To measure a 3-phase, 3-wire Delta service, eGauge utilizes the two-watt meter method of power monitoring based on Blondel's Theorem. Blondel’s Theorem is basically a rule for figuring out how many current sensors you need to measure power correctly.
Think of it this way:
Count the wires carrying electricity, then subtract one.
For example:
- 2-wire system = 1 CT
- 3-wire system = 2 CTs
- 4-wire system = 3 CTs
In the following examples, each measurement element represents a CT and a voltage input to the eGauge meter.
Example: Three-phase, four-wire system
Imagine a building has:
- Phase A
- Phase B
- Phase C
- Neutral
That is four wires, so Blondel’s Theorem says you need three measurement elements. The CTs are installed on Phase A, Phase B, and Phase C. A CT is not needed on the neutral.
In this type of installation we also need three Voltage readings, all in reference to Neutral.
- Phase A to N
- Phase B to N
- Phase C to N
The meter combines the measurements from the three phases to calculate the total power used by the building. Any imbalance that causes current to flow on the neutral is already reflected in the three phase measurements.
Example: Three-phase, three-wire system
Imagine a building has:
- Phase A
- Phase B
- Phase C
That is three wires, so Blondel’s Theorem says you need two measurement elements. The CTs are installed on Phase A and Phase B. Phase C is used as the common and does not need a CT.
In this type of installation we also need two Voltage readings. Because there is no Neutral the voltages are relative to the third (common) phase:
- Phase A to Phase C
- Phase B to Phase C
The meter combines the measurements from the two monitored phases to calculate the total power used by the building. Any imbalance that causes current to flow on the third phase is already reflected in the measurements of Phase A and Phase B.
"If you're not measuring the power on Phase C, how could you possibly know the total?"
In a three-wire system, there is no Neutral. So, we know:
(Phase A Current) + (Phase B Current) + (Phase C Current) = 0
We also know:
Current in = Current out
The two measurement elements are measuring the system from a common reference, and the currents in a three-wire system have to balance. When you add the two measurements together, the parts associated with that common reference cancel out mathematically, leaving the power of the entire load as the result.