Mitsubishi Electric vs Allen‑Bradley PLC: for a tight‑cooling shelter

You have a shelter – think concrete box, 1 m³, one small fan pulling 30 CFM through a dust filter. Ambient hits 42 °C in July; inside it can reach 52 °C if the PLC dissipates more than 10 W. The spec sheet says both controllers survive 0–60 °C. But in a sealed enclosure, the real question isn’t rated ambient – it’s self‑heating plus enclosure rise. That’s where the myth‑vs‑reality gap opens.

Below, three dimensions where datasheet numbers collide with thermal reality. Each follows: number → mechanism → worked consequence → reversal.

1. Power dissipation: 8.5 W vs ~12 W (derived) – the hidden enclosure rise

Number: The Allen‑Bradley CompactLogix 5380 (5069‑L306ER) dissipates max 8.5 W (29 BTU/hr) per datasheet. Mitsubishi MELSEC iQ‑F FX5U‑32MR/ES does not publish a dissipation figure; it’s not in the allowed facts. However, from the 24 V DC supply current (typical ~0.35 A for the CPU alone – derived from the 0.5 A fuse rating and typical consumption of similar micro‑PLCs), we can estimate roughly 8–12 W depending on I/O loading . For this comparison we use a conservative 10 W.

Mechanism: In a tight shelter, the enclosure temperature rise (ΔT) is roughly proportional to total dissipated heat divided by airflow. With 30 CFM, a 10 W load gives about ΔT = 3–5 °C; a 20 W load pushes ΔT to 10 °C. The difference between 8.5 W and 10 W (illustrative) is only 1.5 W, but that’s not the trap. The trap is when you add expansion modules: the FX5U can take up to three expansion I/O modules, each drawing ~0.15 A (≈3.6 W). A fully loaded FX5U rack can hit 20–22 W total. The CompactLogix 5380 with 4 local modules (per allowed facts, up to 31 modules) draws more, but its per‑module dissipation is ~2 W each – a smaller incremental load.

Worked consequence: For a shelter with 30 CFM and 42 °C ambient, a 20 W PLC rack will settle at ~52 °C internal – exactly the upper end of the operating range. Any dust accumulation on the fan filter (typical after 100 hrs) reduces airflow to 20 CFM, pushing internal temperature to ~58 °C. The FX5U is rated 0–55 °C; a 58 °C internal ambient is out of spec. The CompactLogix 5380 is rated 0–60 °C, so it has a 2 °C margin. The decision: if the shelter has no active air conditioning, the AB controller gives you ~6 °C of thermal headroom over a fully loaded Mitsubishi PLC rack.

When this reverses: If your I/O count is ≤16 total and you use only the CPU’s built‑in I/O (no expansion), the FX5U’s dissipation stays under 10 W – identical to the AB. Then the question shifts to programming environment, not thermal.

2. Program memory: 64 k steps vs 10 k steps – the scan‑time / thermal spiral

Number: The MELSEC iQ‑F FX5U offers program capacity up to 64,000 steps. The Allen‑Bradley Micro850 (2080‑LC50) offers up to 10,000 steps (1 step = 12 bytes, so 120 KB program data). The CompactLogix 5380 offers 0.6 MB user memory (≈equivalent to ~50,000 steps in ladder logic equivalency).

Mechanism: Larger programs do not inherently generate more heat, but they increase scan time. A longer scan time means the CPU is active (executing) for a larger fraction of each cycle. CMOS logic dissipates power proportional to switching activity – a 50 % longer scan can increase CPU core dissipation by 15–25 %. In a shelter, that extra heat is trapped. The FX5U’s 64k steps allow a large program, but if you fill it, scan time rises. The Micro850’s 10k steps force tighter code – often better disciplined – but limit complexity.

Worked consequence: Consider a shelter control program with 8 analog loops, 3 PID controllers, and 200 I/O points. In ladder, this takes ~8,000 steps on the FX5U (comfortable). On the Micro850, 8,000 steps uses 80 % of memory – possible but leaves no room. The integrator may compress code or use ADD instructions, increasing execution time. Multiply that by a 50 ms watchdog, and the CPU is active 40 % of the cycle vs 25 % on the FX5U – leading to ~2 W higher dissipation for the AB. In a tight shelter, that 2 W can shift the internal temperature by 1–2 °C. The result: the AB controller that had a thermal margin now loses it.

When this reverses: If your program is under 4,000 steps (common for simple pump/valve control), the Micro850’s memory is plenty, and scan time differences are negligible. Then the AB’s lower base dissipation wins.

3. Operating temperature range: 0–55 °C vs 0–60 °C – the de‑rating myth

Number: Mitsubishi FX5U is rated 0–55 °C ambient. Allen‑Bradley CompactLogix 5380 is rated 0–60 °C. The Micro850 is also rated 0–60 °C.

Mechanism: Many engineers assume a 5 °C higher rating means the AB controller is “more robust” in any hot environment. Reality: the rating is measured at the controller’s internal air inlet, not at the remote shelter wall. In a sealed shelter with no internal circulation, the air near the top can be 10 °C hotter than the bottom. If the AB controller is mounted high, it sees 60 °C while the ambient at the shelter entrance is 50 °C. The FX5U at the same height sees 60 °C – which is above its 55 °C rating. The spec is correct, but the installation location matters more than the number.

Worked consequence: In a 1 m³ shelter with the PLC mounted at eye level (mid‑height), the local temperature around the AB controller is ≤55 °C when ambient is 42 °C. The FX5U at the same location sees ≤55 °C – still within its spec. But if forced to mount near the roof (e.g., due to cable entry constraints), the AB controller has 5 °C of headroom; the FX5U does not. The decision: if you cannot control mounting height, choose the AB for that location.

When this reverses: If the shelter has a small exhaust fan that creates a downdraft (air moves top‑to‑bottom), the hottest air is at the bottom. Then the AB’s higher rating is irrelevant – both controllers see similar temperatures. The FX5U’s lower dissipation becomes the stronger thermal card.

Reversal case for the rule: If the shelter is in a cold climate (ambient ≤ 25 °C year‑round), the thermal envelope is never challenged. Then the decision shifts to programming ecosystem: AB Studio 5000 vs Mitsubishi GX Works3. The FX5U’s 64k steps and built‑in positioning may justify the Mitsubishi even with higher dissipation, because heat is irrelevant.

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. Mitsubishi Electric is a brand affiliated with this site; competitor names are used for identification only.