# Measuring three wire circuits in a two phase supply (120/208v)

I have an iotawatt on the way and am preparing to install it, but have been thrown for a bit of a loop in finding I have a two phase supply (2 phases + neutral, 120/208v) instead of a standard split-phase 120/240v supply.

I’m a little confused when it comes to measuring three wire circuits that use both 208v and 120v.

How is power measured for both voltages?

Assuming my phases are A+B:

A single CT on a phase conductor set to A-B for example will measure just 208 (phase to phase), but can’t properly account for 120v phase-neutral, especially on the unmeasured phase conductor.

Two CTs, one on each phase conductor set to each phase respectively, can measure the power for 120 phase-neutral on each phase, but how do they know to measure phase-phase at 208 and not 2x120=240?

Is there something other than just adding the two together that should be done in an output equation?

This applies to both 120/208 branch circuits, but also the mains monitoring as it’s essentially the same with the whole panel as a 120/208v load.

For referencing, I don’t have access to my meter (apartment building), I only get readings bi-monthly on my bill.

Maybe I’m wrong, but it seems like I should be using something along the lines of:

(A min B x (sqrt3)) + (A max B - (A min B))

Ie: (smallest of the two x 208v) + (largest minus smallest x 120v)

Good questions.

What you have essentially is a four-wire three-phase service absent one of the three phases. You will be using derived reference. Forget about split-phase, you should be using the Three-phase Power section of the documentation.

Install your VT on a 120V circuit, install your two mains, oriented the same way, as in the docs. Install all of your branch circuits, oriented the same way but opposite that of the mains.

Check “Enable derived three-phase” in the inputs setup. Phase A will be the phase to which the VT is connected. The other phase will be B or C, determined using the method described in Configuring Derived Reference. Once you do that, you will have the mains installed and your total power will be the sum of the two mains.

For the rest of this, I’ll assume the second phase is B. If it turns out that it’s C, then substitute C in the following discussion.

When derived reference is enabled, all of the CTs will have an additional “Mains Phase” selection that defaults to A. Identify the 120V circuits that are on phase B and select Mains Phase B (or C if your other main is C).

That will take care of your 120V branch circuits. The 240V circuits are a little different:

For two-wire circuits, use one CT and select Mains Phase A-B (or C-A if your other phase is C).

For three-wire circuits, treat it as two 120V circuits: Use two CTs, and set the Mains Phase to A for the A circuit and B (or C) for the other.

That’s it. I know it sounds complicated, but just follow step-by-step and it will work fine. The folks in Australia do derived three-phase all the time with no problems. Check back with any questions, and let me know how you make out.

“For three-wire circuits, treat it as two 120V circuits: Use two CTs, and set the Mains Phase to A for the A circuit and B (or C) for the other.”

I understand the setup; what confuses me is how power is actually calculated on these three wire circuits.

How does the math differ between split and two phase installations? Can you provide equation examples?

If I’m treating them as two 120v circuits, won’t phase to phase power be multiplied by 120v twice, ie 240v instead of the expected 208v?

Sry if I’m missing something obvious, really not familiar with three phase math.

It doesn’t differ, there is no special code for this, the phase of the current differs. Watts are the integration of voltage x current. When the current of any 120V component of the load flows to the neutral, it is in phase with the voltage reference. When the current of any 208V component of the load flows to the other phase, it is out of phase with the voltage reference by 30°. 240 x cos 30° = 208.

It’s baked in the cake. Each leg is measured independently. It just works.

1 Like