Siemens SIMATIC S7 System Manual page 762

554BCalculating a power budget
Some of the 24V power input ports in the PLC system are interconnected, with a logic
common circuit connecting multiple M terminals. The CPU 24V power supply input, the SM
relay coil power input, and a non-isolated analog power supply input are examples of circuits
that are interconnected when designated as not isolated in the data sheets. All non-isolated
M terminals must connect to the same external reference potential.
Connecting non-isolated M terminals to different reference potentials will cause unintended
current flows that may cause damage or unpredictable operation in the PLC and connected
equipment.
Such damage or unpredictable operation could result in death, severe personal injury
and/or property damage.
Always be sure that all non-isolated M terminals in a PLC system are connected to the
same reference potential.
Information about the power budgets of the CPUs and the power requirements of the signal
modules is provided in the technical specifications (Page 657).
Note
Exceeding the power budget of the CPU may result in not being able to connect the
maximum number of modules allowed for your CPU.
Sample power budget
The following example shows a sample calculation of the power requirements for a
configuration that includes one CPU 1214C AC/DC/Relay, one SB 1223 2 x 24 VDC Input/ 2
x 24 VDC Output, one CM 1241, three SM 1223 8 DC In/8 Relay Out, and one SM 1221 8
DC In. This example has a total of 48 inputs and 36 outputs.
Note
The CPU has already allocated the power required to drive the internal relay coils. You do
not need to include the internal relay coil power requirements in a power budget calculation.
The CPU in this example provides sufficient 5 VDC current for the SMs, but does not provide
enough 24 VDC current from the sensor supply for all of the inputs and expansion relay
coils. The I/O requires 456 mA and the CPU provides only 400 mA. This installation requires
an additional source of at least 56 mA at 24 VDC power to operate all the included 24 VDC
inputs and outputs.
762
WARNING
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05