DMS APPLICATION NOTE 23 - Murata Power Solutions

3-Wire, 240V Single-Phase Power Measurements
Detailed Instructions
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 configuration are provided in this application
1. Using a suitable measuring instrument, confirm 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 first 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.
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 figure 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 configurations described in this application note involve hazardous—and potentially lethal—voltages
and currents. Therefore, all associated work must only be performed by technically qualified personnel, while observing and adhering
to all applicable regulatory codes and product data sheets, and with all
AC power sources de-energized (OFF).
2. The other 120V leg (L2 in diagram) must be wired such that it first 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 flowing 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.
120 V & 240 V LOADS
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.
Polarity dot
on bottom
120 Vac
120 Vac
240 Vac (50/60 Hz)
Figure 1. Wiring diagram for 3-wire 120V/240V single-phase systems with neutral wire, monitoring total current and total power
here to view the ACM20 data sheet for more information.
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3-Wire, 240V Single-Phase Power Measurements
Precautions and limitations applicable to 3-wire, single-phase wiring
1. The ACM20’s VOLTS reading will always be that of leg L1 (120Vac), and not
the full 240Vac supply. The voltage of leg L1 is used for all power and power
factor calculations. Therefore, any voltage imbalances between legs L1 and
L2 will introduce WATTS reading errors, with the magnitude of these errors
being proportional to the voltage imbalance. In most applications, voltage
imbalance errors are usually relatively low. However, imbalance errors are in
addition to those specified in the ACM20 data sheet’s “Performance/Functional Specifications” section.
2. Due to the unique nature of this wiring configuration, Murata Power Solutions cannot guarantee that the ACM20 will meet all of its standard published
specifications. IMPORTANT: the sum of the currents flowing through the
ACM20’s built-in CT cannot exceed the ratings for that particular model’s
input range. For example, when using the 100A model in this configuration,
the sum of the currents in legs L1 and L2 cannot exceed 100 Amps.
Murata Power Solutions, Inc.
11 Cabot Boulevard, Mansfield, MA 02048-1151 U.S.A.
ISO 9001 and 14001 REGISTERED
3. The 100 Amp model (ACM20-4-AC1-R-C) should be used for this configuration because of its higher current rating and ability to display power in
Kilowatts. This particular model is rated for 100A, but has an overcurrent
capability of 150A.
4. Fusing the ACM20's TB1 input lines is always required; complete fusing
specifications and instructions are included in the ACM20 product data
sheet. For reference, a link to the ACM20 data sheet is provided here:
5. The products described in this application note are subject to Murata Power
Solutions’ “Operating Requirements” and the “Life and Safety Critical Applications Sales Policy” described at
6. Any mechanical or electrical modifications to the ACM20 (adding CT lead
extensions, use of substitute hardware or components, etc.), or failure to adhere to the data sheet’s specifications and instructions, will void its warranty
and safety-agency certification status.
Murata Power Solutions, Inc. makes no representation that the use of its products in the circuits described herein, or the use of other
technical information contained herein, will not infringe upon existing or future patent rights. The descriptions contained herein do not imply
the granting of licenses to make, use, or sell equipment constructed in accordance therewith. Specifications are subject to change without
© 2013 Murata Power Solutions, Inc.
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