shelly Pro 3EM-3CT63, step by step.
Will my cable fit? This is mistake number one
The clamp has a Ø9 mm opening, and this is where nearly everyone trips: they confuse the cable’s cross-section with its outer diameter.
They are not the same thing. A 16 mm² conductor measures about 8 mm across the outside with its insulation. It fits, but only just. Measure the cable before you buy, not the cross-section written on the drawing.
How much current will it take?
63 A, against 120 A on the Pro 3EM. That is the other side of the compact format.
Accuracy:
- 1% between 2 and 63 A
- 2% between 1 and 2 A
- 5% below 1 A
Energy measurement is class B.
How does it differ from the Pro 3EM?
| Pro 3EM | Pro 3EM-3CT63 | |
|---|---|---|
| Current | 120 A | 63 A |
| Clamps | Three separate | One combined, Ø9 mm |
| History | 45 days minimum | Up to 60 days |
| Protocol | MQTT | Modbus |
| Relay | No — Switch Add-on | No — Switch Add-on |
Modbus is not a minor detail: if you are integrating into a PLC or an industrial system that already speaks Modbus, this one drops in without a gateway. If your world is MQTT and home automation, the Pro 3EM is the natural choice.
Is it three-phase only?
No, despite the name. It works single-phase and three-phase. The “3CT” refers to the three transformers inside the clamp, not to a requirement for three phases.
Does it have a relay?
No. Like the Pro 3EM it only measures. To switch loads you need the Switch Add-on. The only one in the Pro range with a relay of its own is the Pro EM-50, and with the 2 A limit worth remembering.
Why does calibration ask for 500 W?
For the same reason as the Pro 3EM: it needs 500 W per channel to have something reliable to calibrate against. It is a one-off, not a daily requirement — once done, it measures far smaller loads.
It shares the 30 VA per channel threshold too: below that it shows power but does not accumulate energy.
The red and black connectors
The clamp joins the unit with three two-pin MX3.0 connectors, colour-coded red and black. The colour matters: it decides which channel each phase is assigned to. Swapping them breaks nothing, but it attributes one phase’s consumption to another — and that does not show up by looking at the numbers.
The recurring mistakes
- Cross-section versus diameter, as above. It causes more returns than anything else.
- Clamp reversed: follow the K → L marking or power reads negative.
- Phase sequence: if the warning appears, two phases are crossed in the board.
The 60 days of history can be read from the console and exported to CSV and JSON. The app and the console work with Gen2, Gen3 and Gen4.
How to use both, section by section: mobile app manual and web console manual.