Connection Diagram: Wiring a Digital Wattmeter
A wattmeter has two circuits. The current circuit (I terminals) carries the load current and is wired in series with the load. The voltage circuit (V terminals) senses the supply voltage and is wired in parallel (across) the load or supply. Power is the product of the two, so getting either one wrong gives a wrong or reversed reading.
Steps
- Switch off and isolate the supply.
- Connect the Line to the I (in) terminal, and I (out) onward to the load. This puts the current coil in series.
- Connect the V (in) terminal to the same Line point as I (in), and V (out) to Neutral. This puts the voltage coil across the supply.
- Connect the auxiliary supply terminals if the model has a separate one (see Meaning of Auxiliary Supply).
- Check polarity and tighten all terminals, then energise and confirm the display reads a positive value.
Using a CT for higher currents
If the load current is above the meter's direct range, pass the load conductor through a current transformer (CT) and connect the CT secondary (usually 5 A or 1 A) to the I terminals. Set the CT ratio in the meter's set-up so the display shows the actual primary power.
Safety: never open the secondary of a CT while the primary is carrying current. Dangerous voltages can appear across the open secondary.
Terminal names and numbers differ by model, so check the wiring label on the unit and the datasheet before connecting.