Siemens SIMATIC S7-1500 System Manual page 42

Infeed via CPU and system power supply
For larger hardware configurations, infeed into the backplane bus by the CPU alone no
longer suffices. If modules consume more than 10 or 12 W in total (depending on the CPU
used), insert an additional system power supply.
Supply the system power supply with the permissible supply voltage and the CPU with
24 VDC from a load current supply.
Both the system power supply and the CPU feed current into the backplane bus. The
supplied power is summed.
Power addition: "Infeed power of the system power supply" + "Infeed power of the CPU"
Parameter assignment of the CPU: In STEP 7, select the "Connection to supply voltage L+"
option in the "Properties" tab in the "System power supply" area navigation so that STEP 7
can calculate the balance correctly.
Figure 4-5
S7-1500 Automation System
System Manual, 02/2014, A5E03461182-AB
Supply voltage via the CPU and system power supply
Application planning
4.2 System and load power supply
41