Regarding the Number of CTs per branch circuit for HDPM in a configuration that takes single-phase branches from a three-phase, three-wire circuit

[T.Okada]

Hi, 

We are considering applying HDPM to a panel where the main bus is a three-phase, three-wire (R, S, T) circuit that branches into individual single-phase circuits. In this case, we believe that two CTs are required per branch circuit for power measurement. However, if we only use the current value and do not use other measurement values (even if they are not measured accurately), is it possible to use just one CT per branch circuit? If this is difficult due to specifications or settings, please let us know.

Regards，

@T.Okada 

[Mehran_Mehrnia]

Hi @T.Okada 

Your panel uses a three-phase, three-wire (Delta) main bus, and you are taking single-phase branches from it. This is an uncommon setup for general branch circuits.

Measuring the three-phase main circuit: A three-phase, three-wire main circuit requires two CTs (connected on any two of the three phases) for accurate power (kW, kWh) measurement using the Two-Wattmeter Method.

The primary goal of the HDPM6000 is power monitoring. It is fundamentally an energy and power quality meter designed to calculate real power (kW) and energy (kWh), which requires both voltage and current measurements.

If the connection is not single-phase, three-wire (split-phase) but instead a single-phase, two-wire branch circuit, for example, where branch wire 1 (Phase R) carries current to the load and branch wire 2 (Phase S) returns current from the load, you are dealing with one current path. In this case, using one CT is sufficient for measuring amperage, provided you configure the HDPM channel to reference the correct phase-to-phase voltage (e.g., R–S) so it can calculate kW and kWh based on that single current reading.

This one-CT rule works only for single-phase, two-wire circuits. It does not work for Three-Phase, Three-Wire Circuit (Main Bus).

Best Regards,

-Mehran

L3 Expert Advanced metering 

[Miyai]

Hi @Mehran_Mehrnia 

I'd like some advice on this matter.

I would like to measure V, I, W, and kWh of 42 circuits using configuration below, so I would appreciate some advice.

aa) If I want to measure 42 circuits, do I need to purchase an 84-circuit R unit? Or can a 42-circuit device handle it?

bb) Is it correct that I understand that one CT is required for one circuit as shown in the diagram below?

cc) Can I configure each circuit to measure power and energy separately? I've tried various settings but I can't get the power/energy consumption for each circuit to be displayed.

Best regards,

Miyai

[T.Okada]

Hi @Mehran_Mehrnia 

Thank you for your prompt reply.

I understand the setup for monitoring only the current on a single‑phase, two‑wire branch circuit. In this case, if I want to reference the R–S phase‑to‑phase voltage, do I need to use two CT ports on the HDPM6000R, or can I use just one? I believe one CT port would remain unused because the "Exp phase" setting cannot be configured to 1–2, 2–3, or 3–1; it can only be set to 1, 2, or 3.

Best Regards,

Okada

[Mehran_Mehrnia · Accepted solution]

Hi @Miyai @T.Okada 

When the meter is configured as a 3-phase Delta system, as shown in the installation guide, it (HEAD Unit) uses the Two-Wattmeter Method. In this method, phases 1 and 3 are measured relative to phase 2, which is connected to the reference N terminal:

aa) In the image you shared, each circuit uses two channels: one measuring the CT for phase 1 (L1–L2) and the other for phase 3 (L3–L2). Therefore, if you need to monitor 42 circuits, you will require 84 channels.

bb) In Delta mode, under Channel Cfg tab, each branch needs one CT in order calculate correctly the total W and kWh of the related 3 phase Delta circuit. Therefore you need to configure the channels as below. In this example, the 3 channel circuit is considered as circuit 1:

cc) As I tested, in 3 phase, Delta mode, it's not possible to have each channels' parameter individually:

In this configuration, only in 3-phase Wye mode, whether channels are bundled into a single circuit or configured individually, can the channel parameters be measured and displayed:

Best Regards,

-Mehran

L3 Expert Advanced metering

[Miyai]

Hi @Mehran_Mehrnia 

Thanks for your advice.

I understand.

Best regards,

Miyai

[Miyai]

Hi, @Mehran_Mehrnia 

I'd like some more advice.
This configuration is three-phase delta system.

aa) Is my understanding correct that each circuit from C1 to C3 requires 1CT?
      The measurement items are assumed to be V, I, W, PF, and kWh.

bb) If THDi measurement is required for each phase in the C1 to C3 circuit, are 2CTs needed for each circuit?

cc) Is there any problem with mixing C1 through C4 together?

dd) Regarding the C4 circuit, is 3CT required in the case of a three-phase branch?
The measurement items are assumed to be V, I, W, PF, and kWh.

Best regards,

Miyai

[Mehran_Mehrnia]

Hi @Miyai 

aa) Even though each of C1 to C3 is shown as a single‑phase branch derived from the delta bus, the way the HDPM6000 is designed means that each channel would need its own CT, although they would measure the same value.

bb) As mentioned above, 2 CTs are required.

cc) The channel configuration looks correct. Mixing circuits (C1–C4) is allowed in the HDPM6000 when each circuit is properly grouped and phase-mapped.

dd) Correct. For C4 (3-phase branch), as explained in aa, 3 CTs are recommended to enable full metering, including per-phase current, THDi, etc.

Best Regards,

-Mehran

L3 Expert Advanced metering

[Miyai]

Hi @Mehran_Mehrnia 

Thanks for your advice.

Do you know of any documentation on examples of HDPM6000 and CT combinations?

I checked the manuals for the head unit and R unit, but I couldn't find any examples of CT connection circuits.

Best regards,

Miyai

[Mehran_Mehrnia]

Hi @Miyai 

There is no detailed example-based CT wiring guide specifically for HDPM6000 in current manuals. Each measurement channel that is intended to monitor current per each phase must be connected to an appropriate CT input on the branch module.

Best Regards,

-Mehran

L3 Expert Advanced metering