# Measuring 3-phase power

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#### Endstyle

##### New Member
Hello,

I had an idea to measure the consumption of my our newly installed "Air pump" (sorry dont know the exact word in english). It runs on three phase power and I was wondering if I could use 3 ,single phase meters, like this one SONOF POW . So I would use one for each phase. And since the air pump says it has 9kW power, and these handle 3.5kW each I think it should be fine.
Would that be even remotly possible?

Note that I dont know that much about electricity so If that questions sound wierd to you, thats why

Thank you and have a nice day

#### dknguyen

##### Well-Known Member
You can use one, two, or three watt meters depending on the details of the thing you're measuring.

One Watt Meter: only valid if you know for a fact that the load on all three-phases are identical. Then you just measure power on one phase and multiply by three. Since you can never be sure they are identical without always monitoring them, this method is inadvisable.

Two Watt Meters: If no neutral wire is available (i.e. delta-connected, three wires only). Add the value from both wattmeters.

Three Watt Meters: If a neutral wire is available (aka. Wye-connected, three wires plus a fourth neutral wire). Add the readings from all three watt meters. Must place wattmeter to measure between each phase wire and neutral.

https://meettechniek.info/measuring/three-phase-power.html

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#### Diver300

##### Well-Known Member
To use two single phase meters they have to be rated to the phase-phase voltage. I guess the motor is rated at approximately 400 V phase - phase, and the supply is approximately 230 V phase - neutral. Therefore to use two meters, both need to work at 400 V. It is probably cheaper to use 3 meters rated to 230 V.

Also, you shouldn’t use the power meters that you linked to, because they are also able to switch off the load. Switching the three phases individually with switches rated to 230 V can result in problems.

#### tcmtech

##### Banned
Why do you need to know the power consumption?

One simple method to estimate the average power consumption is to measure the running currents of each phase then add them together and multiply that number by the line voltage then divide that by the Power Factor (PF and usually around .6 - .8 and listed on the motor nameplate) on the motor.

That will give you a fairly accurate wattage draw value to which when multiplied by the run time will tell you the estimated KiloWatt Hour consumption per hour of run time which can be kept track of with a simple hour meter.

#### Endstyle

##### New Member
Thank you for all the anwsers!
1. I actually meant Heat Pump, the one with the outside and the inside unit, that heats the radiators in the house.

2. I belive it is connected with three phases with no neutral ( I can only see the three wires with 16A 400V fuses), but I'll have to check. Would measuring the power still be possible without the neutral wire, if I were to use those, since they only handle 250v? If yes, how would I connect it?

3. I have no intent of switching off any of the devices, only use them for measuring the power.

4. I found ordered a bundle of 3 of them already. I saw on a couple of videos, that you can flash the chips inside, so they send the information (Voltage, amperage and freqency) to your server and you can use that to make graphs or whatever you please. I think It would be a fun project and measuring the power consumption of that Heat Pump was just something I thought of, since I am interested how much it actually spends in those really cold days vs on milder days. Of course I would only begin wiring up those after extensive testing with large loads ( I mean they are pretty cheap so I need to make sure, they can handle the rated power ) and after setting up the server and graphs and everything.

So my main question is , If it is possible to measure three phase power with those, and one per phase and how would I go about wiring those up.

#### tcmtech

##### Banned
You can also buy standard issue commercial KWH meters just like your utility company uses for cheap if you don't mind doing a bit of online or local searching. Where I live my local utility company will sell me a reconditioned meter plus socket panel for $50 -$100 depending on the phase rating and amp capacity. I have also picked up countless others from the salvage yards that deal with their scrap for a few dollars each. Local electrical contractors usually carry them as well but the price can be pretty high though (they get greedy on this sort of stuff) . So do most home building supply stores as special order items. (not so greedy)

As for the three wires running the unit I have doubts it's a three phase unit because there would be a 4th wire for either common or earth ground going by any electrical codes I am aware in any civilized and developed country.
More than likely its a single phase unit that feeds off two live lines or a live and common set and the third wire is the earth ground line. A quick look at the power panel that it feeds form would tell you that. If it only has 1 or 2 breakers as either a single or a tied together set its single phase. If it has three tied together its three phase which if where it's at is not a commercial building the odds of having three phase power to private residence is highly unlikely.

#### Endstyle

##### New Member
We already have a internal electricity meter, which is ours and not the electric company's and that works great, but its wired to the whole house and the main purpuse of those Sonof meters is to measure and send that data to my pc where I can use it for stuff and I figured that heat pump would be a great candidate to measure the consumption.

As for the phases, Im 100% sure we have three phase power to the house. I live in slovenia and on my street I belive jsut about everyone has 3 25amp phases, even tho mostly older people live here and they dont understand that they are paying for "conncetion cost" alot more than they use. (I would be surprised if they use one 25amp line, let alone 3)
In the breaker box, I see three wires that are fused and are going to the internal unit of the heating pump. You are most likely right about having a ground wire, but Im not sure about a neutral, I'll open up the back of the fuse box tommorow and check.
Here is how the fuses look like for the heat pump. PICTURE . The bottom is a main switch that shuts off the intire house including the solar power and you can see the three phases on the switch on the left and right next to it asweel. The blue I belive is neutral and green is ground.

#### Diver300

##### Well-Known Member
In Europe houses are usually only single phase 220 - 240 V. The only other commonly used supply is 3-phase, which are just three single phase supplies, each 120° apart, so 380 - 415 V between phases and 220 - 240 V phase to ground. That is common in commercial premises. It is usual to have all three phases run down roads, either overhead or underground, and a single phase is taken to each house, so every 3rd house is on the same phase.

A three phase motor would usually be 380 - 415 V between phases. I've never come across any other phase arrangements than that, except 110 V split phase for low voltage power tools

If those power meters are what you want to use, it's fine to connect them all to one phase and the neutral, and it doesn't matter that the motor has no neutral connection. You just connect them like this:-

but without the neutral connection to the star point of the load. The three meters will measure balanced or unbalanced loads. With a simple 3-phase motor, I would expect all three meters to be within a few percent of each other.

I suggest that you bypass the switches within the power meters so that they can't turn off.

Edit - It seems that in Slovenia having all three phases to a house is common. I, for one, am rather envious of that arrangement!

#### Endstyle

##### New Member
Yep I belive that is the system we have wired in our house. Between the phases its about 410ish and between one phase and ground or neutral around 240.

Using this PHOTO , how would it be connected, so It would be one meter per phase? Do they all have the same neutral wire going to the heating pump like on the drawing? Where would I get the neutral wire going to the meter? Would it be from that blue line like it is in the picture I posted one post above or would it be the one which is supposed to already be going to the pump?

Im sorry for asking these questions, but I really want to understand it completly.

#### Diver300

##### Well-Known Member
There is probably no neutral wire going to the pump. The meter in the photo with the plug and the heater will have the neutral in and neutral out connected together, like the earths are, so you only need one neutral connection to each meter. So each meter will have live in, live out, neutral and earth. Connect all three neutrals together and to the main blue line into the house. Connect all three earths together and earth them. Then connect the three 3-phase live wires from the circuit breaker to the three live in wires, one on each meter. Then connect the heat pump to the three live out connections.

You would need to put the meters where they can have a neutral connection.

#### Endstyle

##### New Member
Ohh, right that makes a lot of sense, thank you very much!
I'll check tommorow to see, exactly how it is all wired up and I'll post some pictures just in case I miss-understood anything.

Also side note, the delivery date on those meters is around dec 5-18, so I have quite some time to study three phase systems and everything carefully

Thank you to everyone again!

#### tcmtech

##### Banned
Im 100% sure we have three phase power to the house. I live in slovenia and on my street I belive jsut about everyone has 3 25amp phases, even tho mostly older people live here and they dont understand that they are paying for "conncetion cost" alot more than they use.

Interesting. I was not aware of any country that used 3 phase for residential power on a large scale let alone a 400 volt system. I'm jealous.

#### Endstyle

##### New Member
So its been a while and I have been flooded with school work and forgot about this project...
I found the user manual for the heat pump ( Fujitsu Waterstage 11kw) and this is how its connected PICTURE . It has a neutral and the power goes to the outside unit first, then to the inside one, that I find kinda strange but whatever. So would that technique still work? I would have to connect the same neutral wire that goes to the outside unit to all three power meters right?

#### Diver300

##### Well-Known Member
It's not a surprise that the power goes to the outside unit first. It looks like the outside unit uses all the power and the inside unit takes very little so that arrangement makes sense.

Metering the power to the outside unit will include power to the inside unit but that will be very little and you are probably interested in the total power consumption anyhow.

For metering purposes it doesn't make any difference which neutral you connect to, but for safety and if there are any ELCBs you should not connect anything from the live or lives of one circuit to a neutral from another circuit.

You should connect the metering circuit suggested in this discussion into the wires feeding the outside unit.

#### RODALCO

##### Well-Known Member
Simplest and most reliable way is to get three identical ex utility meters. Have one meter measure each individual Phase and the Neutral to the Neutral.
You can check for imbalances as the meters should more or less read all the same usage.
Power boards used often three single phase meters too, on switchboards. It was cheaper then a three phase meter and big imbalances were easily detected.

#### kubeek

##### Well-Known Member
Interesting. I was not aware of any country that used 3 phase for residential power on a large scale let alone a 400 volt system. I'm jealous.
actually I think that at least central Europe has three phase power everywhere in residential buildings.

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