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Overview
Overview
WattNode
Congratulations on your purchase of the WattNode
®
, the most compact instrumentation-grade watt/watt-
hour transducer available. Using state-of-the-art ASIC and surface mount components, the WattNode offers
precision energy and power measurements in a compact package. The WattNode enables you to make
precise power and energy measurements from within existing electric service panels avoiding the costly
installation of subpanels and associated wiring. The WattNode is designed for use in demand side
management (DSM), sub-metering, and energy monitoring applications where accuracy at reasonable cost
is essential. The WattNode outputs a stream of pulses whose frequency is proportional to the instantaneous
power and whose count is proportional to total watt-hours. Models are available for single-phase, three-
phase wye and three-phase delta configurations for voltages from 120 VAC to 600 VAC at 50 to 60 Hz.
Current Transformers
The WattNode uses either toroidal or split-core (opening) current transformers (CTs). The WattNode
requires CTs with burden resistors generating 0 – 0.333 VAC. Split-core CTs offer greater ease of
installation, because they can be installed without disconnecting the circuit being measured (although
connecting the voltage terminals on the WattNode requires that at least one circuit in the service panel be
turned off). Toroidal CTs are more compact, more accurate and less expensive, but installation requires
that the measured circuit be disconnected.
The rated current of the CTs should normally be chosen at or above the maximum current of the circuit
being measured. However, if the circuit will normally operate at some fraction of the maximum current and
greatest accuracy is desired at the normal operating power levels, then a CT rated somewhat above the
normal operating current may be a better choice. Take care that the maximum allowable current for the CT
can not be exceeded without tripping a circuit breaker or fuse (see Table 4: Scale Factors) WattNode can
measure up to 150% of rated maximum power for the chosen voltage range and CT, but accuracy will
suffer as the CT’s core begins to saturate. CTs are also nonlinear at very low power levels and may report
less than the true current.
CTs can measure lower currents than they were designed for by passing the wire through the CT more than
once. For example, to measure currents up to 1 amp with a 5 amp CT, pass the wire through the CT once,
then loop back around the outside of the CT, and pass the wire through the CT again. Repeat until the wire
passes through the CT 5 times. The CT is now effectively a 1 amp CT instead of a 5 amp CT. In general,
the current rating of the CT is divided by the number of times that the wire passes through the CT.
Optoisolator Output
The pulse output of the WattNode passes through an optoisolator to provide 2500 volts of isolation. This
allows the WattNode to be interfaced to monitoring or data logging hardware without concern about
interference, ground loops, shock hazard, etc.
The standard output frequency WattNodes (2.667Hz and 4.000Hz) produce a square-wave output with a
duty cycle very close to 50%. The higher output frequency models generate a less regular output
waveform, since variations in the measured power during a single cycle appear in the output. The medium
output frequency WattNodes (193.3Hz and 290.0 Hz) produce a varying square-wave with random
fluctuations from 25% to 75% duty cycle. The highest output frequency WattNodes (773.3Hz and 1160Hz)
produces a waveform with narrow pulses and varying pulse spacing.Installation
