Siemens SIMATIC S7-300 Hardware And Installation Manual page 58

Configuring
How to determine load current
The required load current is determined by the sum load current of all sensors and
actuators connected to the outputs.
In case of short-circuit the DC outputs are briefly loaded with twice to three times
the rated output current before the clocked electronic short-circuit protection comes
into effect. Thus, you must consider this increased short-circuit current when
selecting your load power supply unit. Uncontrolled load power supplies usually
provide this excess current. With controlled load power supplies - especially for low
output power (up to 20 A) - you must ensure that the supply can handle this excess
current.
Example: S7-300 with load power supply unit from PS 307
The figure below shows the overall S7-300 configuration (load power supply unit
and grounding concept), with TN-S mains supply.
Apart from powering the CPU, the PS 307 also supplies the load current for the 24
VDC modules.
Note: The arrangement displayed does not correspond with the physical
arrangement; it was merely selected to give you a clear overview.
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S7-300 Programmable Controller Hardware and Installation
A5E00105492-02