Operating Instructions 45
Current
The WattNode Modbus meter measures the RMS current for each phase. This is an indirect
measurement and does not include all harmonic content, so the current is not as accurate as the
power and energy measurements.
CurrentA, CurrentB, CurrentC
Technically, AC current does not have a sign (positive or negative), but the WattNode meter sets
the sign of the current to match the sign of the real power for the same phase. For example, if the
power on phase A is negative, then the current for phase A (
CurrentA
) will also be negative.
The floating point current registers are in units of amps. The integer current registers are in scaled
amps (
, default value 20000), so the following equations will convert to amps.
Ia = CurrentAInt * CtAmpsA / CurrentIntScale
Ib = CurrentBInt * CtAmpsB / CurrentIntScale
Ic = CurrentCInt * CtAmpsC / CurrentIntScale
For example, with 200 amp current transformers and
CurrentIntScale
= 20000, if
CurrentAInt
(
1351
) reports 5000, the actual current is 5000 * 200 / 20000 = 50.00 amps.
Demand
Demand is defined as the average power over a specified time interval. Typical demand intervals
are 5, 10, 15 (default), 30, 60, etc. up to 720 minutes, but the WattNode meter supports arbitrary
demand intervals from 1 to 720 minutes (12 hours). The meter records the peak demand for
metering applications where the measurements may only be accessed weekly or monthly.
Since the WattNode meter can measure bidirectional power (positive and negative), and the
demand is the average power over an interval, demand can also be positive or negative. This is
only likely to occur with something like a grid-tied PV system, where you may put energy back
into the grid at certain times of the day (negative power). In this case, you would see negative
demand. If you have both positive and negative power during a demand interval, both the positive
and the negative data will be averaged together, such that the negative power subtracts from the
positive, reducing the overall demand.
P
o
w
e
r (
w
a
tt
s)
Demand
Interval
Demand
Interval
Demand
Interval
Demand
Interval
Demand
Interval
Power
Curve
Demand
Figure 10: Demand Measurement
WattNode meters also supports rolling demand (also called “sliding window”), in which the
demand intervals are evenly divided into a fixed number of subintervals. At the end of each
subinterval, the average power over the demand interval is computed and output. This results in
better accuracy, especially for demand peaks which would not have lined up with the demand
interval without subintervals. On power up, the demand measurements will report zero until one
full demand interval is completed. From 1 to 10 subintervals are supported. A subinterval count of
