S7-300 Power Configuration: 24VDC Supply and 5V Backplane Guide

Overview of S7-300 Power Architecture

An S7-300 station has three electrically isolated power domains that must be considered together when sizing a panel: the line-side PS 307 load power supply (120/230 VAC to 24 VDC), the CPU-internal 5 V backplane that feeds signal modules (SM), function modules (FM), and communication processors (CP) attached to the right of the CPU, and the 24 VDC load current consumed by the field-side input and output circuits of each SM and CP. Unlike S7-400, there is no separate backplane power supply to calculate because the CPU itself sources the 5 V needed by the right-side I/O bus. The result is that a properly configured S7-300 typically requires no manual power-budget calculation for the 5 V rail, but every output card and every CP still requires a fuse-protected 24 VDC feed sized to its nameplate current. The official hardware reference is the SIMATIC S7-300 Automation System, Hardware and Installation manual.

Prerequisites

• STEP 7 V5.5 or TIA Portal V13+ project with the target S7-300 station configured (rack 0, slot 1 PS 307, slot 2 CPU, slots 4–11 SM/FM/CP).
• Hardware catalog list of every module ordered with order number (6ES7 …), firmware version, and associated manual.
• PS 307 selection: 2 A (6ES7307-1BA01-0AA0), 5 A (6ES7307-1EA01-0AA0), or 10 A (6ES7307-1KA02-0AA0) load supply.
• 24 VDC field distribution terminals with group fuses (typically 8 A glass or blade).
• Multimeter capable of measuring 5.0 VDC ±0.5 V at the backplane and 24 VDC ±5 % at each SM terminal.

The Three Power Domains of an S7-300 Station

Domain 1 — PS 307 Line Supply

The PS 307 module converts the line voltage to 24 VDC and feeds the CPU through the backplane connector on slot 1. The CPU does not pass 24 V out to the modules on slots 4–11; instead, it converts that 24 V internally to the 5 V used by the I/O bus. Selecting the PS 307 is therefore based on the sum of all 24 V load currents drawn by SMs and CPs, not the 5 V budget.

Domain 2 — 5 V Backplane

Every CPU 31x provides a regulated 5 V rail for the right-side I/O bus. Typical maximum output current for the standard CPUs is 1.2 A (CPU 312/313/314/315-2 DP/317-2 DP), which is more than enough for any combination of up to eight SM/FM/CP modules. Per the S7-300 installation manual, every catalog module offered for the S7-300 has been validated to fit inside this budget, so you do not normally have to add 5 V currents manually. The exception is the ET200S distributed I/O, where the interface module IM 151 must be sized against its own power-feed modules (see Section 8).

Domain 3 — 24 VDC Load Supply

The 24 V field supply is what powers the input sensors wired to an SM 321, the output actuators wired to an SM 322, the encoder supply of an SM 338, and the front-end electronics of every CP. It is supplied from the same PS 307 (or a separate 24 V supply if galvanic isolation between field and logic is required) and is routed into the module through two or more grouped terminal pairs:

Sizing the 5V Backplane Current Budget

Although Siemens guarantees that any valid S7-300 configuration fits inside the 5 V budget, during detailed panel engineering you should still confirm the figure because some FM and CP modules draw close to the practical margin (≈ 200 mA each). Use the formula:

The 24 V values shown for the digital I/O are not the operating current of the input sensor or the output actuator — they are the maximum currents that the module itself routes through its internal 24 V bus, used to size the external fuse. For an SM 322 DO16 the 80 mA figure refers to the power dissipated inside the module electronics, not the switched load current; a separate 24 V feed is normally used to power the loads themselves through the output terminals.

24VDC Load Supply for Communication Processors

The CP 341 (point-to-point) and CP 343-1 Lean (Industrial Ethernet) modules each need an external 24 VDC connection to the front X1 connector. The connector pins are:

Without the external 24 V, the CP will power up on the 5 V backplane (so PROFINET/PROFIBUS diagnostics will appear) but the field-side interface will not function and the CP will report SF (group fault) and, on the CP 343-1 Lean, the LINK LED will remain off. Confirm this by measuring 24 V ±5 % directly at pin 1 of X1.

24VDC Supply for SM 321 / SM 322 / SM 323 Digital I/O

SM 321 digital-input and SM 322 digital-output modules expose grouped 24 V terminals on the front connector. The grouping is the mechanism that lets you wire two halves of an SM to different 24 V supplies so that a fault in one group does not pull down the other. A 16-channel SM has two groups (1L+/1M and 2L+/2M), a 32-channel SM has four (1L+/1M … 4L+/4M). The typical assignments for an SM 322 DO16×24V/0.5A are:

The 24 V terminal is internally bonded to the module's 24 V electronics supply, so each group must be fused. For an SM 322 DO16 the recommended group fuse is 8 A (slow-blow) at the panel, which protects the wiring while leaving headroom for simultaneous inrush on eight 0.5 A outputs. The 80 mA figure quoted by Siemens for the SM 322 is the internal 24 V current drawn by the module's driver stages — it is what the fuse on the module side must allow to pass; it is not what the loads draw.

Wiring example for SM 322 DO16

24VDC Supply for SM 331 / SM 332 Analog Modules

Analog modules have a single 24 V supply pair (1L+/1M) used to feed the front-end amplifiers, the transducer excitation (when supplied internally), and the sensor return path. For a 2-wire transmitter on an SM 331 AI8 the module's 1L+ powers the transmitter through the same terminal pair used for the analog input, which is why the analog input configuration in STEP 7 has a "Measuring range" tab that also controls whether 1L+ is used as the 24 V source. The 24 V current per analog input can be as high as 25 mA (current-input mode with internal supply) — multiply by the number of active channels and add 60 mA for the module's own consumption to obtain the 24 V load for the PS 307.

ET200S Distributed Backplane Power Sizing

ET200S is the only S7-300 family member where the 5 V backplane must be calculated. The IM 151 interface module feeds a separate backplane that can carry up to 30 electronic modules, each with its own 5 V consumption. The sizing rule from the ET200S manual is:

The IM 151-1 Standard provides about 700 mA on the 5 V backplane; the IM 151-1 High Feature provides about 1000 mA. Electronics modules typically draw 10–25 mA each, but motor starters and certain power modules can draw 40 mA or more. The 24 V load supply on ET200S is broken into segments by PM-E power modules, each of which is rated 10 A continuous for the segment it supplies. Always sum the worst-case inrush of all the loads fed by a single PM-E and select a slow-blow fuse that is below the 10 A rating but above the inrush peak.

Wiring Topology, Grounding, and TN-S Systems

The S7-300 hardware manual shows the canonical TN-S grounding topology: the PS 307 input M terminal is bonded to the protective earth (PE) at a single point inside the PS 307, so the 24 V negative rail is grounded. The 24 V return used by the SM field circuits must therefore also reference this same ground; do not float it. Use the green-yellow PE conductor from the panel star ground to the PS 307 PE terminal, and run a separate green-yellow bonding conductor from each DIN-rail segment to the same star ground. If you use a TN-C-S (PME) supply, ensure the neutral-earth bond is at the service entrance only; do not add a second N-E bond at the panel — this is the most common source of nuisance ground-fault trips on S7-300 panels.

Step-by-Step Power Commissioning Procedure

1. Verify the line voltage at the PS 307 input terminals (L1, N, PE) with the PS 307 de-energized. Confirm 120 VAC ±10 % or 230 VAC ±10 % as configured by the selector switch on the front of the PS 307.
2. Energize the PS 307 only (CPU not yet plugged). Measure 24 VDC ±5 % at the PS 307 output terminals. Current should be < 50 mA (PS 307 internal consumption).
3. Power down. Insert the CPU. Power up. Observe the CPU LEDs: SF off, BF off (no bus faults), DC5V on. Measure 5.0 V ±0.5 V between the backplane test points (CPU front, top row).
4. Insert each SM one slot at a time. After each insertion, check that SF stays off on the CPU and on the SM. If SF lights, remove the SM immediately and verify both the slot assignment in HW Config and the 24 V wiring on the front connector.
5. Apply 24 V to the SM terminal pairs (1L+/1M, etc.). Confirm the sensor-supply LED on the SM (if present) is on. Measure the current drawn from each group — it should be the sum of the sensor quiescent current plus the module's internal 24 V consumption (50–80 mA).
6. Insert and power each CP. Confirm the X1 24 V pin is at 24 V ±5 % and that the SF LED extinguishes within 5 s.
7. Download the STEP 7 / TIA Portal hardware configuration. From the online view, open PLC → Module Information → Diagnostic Buffer and confirm no "Module removed/inserted" or "External voltage missing" entries.

Verification and Measurement

After commissioning, perform the following four verification checks. Each one has a single numeric acceptance criterion so the result can be signed off without ambiguity:

Troubleshooting Matrix

Field-Proven Caveats

• The 24 V returned to the SM M terminals must be the same 24 V reference as the one powering the CPU's PS 307, otherwise ground-loop currents can flow through the backplane and produce random SF entries.
• When you mix relay output SMs (SM 322 DO8×Relay) with 24 VDC SMs, the relay modules do not require a 24 V supply for the contacts, but they still need 24 V at 1L+/1M to power the driver coils. Confirm this on the wiring diagram before commissioning.
• If you are using a CP 343-1 (not the Lean variant), the 24 V current at X1 rises to about 400 mA during link-up because the integrated switch powers up all eight ports. Use a 500 mA slow-blow fuse rather than the 200 mA used for the Lean.
• Do not run the 24 V field wiring in the same conduit as the 120/230 VAC line feeding the PS 307. Even with TN-S grounding, capacitive coupling can inject noise that the SM 321 will interpret as a sticky input.

FAQ