If I use a delta WattNode® model like the WNC-3D-240-MB to monitor a circuit with neutral, do I need to connect neutral?

Yes, for accurate results, you must connect neutral if your load or panel has a neutral connection. More specifically, if any neutral current can flow, you must connect neutral.

However, if your service does not provide neutral or your load does not connect to neutral, then you do not need a neutral connection. However, we still recommend connecting the neutral N terminal on the WattNode meter to safety ground for maximum accuracy. See the manual for details and circuit diagrams. Note that the terminal labeled N on delta (3D) model WattNode meters is not used to power the meter—it is only used as a common measurement reference point.

Why?

According to Blondel’s theorem, if power is flowing through a system of N wires, you can measure the power with N meter elements. You can reduce this to (N – 1) meter elements, provided the common voltage measurement point is one of the N wires.

In the case of the WattNode meters (WNB and WNC series), the neutral connection is the common voltage point for the three meter elements. If you have a three-phase four-wire load (the fourth wire being neutral), then because you have four wires, you need three meter elements (WattNode meter phases and CTs) assuming that neutral is the common voltage point. If you do not connect neutral, you would need four meter elements, the fourth to measure the power flowing in the neutral conductor.

In practice, the error due to not connecting neutral will be very small if the neutral current is small and if the three phase voltages are very well balanced. As the neutral current increases or the phase voltage imbalance increases, not connecting neutral will cause more significant errors.

True three-wire delta services without a neutral conductor do not require a neutral connection, because there are three wires and three meter elements in the WattNode meter (one for each phase).