Aeotec Heavy Duty Switch _and_ 3-phase powered appliances (e.g: big A/C)

Hi folks,

I’m looking at the specs of the Aeotec Heavy Duty Switch Gen 5 ( and it claims to be able ot handle up to ~40 Amps of current and 2-Phases.

I have a big ducted A/C I’d like to monitor, but unfortunately it is 3-Phase – Although its total maximum rated power is only 8100W. It even has a 20A circuit breaker to the outside unit.

I’m not even sure what I’d like to do is “technically feasible” at all. Can I somehow hack the Aeotec Heavy Duty Switch Gen5 to measure the power consumption of this A/C?

Kind regards


Without knowing where in the wold you are this is meaningless.

  • 8100W/240V = 34A (single phase)
  • 8100W/120V = 68A (single phase)

Keep in mind the power ratings on smart devices are for resistive loads. An A/C unit has a compressor and fan inside. These are not resistive loads and you can see peaks of up to 5x their power rating if it does not have some sort of soft starter.

To monitor the power you could use a “non-contact” method like current transformers. The Shelly EM3 is good for measuring up to 120A per phase (three phases). It also has a 10A relay you could use to control an appropriately sized three phase contractor if you require power control.

Sorry I should have stated.

I’m in the AU. So 240VAC here @ 50Hz.

Apparently the side of the A/C claims 440VAC with a maximum rated i nput of 8100W – But its not clear to me why it even needs 3-Phase power or how this works out.

The A/C unit outside also has 440V written on the side too.

So this means 8100W/440V = ~18.5A right?

Not Quite.

For three phase the three powers are 120 degrees phase shifted with respect to each other and you have to include a sqrt(3) factor because of this.

Current = 8100/415/sqrt(3)

= 11.3A.

Note: This is for a purely resistive load, which your load is not. as I pointed out in my last post above.

Also the 440V is a maximum rating. For 240V line to neutral the voltage between phases will be closer to 415V (240V*sqrt(3)).

There’s a calculator here:

Ahh I see.

So to answer my original question; the Aeotec Heavy Duty Switch will not work for this use-case. Right? I believe its only design for 1 or 2 phase according to the wiring schematics anyway (

Wrong. At least possibly.

As I keep saying, your A/C is not a resistive load. The Aeotec switch power rating is only for a resistive load.

An A/C has a compressor motor and a fan motor these can draw 5 times their rated switch on current if they do not have soft starters.

Do you actually need to control the power to the A/C or do you just want to monitor its power use?

I gave you options for both:

This is a wifi device that can be integrated into home assistant easily with mqtt. No cloud connection required. No Home Assistant Z-wave network initialisation after a restart required.

Yeah for the A/C especially I just want to monitor its power draw and usage.

Then the Shelly EM3 fits the bill nicely, and no direct connection between the supply and your load.

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Yeah it looks like a 3-phase clampeter by the looks.

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A smart 3 phase clamp meter is exactly what it is.

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