# Three phase with resistive loads

I have one place with electric stove and oven combo which takes 3 phases and sauna which also takes 3 phases. I will use the derived phase method, but I was wondering why do I need to do that if the all the loads are more of resistive loads. Also in case of the oven/stove each phase might be powering different thing inside of it. So if I see 5 amps on phaseA and 2 amps on phaseB how do I know that this is the correct value due to different consumption or it is wrong due to different phase and needs to be derived?

Another question, if I put the 100A sensor on the main N line would that give me total of all the phases?

Not sure I follow you here. Are you saying that because at unity power-factor Watts = VA you don’t need the real power calculation? If that’s it, then OK, you can configure each phase as in a single-phase system and although the real-power calculation (Watts) will be way off, the VA will accurate. If you are satisfied to look at VA confident in the knowledge that the load is purely resistive, that’s fine, you don’t need to configure derived. But I don’t get why you wouldn’t use derived. It’s the exact same hardware and the VA will be exactly the same. The difference is that Watts will be accurate as well.

That is true.

This is a resistive load, so the Watts will be correct with the correct phase designation, and it will be half with either of the incorrect designations. When you get one correct, the second will be down to two choices, one twice the other, and the third will be known by the process of elimination.

No. Long story short, you could be using 10,000 Watts and have no current in the neutral wire. Also, there is no appropriate voltage reference for it because it’s associated with all three phases.

The homes I am working with has 3 phase coming to them. And I think they distribute phase equally over the 14 circuits. some appliances like (stove/oven combo) gets the 3 phases all together. So you mean I start by deciding which one is phaseA and plug the volt ref for it. Then for each wire of the 14, I select first that it is phaseA then B then C. and then the highest value I see among those three values is the right phase, correct?

I’m reluctant to start trying to explain how to do this as a series of Q&A in the forum. The procedure is thoroughly documented here: https://docs.iotawatt.com/en/master/threePhase.html#configuring-derived-reference

If you find a specific part of the documentation unclear or confusing, please reference that in a post and I’ll try to address the deficiency in the documentation. The goal is to have a single place where accurate comprehensive information is to be found.

Sure, here are the confusing parts for me:
“and roughly half power for all of the circuits on phases B and C.” How do I know that I am looking at half the power or maybe it is actually the full power?
“and evaluate which of the inputs appear to be showing power that is half what is expected.” That is the thing, I cannot have the expected value due to a an appliance like the stove has 4 cooking areas and oven under. Which I do not know which load is going on what phase ( inside the appliance).

Also in my case the 3 Phases are distributed over 14 wires ( not only 3 wires as the docs assume) . So to get the right phase for each it means I check under what designation the watt will be the highest, correct?

This is why an electrician is recommended. Once the mains are identified, the mains associated with each branch circuit can be observed at the distribution point. It’s possible to fumble around with power measurements to try to deduce each phase, but that is fraught with opportunities for confusion, as you point out with the varying stove loads.

This is true not only for derived reference, but for direct reference as well. There is no easy solution and you need to understand what to expect, as an electrician would.

I hope that you have come to appreciate the fundamental issues to success, most importantly that all of the CTs are oriented the same way in the panel. Without that assurance, the problem is many times more complex.

The documentation aims to identify the phase associated with each main. That is the starting point and the key to three-phase regardless of using direct (three VTs) or derived (one VT). Typically the other branch CTs are installed in the load center in proximity to the mains. In a good installation, each phase will be color coded. Some do not color code, so the conductors must be traced back to the corresponding main, where the phase is known.

So identifying the mains is the key to three-phase.

Do you have a picture of the panel?

There will be an electrician handling the the actual clamping of sensors, but his knowledge of IoTawatt will be zero. I assume that I can ask him to find out which phase each of the 14 breaker circuit belong to. But the configuration of IoTawatt is on me. Which boils down as you said to marking which main is B and which one is C.
In the docs “Go to the status display and evaluate which of the inputs appear to be showing power that is half what is expected.” so I guess my problem is with how do I know what “appears” to be showing less than expected? Do you have other tips that I can tell the electrician that would help me in matching the virtual B and C to the physical phases ?

I was thinking to put three CTs on the three mains. Then using multimeter check which one is A ( same as the socket that VT is plugged in) . Now for B and C. I put both of them B and then both of them C. Now which ever gets higher watt value, gets that designation. What do think about this idea?

That strategy relies on significant resistive loads for best results.

I had a similar issue as well while setting up my IotaWatt meters for a monitoring projects. I measured each of the phase active power after the set up and realized that using the derived measurements, setting the phases at 120 and 240 degrees apart did not work. The measured power was significantly lesser than the actual power that multimeter measured on each of the phases by close to 50%. When I fiddle with some of the other phase differences (B-C) or 150 for a second phase, its readings for that phase became the same. The other phases still had about 50% of the actual line readings. The question is, is it always 120 and 240 degrees phase difference to be applied and why do I see to get wrong readings by using these? Secondly, why would a phase difference of 150 degrees work? and lastly, how do I get the right readings on at the three phases? How does the orientation of the CTs affect the outcome of the derived three phase measurements?

It sounds very much like you may have some CTs reversed, and it has nothing to do with derived reference. Direct measurement would have the same issues.

Yes, the phase difference is always +/- 120° (+240° is also -120°).

In any three phase measurement, if the correct voltage reference (derived or direct) is specified, the power will be correct but it will be off by cos(180°) = -1.0 if the CT is reversed. When the wrong voltage reference is used, it will be off by cos(120°) -0.5.

I don’t know what type of multimeter you used for this, presumably something like a kill-a-watt. The question that I have is what the power factor was. The 50% decrease is only reliable with a purely resistive load (power factor 1.0). When you use reactive loads, all bets are off. You would need to post the details of this issue, including the equipment used to measure, pictures of the install, and screenshots of the various observations in order for me to speculate on this. I am confident it will turn out to be an installation issue, but there is also the possibility that you have phase-to-phase loads which do require the A-B, B-C, or C-A reference. Unless you have a three-wire service, that should not be the case with the mains.