Siemens SIMATIC S7 System Manual page 22

Installation
When planning your layout for the S7-1200 system, allow enough clearance for the wiring
and communications cable connections.
①
②
Power budget
Your CPU has an internal power supply that provides power for the CPU, the signal
modules, signal board and communication modules and for other 24 VDC user power
requirements.
Refer to the technical specifications (Page 279) for information about the 5 VDC logic budget
supplied by your CPU and the 5 VDC power requirements of the signal modules, signal
board, and communication modules. Refer to the "Calculating a power budget" (Page 321) to
determine how much power (or current) the CPU can provide for your configuration.
The CPU provides a 24 VDC sensor supply that can supply 24 VDC for input points, for relay
coil power on the signal modules, or for other requirements. If your 24 VDC power
requirements exceed the budget of the sensor supply, then you must add an external 24
VDC power supply to your system. Refer to the technical specifications (Page 279) for the 24
VDC sensor supply power budget for your particular S7-1200 CPU.
If you require an external 24 VDC power supply, ensure that the power supply is not
connected in parallel with the sensor supply of the CPU. For improved electrical noise
protection, it is recommended that the commons (M) of the different power supplies be
connected.
22
Side view
Horizontal installation
③ Vertical installation
④ Clearance area
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02