Siemens Simatic S7 Series

Calculating a power budget
B.2 Calculating a sample power requirement
S7-200 SMART
System Manual, 09/2015, A5E03822230-AC
655
B.2
Calculating a sample power requirement
Calculating a sample power requirement
The following table shows a sample calculation of the power requirements for a CPU that
includes the following:
● CPU SR40 AC/DC/Relay
● 3 each EM Digital Output 8 x Relay (EM DR08)
● 1 each EM Digital 8 x Inputs (EM DE08)
This installation has a total of 32 inputs and 40 outputs.
Note
The CPU has already allocated the power required to drive the CPUs 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 V DC current, but does not provide enough
24 V DC current from the sensor supply for all of the inputs and expansion relay coils. The
I/O requires 392 mA and the CPU provides 300 mA. This installation requires an additional
source of at least 92 mA of 24 V DC power to operate all the included 24 V DC inputs and
outputs.
Table B- 1 Calculation of the power budget for a sample configuration
CPU power budget
5 V DC
24 V DC
CPU SR40 AC/DC/Relay
1400 mA
300 mA
minus
System requirements
5 V DC
24 V DC
CPU SR40, 24 inputs
24 * 4 mA = 96 mA
Slot 0: EM DR08
120 mA
8 * 11 mA = 88 mA
Slot 1: EM DR08
120 mA
8 * 11 mA = 88 mA
Slot 2: EM DR08
120 mA
8 * 11 mA = 88 mA
Slot 3: EM DE08
105 mA
8 * 4 mA = 32 mA
Total requirements
465 mA
392 mA
equals
Current balance
5 V DC
24 V DC
Current balance total 275 mA (92 mA)