3-Wire, 240V Single-Phase Power Measurements
http://www.murata-ps.com/data/meters/acm20.pdf
Introduction
We are often asked if one ACM20 power meter can be used to measure the total
current and total real power (watts) in a 3-wire, single phase, 240V AC mains
supply with a neutral conductor and two 120V hot legs. The answer to this question
is a conditional “yes,” and instructions and limitations—along with important safety
precautions—pertaining to this type of confi guration are provided in this application
note.
This type of 3-wire single-phase system typically supplies power to either 120V
loads or 240V loads, or any combination of the two. A wiring diagram showing
connections for a typical installation is shown in fi gure 1 below. The supply wire
colors in the diagram (red, black, and white) were chosen for reference only and
can vary depending on the installation. Also, while the diagram illustrates a 240V
mains supply, it’s applicable to most 3-wire, single phase, AC mains in the range
of 170Vac to 264Vac (47-63Hz). Detailed instructions for the wiring diagram are
provided below.
Please note, the wiring confi gurations described in this applica-
tion note involve hazardous—and potentially lethal—voltages
and currents. Therefore, all associated work must only be per-
formed by technically qualifi ed personnel, while observing and adhering
to all applicable regulatory codes and product data sheets, and with all
AC power sources de-energized (OFF).
!
Detailed Instructions
1. Using a suitable measuring instrument, confi rm that the 240Vac power
source is de-energized (OFF) prior to making any connections to the ACM20
and all associated loads. To measure the total current and total real power
in this type of system, connect the ACM20's rear TB1-A terminal to the
neutral conductor and its TB1-B terminal to the 120V hot leg labeled L1 in
the diagram. For proper current transformer (CT) phasing, the 120V leg L1
connected to TB1-B must fi rst be routed through the ACM20's built-in CT,
with its white polarity dot facing down, and then to the loads it is feeding,
following the orientation shown in the diagram. It is extremely important to
make sure the CT wiring orientation for both 120V legs is correct.
2. The other 120V leg (L2 in diagram) must be wired such that it fi rst passes
through the CT from top to bottom and then to the loads it's feeding, i.e., in
a direction opposite to leg L1. This is to ensure proper CT phasing, which
in turn will ensure that the two leg currents will be added correctly, and
not subtracted. If the phasing of leg L1 with respect to TB1, or L1 to L2
legs is incorrect, the WATTS and AMPS readings will have very large errors,
regardless of how much current is fl owing in either leg. If the watts and
Amps readings appear to be much lower than anticipated, turn the AC power
source off and carefully check for correct CT phasing.
Figure 1. Wiring diagram for 3-wire 120V/240V single-phase systems with neutral wire, monitoring total current and total power
L1
Neutral
L2
120 V & 240 V LOADS
B
A
TB1
Fuse*
0.5A/250V
ACM20-4-AC1-R-C
Note: If the ACM20 displays zero
Watts with power applied to the
load, reverse the direction of the
wire passing through the CT’s
center hole. See Technical Note 6*
for more information.
Epoxy
encapsulation
Polarity dot
on bottom
240 Vac (50/60 Hz)
120 Vac
120 Vac
*Click here to view the ACM20 data sheet for more information.
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DMS APPLICATION NOTE
23
DMS_AN_23_AppNote
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