Calculating A Sample Power Requirement - Siemens SIMATIC S7 System Manual

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
CPU SR40 AC/DC/Relay
System requirements
CPU SR40, 24 inputs
Slot 0: EM DR08
Slot 1: EM DR08
Slot 2: EM DR08
Slot 3: EM DE08
Total requirements
Current balance
Current balance total
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
5 V DC
1400 mA
minus
5 V DC
120 mA
120 mA
120 mA
105 mA
465 mA
equals
5 V DC
275 mA
Calculating a power budget
B.2 Calculating a sample power requirement
24 V DC
300 mA
24 V DC
24 * 4 mA = 96 mA
8 * 11 mA = 88 mA
8 * 11 mA = 88 mA
8 * 11 mA = 88 mA
8 * 4 mA = 32 mA
392 mA
24 V DC
(92 mA)
789