Measurement categories are defined by the CE and UL safety standard IEC 61010-1 and are used to indicate the ability of an instrument (like the WattNode^® meter) to withstand voltage spikes without posing a shock hazard to the operator. An instrument should only be used at or below its rated measurement category and voltage. For example, most WattNode models are CAT III rated and can be used to monitor CAT I, CAT II, and CAT III circuits.

• CAT I: This category is for measurements of circuits not directly connected to the mains. This is generally not relevant for the WattNode since it monitors mains circuits.

• CAT II: This category is for measurements of branch circuits that power plug loads such as household appliances, portable tools, and similar equipment that plugs into a outlet

• CAT III: This category is for measurements performed on equipment permanently installed in the building. Examples are measurements on motor control centers, circuit-breaker panels, junction boxes, switches, lighting fixtures, and wiring, including cables, bus-bars, and equipment for industrial use and some other equipment, for example, stationary motors with permanent connections to the electrical distribution system.

• CAT IV: This category is for measurements performed at the service entrance or on power lines (overhead or buried).

There are situations where the WattNode meter needs to monitor the main service entrance, which is classified as CAT IV. In this situation, the WattNode voltage inputs (green screw terminal block) must be connected in a breaker panel or panel board, downstream of a the main disconnect, breaker, or fuses. The line voltages at this location will be essentially identical to the voltages measured at the main disconnect, so the WattNode measurements will be accurate. There is another reason for connecting the WattNode voltage inputs downstream of the service entrance. The largest wire gauge that the voltage inputs terminal block will accept is #12 AWG and there is usually no safe way to connect this small of a wire to the large terminations (lugs) inside the main panel