16 Installation
Selecting Current Transformers
The full-scale rated current of the CTs should normally be chosen somewhat above the maximum
current of the circuit being measured (see
below for more details). In some
cases, you might select CTs with a lower rated current to optimize accuracy at lower current
readings. Take care that the maximum allowable current for the CT can not be exceeded without
tripping a circuit breaker or fuse; see
We only offer CTs that measure AC current, not DC current. Significant DC current can saturate
the CT magnetic core, reducing the AC accuracy. The most loads only have AC current, but
some rare loads draw DC current and may not be measured correctly. See our website for more
information:
http://www.ccontrolsys.com/w/DC_Current_and_Half-Wave_Rectified_Loads
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, loop the
wire through the CT five times. The CT is now effectively a 1 amp CT instead of a 5 amp CT. The
effective current rating of the CT is the labeled rating divided by the number of times that the wire
passes through the CT.
If you are using the measurement phases of the WattNode (
Ø
A
,
Ø
B
, and
Ø
C
) to measure dif-
ferent circuits, you can use CTs with different rated current on the different phases. Instead of
setting one
CtAmps
value for all phases, you can use different values for each phase:
CtAmpsA
,
CtAmpsB
, and
CtAmpsC
.
Current Crest Factor
The term “current crest factor” is used to describe the ratio of the peak current to the RMS cur-
rent (the RMS current is the value reported by multimeters and the WattNode meter). Resistive
loads like heaters and incandescent lights have nearly sinusoidal current waveforms with a crest
factor near 1.4. Power factor corrected loads like computer power supplies typically have a crest
factor of 1.4 to 1.5. Many common loads can have current crest factors ranging from 2.0 to 3.0,
and higher values are possible.
The meter current transformer inputs will clip and become inaccurate if the peak current is too
high. This means you may want to be conservative in selecting the CT rated current. For example,
if your load draws 10 amps RMS, but has a crest factor of 3.0, then the peak current is 30 amps.
If you use a 15 amp CT, the meter will not be able to accurately measure the 30 amp peak cur-
rent.
Note:
this is a limitation of the meter measurement circuitry, not the CT.
The following graph shows the maximum RMS current for accurate measurements as a function
of the current waveform crest factor. The current is shown as a percentage of CT rated current.
For example, if you have a 10 amp load with a crest factor of 2.0, the maximum CT current is
approximately 85%. 85% of 15 amps is 12.75, which is higher than 10 amps, so your measure-
ments should be accurate. On the other hand, if you have a 40 amp load with a crest factor of
4.0, the maximum CT current is 42%. 42% of a 100 amp CT is 42 amps, so you would need a
100 amp CT to accurately measure this 40 amp load.
hole (3.2 mm). Or you may use hook-and-loop fasteners. If you use screws, avoid over-tightening
which can crack the case. If you don’t use the supplied screws, the following sizes should work
(
bold
are preferred); use washers if the screws could pull through the mounting holes
Screw Style
U.S.A. UTS Sizes
Metric Sizes
Pan Head or Round Head
#6,
#8
, #10
M3.5,
M4
, M5
Truss Head
#6
, #8
M3.5
, M4
Hex Washer Head (integrated washer)
#6,
#8
M3.5,
M4
Hex Head (add washer)
#6,
#8
, #10
M3.5,
M4
, M5
Table 2: Mounting Screws
