“A 40 W PLC in a 35 W panel is not a thermal problem, until it is — how many watts do you really need?”

You are reviewing a control cabinet cooling budget, and the PLC datasheet says “power dissipation max 8.5 W” [CompactLogix 5380]. But the Siemens S7‑1200 CPU 1214C datasheet lists a typical current draw of about 0.9 A at 24 V — roughly 22 W. Which number do you use for your panel heat load? The answer determines whether your cabinet needs a fan, a bigger heatsink, or nothing at all. This article runs three proof-by-cases that separate datasheet spec from real panel load, using the two most common mid-range architectures: the Allen‑Bradley CompactLogix 5380 and the Siemens S7‑1200 series. Each case forces a different decision rule.

Case 1: The processor‐only panel — when “max” means “without I/O”

The CompactLogix 5380 (5069‑L306ER) datasheet states a power dissipation of 8.5 W max and a thermal dissipation of 29 BTU/hr (≈ 8.5 W). That figure is the bare CPU module, no expansion, no I/O. The Siemens S7‑1200 CPU 1214C datasheet gives a typical current draw of ~900 mA at 24 V DC, which is roughly 22 W — but that includes the on‑board 14 DI / 10 DO and 2 AI. The Siemens PLC number already accounts for the built‑in I/O power, while the Allen‑Bradley PLC number is only the logic module.

Mechanism: The processor alone consumes ~3–5 W for its core logic; the rest is the I/O field‑side power. The CompactLogix 5380 shifts I/O power to the I/O modules themselves (each module has a separate dissipation rating), whereas the S7‑1200 bundles the I/O supply into the CPU spec. If you compare the two numbers head‑to‑head, you are comparing the CPU-only portion of the Allen‑Bradley against the full I/O‑loaded portion of the Siemens — a mismatch that can mislead by a factor of 2–3.

Worked consequence: For a panel containing only the CPU and no remote I/O, the Allen‑Bradley draws about 8.5 W while the Siemens draws about 22 W. If you naively pick the lower number for your thermal budget, you might plan a 10 W dissipation slot only to find the Siemens controller adding 12 W more heat. The decision: in a sealed, fan‑less cabinet of about 3 litres internal volume (roughly 200×200×75 mm), the difference between 8.5 W and 22 W is the difference between a surface temperature rise of ~7 °C versus ~18 °C above ambient (assuming natural convection; illustrative). The 22 W load can push the internal air temperature above the 60 °C storage limit of the S7‑1200 if the ambient is already 45 °C.

Reversal: If you are using the Siemens CPU with no external I/O — just the 14 DI / 10 DO that are already accounted for — the 22 W is the right number. If you are using the CompactLogix 5380 with even one local I/O module, its total dissipation will jump: a single 8‑point output module can add 2–3 W per module. Once you add 4–5 modules, the total can exceed the Siemens single‑box figure. The rule: for a processor-only cabinet, the Siemens number is self‑contained; the Allen‑Bradley number is only the start of the sum — you must add the I/O module dissipation separately.

Case 2: The fully expanded micro panel — when I/O density changes the load

Now consider a panel with 28 digital inputs and 20 digital outputs — a common configuration for a packaging line or material‑handling cell. The Allen‑Bradley Micro850 2080‑LC50‑48QBB already packs 28 DI / 20 DO in one base unit. Its datasheet does not give a single power dissipation number, but based on the sum of its built‑in I/O (approx. 0.3 A per 8‑point bank typical at 24 V, about 7.2 W for the I/O alone) plus the CPU core (~2.5 W), the total is roughly 9.7 W (illustrative). The Siemens S7‑1200 CPU 1214C with the same I/O count would need a CPU plus three signal modules (assume SM 1221 and SM 1222, each drawing about 50 mA from the CPU bus). The total bus current for three modules is about 150 mA at 24 V = 3.6 W, added to the CPU’s 22 W, giving ~25.6 W (illustrative).

Mechanism: The Micro850 integrates the I/O power more efficiently because its onboard power supply is designed for the full complement of I/O on the base unit; the Siemens architecture draws the I/O module supply from the CPU backplane bus, increasing the total system draw. This is not a flaw — it is a design choice. The Siemens system is modular; the Micro850 is a brick. The efficiency difference comes from the fact that the Micro850’s I/O is on the same PCB with shared power regulation, while the Siemens bus supplies each module through a common 5 V rail that is less efficient at distributing 24 V field power.

Worked consequence: For the 48‑I/O case, the total power difference is about 9.7 W vs 25.6 W — a factor of 2.6. In a panel with a 60 W cooling budget, the Micro850 uses only 16% of the budget, leaving headroom for a small Ethernet switch and an HMI. The Siemens S7‑1200 with the same I/O count consumes 43% of the budget, which may force a larger fan or a cabinet size upgrade. The decision rule: if you know your I/O count exactly and it matches a Micro850 brick, you can size the cooling with a ~10 W thermal load; if you use Siemens, you must budget at least 25 W for the same I/O capacity.

Reversal: If your I/O requirement exceeds the built‑in capacity of the Micro850 — say 64 digital points — you need expansion modules, which add about 2–3 W each. At 64 I/O, the Micro850 with one 16‑point module might reach ~13 W, while the Siemens with two extra modules might reach ~29 W. The gap narrows but does not close. The exception is when you need very high‑speed counters or analog channels: the Micro850 has 6 HSC and 3 PTO built‑in without extra modules, while the Siemens may require a signal board that adds negligible power (~0.5 W). In that case the Siemens total may not increase much, and the gap remains large.

Case 3: The motion‑critical cabinet — when servo drives dominate the budget

This case is common but often overlooked: a panel with one or two small servo axes. The CompactLogix 5380 can control up to 32 axes via EtherNet/IP, but the CPU itself does not drive the motor power — the servo drives handle that. The CPU dissipation stays at ~8.5 W regardless of axis count. The Siemens S7‑1200 has integrated motion via PTO (pulse‑train output) and PID, but it also does not drive motor power; however, the CPU’s 22 W includes the onboard I/O and the bus supply for any drive‑interface modules.

Mechanism: The real heat in a motion panel comes from the servo drives, not the PLC. A typical 400 W servo drive at full load dissipates about 15–20 W of heat (roughly 4–5% of motor power). Two such drives add 30–40 W. The PLC’s contribution becomes a fraction of the total — 8.5 W vs 22 W, but that difference is about 13.5 W, which is less than the heat from half a drive. In a panel with 50 W total dissipation from drives, the 13.5 W difference is about a 20% error in the thermal budget — enough to matter if the cabinet is tightly sized, but not as decisive as the I/O‑dense cases.

Worked consequence: If your panel has two 400 W servos and a small HMI, the total dissipation might be ~80 W. The choice of PLC shifts that by 13.5 W, which could mean the difference between a 75 W budget (with Allen‑Bradley) and an 88 W budget (with Siemens). A 75 W sealed cabinet designed for +50 °C ambient might require a 90 W heatsink specification; the 88 W load would exceed it by ~15%, forcing a heatsink redesign or an active fan. The decision: in a panel where drives already dominate, the PLC choice becomes a second‑order effect — you can often adjust the heatsink size or add a small 50 CFM fan for

Reversal: If the panel has no drives — only the PLC and a few relays — the PLC dominates. In that scenario, the 13.5 W difference is the entire thermal load, and the Allen‑Bradley gives a clear advantage. But if the panel already has a 100 W drive, the 13.5 W difference is noise.

Non‑obvious insight: the “power dissipation” spec hides the I/O power delivery architecture

The datasheet number that looks like a pure thermal spec is actually a system‑architecture clue. The Allen‑Bradley CompactLogix 5380’s 8.5 W is a “core‑only” spec; the Micro850’s 9.7 W is a “full brick” spec; the Siemens S7‑1200’s 22 W is an “I/O included” spec. Compare them only after you re‑allocate the I/O power. Many engineers fall into the trap of comparing the 8.5 W to the 22 W and concluding the Allen‑Bradley is “more efficient,” when in fact the total system efficiency (including I/O) depends on the I/O count.

Failure mode: the PLC that runs hot because you used the wrong I/O supply

If you use the CompactLogix 5380 with a high‑current I/O module that is not rated for the 24 V field supply and instead draws power from the CPU backplane, you can exceed the CPU’s internal bus current limit (typically 1.5 A at 5 V for the backplane). The datasheet does not list a “max I/O power from CPU” — it only lists module limits. The failure mode: the CPU’s 5 V regulator overheats and shuts down, and you lose the entire control system. This is rare but documented field incidents show it happens when engineers extrapolate from the 8.5 W number and add too many modules. The prevention: use the I/O power budget calculator in the CompactLogix selection guide, not the 8.5 W number alone.

Rule‑based conclusion: a three‑step threshold for real‑world sizing

After these three cases, you can form a decision tree that does not depend on vague “depends on your scenario” advice:

• Step 1: Calculate the total panel load (including drives, HMI, and all I/O) in watts. Use manufacturer dissipation values for each component. For the Allen‑Bradley CompactLogix 5380, start with 8.5 W and add each I/O module’s dissipation from its datasheet. For the Siemens S7‑1200, use 22 W as a baseline and add signal module bus current × 24 V (typically 0.05 A per module = 1.2 W each).
• Step 2: Compare the PLC contribution to the total load. If the PLC is less than 20% of the total, any PLC choice adds ≤ 10% thermal error — ignore the PLC heat difference and optimise for other factors (software ecosystem, motion capability, network topology).
• Step 3: If the PLC is more than 20% of the total (typical for a pure‑logic panel with no drives), choose the platform with the lower system‑level dissipation. In the micro‑to‑mid range, the Allen‑Bradley Micro850 or CompactLogix 5380 (with proper I/O module counting) will typically give a 10–15 W lower thermal load than an equivalent Siemens S7‑1200 system, which can save you from adding a fan or up‑sizing the cabinet.

This rule does not crown one brand superior in all cases — it gives you a quantitative threshold to make the decision. If your panel already runs hot, the difference of 10 W may be critical; if the drives dominate, the PLC choice is secondary.

Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Allen-Bradley PLC is a brand affiliated with this site; competitor names are used for identification only.