The Shelly Pro EM-50 is a simple unit to use, but there are two figures that generate most queries and returns: 50 A and 2 A. Understanding what each one means avoids buying the wrong model or damaging the device.
The two limitations you need to know
They're easily confused because they're two different things: one is how much current it can measure and the other how much it can switch.
Frequently asked questions
Can I measure my home's main service?
It depends on your contracted power. The 50 A per channel is enough for about 11 kW single-phase, so it fits in many Spanish homes. If your service exceeds that figure, you need the Pro 3EM, which reaches 120 A per phase.
Can I connect an electric water heater to the relay?
Not directly, and this is the most expensive mistake made with this model. The output is a dry contact of 2 A maximum at 240 V: it's for driving the coil of a contactor, not for powering the load. A 2,000 W water heater draws about 9 A and would destroy the relay. Always install an intermediate contactor rated for the actual power.
Can I use a 120 A clamp?
No. The unit is calibrated for 50 A transformers and its measurement range ends there. Fitting another clamp would give readings scaled by the wrong factor.
Does it work for a three-phase installation?
Not as such. The Pro EM-50 is single-phase: it takes its reference voltage from a single phase, so it can't correctly calculate the power of the other two. For three-phase you need the Pro 3EM or the Pro 3EM-3CT63.
Why does the accuracy get worse at low load?
It's normal and it's in the datasheet: 1% between 5 and 50 A, and 2% below 5 A. At very low consumption the relative error margin grows, something common to all clamp meters.
The values come out negative, is it a fault?
No. It indicates the clamp is fitted against the direction of current flow, or that there's feed-in to the grid if you have solar panels. You can turn the clamp around or reverse the measurement direction from the web interface without touching the installation.
Do I need the add-on like the three-phase models?
No. The Pro EM-50 is the only one in the Pro range with a built-in relay. The Pro 3EM and the Pro 3EM-3CT63 do need the Shelly Pro 3EM Switch Add-on to drive a contactor.
Can I extend the clamp cable?
It's possible, but the longer the cable the more electrical noise it picks up, and phantom readings appear with the installation nearly off. Keep it short and away from power cables.
Does it work without internet?
Yes. It stores the history internally (at least 60 days at one-minute resolution) and has a real-time clock, so timestamps stay correct even if the connection goes down. You can integrate it via MQTT into Home Assistant without depending on the cloud.
It measures fine but doesn't accumulate energy, what's happening?
Check that the load isn't too small. Shelly meters apply a minimum threshold below which they show power but don't add energy to the statistics, so that noise doesn't clutter the data.
If the readings don't add up
The causes of an incorrect measurement are the same across the whole Shelly range: a poorly closed clamp, a clamp over phase and neutral at once, a reference voltage from another phase or a wrongly configured clamp type. We go through them one by one in the guide on why a Shelly meter reads more than expected.
And if the problem is that the device doesn't appear in the app after changing networks, you have the solution in what to do when Shelly devices won't connect after changing routers.
Useful links
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