Can I measure a 240 VAC two-wire (no neutral) U.S. residential circuit with one CT?
WattNode® meters reference their measurements to neutral or earth ground if there is no neutral. So we recommend monitoring both lines and using two CTs for best accuracy. However, if there is no neutral (or no neutral current), then the current is identical in both wires. The voltage (relative to ground or neutral) will be nearly (but not exactly) identical for both phases. So by using one CT, you will measure one-half of the load. You can double the results (or double the effective CT rated amps) to correct for this.
Option One: Neutral Available
If neutral is available, you can monitor one-half of the load by using the WNB-3Y-208-P, WNC-3Y-208-FT10, or WNC-3Y-208-MB models (they require neutral for the internal power supply). Connect ground and neutral as usual, then connect one of the two lines (L1 or L2) to the WattNode meter ØA (phase A) input. Place a CT around the same line being monitored by ØA and connect the CT to the ØA CT input.
You can use the phase B and C inputs to monitor two additional loads if desired. The WattNode Pulse, Option P3 allows each phase to be monitored independently. The WattNode Modbus and WattNode LonWorks meters provide per-phase outputs, which allow each phase to be independently measured.
- You will need to scale the readings to correct for only using one CT. See Scaling below for details.
- See Issues below for notes on the accuracy of this approach and other issues.
Option Two: No Neutral
If there is no neutral present, you must use a “delta” model that is powered from the phase A ØA and phase B ØB terminals. The WNB-3D-240-P, WNC-3D-240-FT10, or WNC-3D-240-MB models can measure a 240 Vac two-wire circuit, by connecting L1 to phase A and L2 to phase B. Both the ØA and ØB Vac inputs must be connected to power the WattNode meter. You can then use a single CT on either phase A or B.
You can also use this configuration to monitor multiple loads, but it is a bit more tricky, because you need to take care to match the CT phase to the voltage phases (or reverse the CT orientation).
Scaling
- For pulse output WattNodes, compute the scale factors as though the CT rated amps were double the actual value.
- For LonWorks and Modbus WattNodes, you can enter a CtAmps value that is double the actual value. This will result in correct values for power and energy, but the reported current with be double the actual value.
- Alternatively, you can post-scale the reported power and energy values by a factor of two.
Issues
- Your accuracy may not be quite as good if the L1 voltage doesn’t match the L2 voltage. Generally, the additional error will be one-half the percentage difference between L1 and L2. For example:
L1 = 119 Vac
L2 = 121 Vac
L1 to L2 % Difference = 1.68%
L1+L2 = 240 Vac
Current = 5 amps
Power = 240 * 5 = 1200 watts
WattNode Phase A Power = 119 * 5 * 2 = 1190 watts
Phase A Power Error = 0.83%
- Note: This power error is only the additional error due to monitoring just one-half of the load. It does not include other errors such as normal WattNode and current transformer accuracy.
- You cannot use a single CT with 208 Vac two-wire circuits with 120 degrees between each phase. For these circuits, you MUST use two CTs.