Sizing the Siemens ET 200S PM24 Power Module (6ES7138-4CA01-0AA0) for I/O Modules

The SIMATIC ET 200S distributed I/O system uses the Power Module (PM) PM-E DC24V, ordered as 6ES7138-4CA01-0AA0 (commonly called "PM24"), to feed the 24 V load supply that powers digital-output actuators, the backplane power-rail, and any auxiliary terminals. The PM does not supply the PROFIBUS/ PROFINET interface module (IM) nor the encoder supply for some analog modules — it only bridges the external 24 V onto the power segments behind the terminal modules. Because every I/O module downstream draws from that segment, the maximum number of modules that can sit behind one PM is governed by the per-module and per-channel current draw, not by a hard module count.

1. PM24 Electrical Envelope

The 10 A figure is the headline number. It is not a "number of modules" rating — it is the maximum continuous load current that the PM's internal bridging contacts and the 24 V power-rail in the terminal modules can carry. Exceeding it trips no electronic fuse; instead, the spring/ screw terminals overheat, the bridging contacts lose contact integrity, and the downstream modules brown-out unpredictably.

2. Power Module vs. Load Group — The Concept Engineers Confuse

The ET 200S backplane has two electrical layers:

1. Backplane / electronic supply (logic): Carried on every terminal module; powers the module's own electronics. Sourced from the interface module (IM) — not the PM.
2. 24 V load supply (power-rail): Bridged through the PM and through every terminal module that follows it, until the next PM or a load-group break terminal is inserted. Digital outputs, AO current/voltage outputs, and any sensor-load terminal draw from this rail.

Insert a 6ES7193-4CK20-0AA0 (TM-P15S23-A1, 24 V load group break, AUX1 bridged) or open the AUX1 jumper to interrupt the power-rail and start a fresh segment. Everything to the right of the break is a new load group, requiring its own PM if it needs power.

3. Reference Current Draws of Common I/O Modules

The values below come from the ET 200S operating instructions, Chapter 3.4 ("Power modules") and the per-module datasheets. Use them as the basis for the per-PM budget calculation.

Two takeaways from the table:

• Digital input (DI) modules draw nothing from the load-rail — only logic from the IM backplane. You can stack ten 8DI modules (6ES7131-4BF00-0AA0) behind a single PM without ever looking at the 10 A budget for the inputs themselves.
• Digital output (DO) modules are what consume the load-rail. A fully loaded 6ES7132-4BF00-0AA0 pulls 4 A from the PM.

4. Per-Module Load Calculation

For every module in the load group, sum the worst-case load current. Use the rated per-channel value, not the average operating current, to stay on the safe side:

I_PM_total = Σ (n_channels × I_per_channel_rated) + Σ I_module_other_loads

Then enforce the constraint:

I_PM_total ≤ 10 A

The constraint that the source explicitly calls out is that three 8DO/0.5A modules (3 × 4 A = 12 A) would theoretically overload the PM. TIA Selection Tool conservatively flags an overload at five 8DO/0.5A modules (5 × 4 A = 20 A) because it sizes against the rating rather than the worst-case coincident load — see Section 7.

5. Worked Sizing Examples

Example A — 10 × 8DI (6ES7131-4BF00-0AA0) on one PM

Inputs only. Total load from PM rail = 0 A. The PM only needs to carry the optional sensor/encoder aux that you wire separately to the AUX terminals of each TM. Fully compatible with PM24 — no current-budget concern.

Example B — 5 × 8DO/0.5A (6ES7132-4BF00-0AA0) on one PM

Worst-case simultaneous load = 5 × (8 × 0.5 A) = 20 A. This exceeds 10 A on the rating sheet and is what the TIA Selection Tool flags red. Invalid configuration by rating — even though a real load will rarely draw 20 A.

Example C — 2 × 4DO/2A on one PM

Worst-case = 2 × (4 × 2 A) = 16 A. Exceeds 10 A on rating sheet. TIA Selection Tool will also flag this. Invalid by rating.

Example D — Mixed: 4 × 8DO/0.5A + 2 × 4DO/2A + 8 × 8DI

Load from PM = 4 × 4 A + 2 × 8 A + 0 A = 32 A. Far over 10 A. This is what the source engineer hit; the TIA Selection Tool would force additional PMs or load-group breaks. Reality will of course draw less, but the PM cannot deliver it.

Example E — Field-observed: 8 × 8DO/0.5A working in the field

Field draw (measured with a clamp meter at the PM terminals) was likely 1–3 A per module rather than 4 A. The actuators were not all firing simultaneously at 0.5 A. The PM survived because the thermal rating is the binding constraint (the spring terminals and bridging contacts can hold 10 A continuously and can over-current for short duty cycles up to ≈15 A). The configuration is still out-of-spec by rating — see Section 7.

6. Auxiliary 24 V Supply for Sensors on DI Modules

A common question in the source thread: where does the 24 V for the sensors wired to a DI module come from? Two valid options:

1. From the same PM via the AUX terminals on the terminal module. The AUX1 (and AUX2 on wider TMs) rails are bridged through the PM and through every terminal module, just like the load supply. This means the sensor current adds to the PM budget.
2. From an external 24 V source wired to a feed-in terminal module (e.g., 6ES7193-4CG20-0AA0 TM-C15S23-A1, AUX1 fed) that breaks the AUX rail but not the backplane. The PM's 10 A is unaffected.

Best practice for DI-heavy stations: keep sensor supply on a separate feed terminal, isolated from the PM, so the PM only feeds the DO load group. This lets the same station accept 10 × 8DI + many DOs behind one PM without the sensor current polluting the budget.

7. Why the TIA Selection Tool and the Field Can Disagree

The source highlights a real engineering puzzle: TIA Selection Tool flags a station with five 6ES7132-4BF00-0AA0 modules as overloaded, yet the same engineer has eight running in production.

The TIA Selection Tool is the design-time tool and is conservative on purpose. It sizes every channel at its worst-case rating so the resulting bill of materials is guaranteed to work regardless of the load. A working eight-module station in the field means the actual coincident load is below 10 A, not that the design is compliant. If you ever put the same station in a 50 °C cabinet with all actuators simultaneously latched, the PM terminals will fail.

8. Configuring ET 200S in TIA Portal (S7-300 / S7-400 / S7-1500)

When you wire the PM into a STEP 7 project, the size of the load group is what the engineering tool checks. The procedure is documented in the TIA Portal ET 200S configuration example and the SIMATIC ET 200S Operating Instructions (Chapter 3.4 — Power modules).

1. In the TIA Portal project tree, open Devices & Networks and add an IM (e.g., 6ES7151-1AA05-0AB0 IM151-1 for PROFIBUS or 6ES7151-3BA60-0AB0 IM151-3 PN for PROFINET).
2. Open the Device view of the ET 200S station. Slot 1 is the IM; slot 2 is conventionally the first PM.
3. Drag the 6ES7138-4CA01-0AA0 from the catalog into the next free slot. Wire the 24 V supply to the front terminals of the PM.
4. Add I/O modules (e.g., 6ES7131-4BF00-0AA0 8DI, 6ES7132-4BF00-0AA0 8DO) downstream of the PM until you change the load group or end the station.
5. Right-click the station and select Properties → Power segment view. TIA lists the worst-case load drawn by every module from each PM. A red bar indicates the 10 A budget is exceeded.
6. To start a fresh load group, insert a load-group break terminal module between two output modules and add a second PM after the break.
7. Compile the hardware configuration. The IM is auto-numbered with the new station address; the analog modules gain channel-level parameters and diagnostic addresses.

9. Verification and Commissioning

After physical install and before handing the station over, verify the PM sizing with a four-step checklist:

1. Visual: Confirm the PM is at slot 2, terminal screws torqued (0.5–0.6 Nm), wire end-sleeves fitted.
2. Voltage check: With the station powered and the IM in RUN, measure 24 V (±5 %) on the output side of the PM with a calibrated DMM.
3. Worst-case load test: Manually force every digital output ON, drive every analog output to its rating, and clamp the 24 V feed to the PM with a true-RMS clamp meter. The reading must be < 10 A continuous.
4. Thermal soak: With the load test still active, run the cabinet at 50 °C ambient for 4 h. PM terminal temperature should stay below 70 °C (most silicone-jacketed wire is rated 90 °C; terminals are the limit). Use a thermal imager or thermocouple on the bridging contact housing.

Diagnostics from the PM

10. Troubleshooting Matrix for PM24 Issues

11. Migration Notes — From ET 200S to ET 200SP / ET 200MP

For new projects, the recommended migration target is the SIMATIC ET 200SP (PROFINET-centric, 3 A base units with integrated load-group management) or ET 200MP for S7-1500 stations. The ET 200SP uses BaseUnits (BU) with integrated light-colored (load group feed) and dark-colored (load group pass-through) variants, which eliminate the manual PM budget calculation by giving each load group a known, finite envelope. Refer to the Siemens phase-out notice for spare-parts and support timelines for the existing ET 200S fleet.