"The coil that sips 2 W vs the one that burns 12 W — but only if the voltage never wavers."

If you’ve ever sized a contactor coil for a 24 V control transformer that also feeds a PLC, you know the pain. The coil is the unit’s always-on load. A 12 W coil in a panel with 50 contactors pulls 600 W of copper-loss heat — and that’s before you count the ventilation fan. But here’s the trap: “efficiency” on a datasheet is usually the coil’s steady-state VA at rated voltage, and that’s almost never the voltage you actually see. This article is for anyone who’s lost sleep over a hot panel or a flickering PLC. It’s not about which contactor has the prettiest curve — it’s about which one keeps its efficiency promise when your line voltage sags.

1. Coil draw: the always-on tax

The numbers (from datasheets): The ABB AF09 contactor uses an electronic wide-range coil; its holding power is ≈ 2–4 W depending on variant and bus voltage. The Siemens SIRIUS 3RT2016 (same frame size, S00) coil is a conventional AC/DC electromagnet with a typical holding power of 8–12 W at 400 V. For a single unit, that’s a 6–10 W difference.

Mechanism — why the number changes the result: The ABB AF electronic coil uses a switched-mode power supply that actively regulates coil current down once the armature seals. The Siemens contactor conventional coil is a direct-wound solenoid—its holding power is roughly $V^2/R$, so a 10% undervoltage (say 360 V instead of 400 V) reduces holding power to ~81% of nominal, which is still ~8–10 W. But a 10% overvoltage (440 V) pushes power to ~121%, i.e. 12–14 W. The ABB contactor electronic coil, by contrast, holds its draw nearly constant across the full 100–250 V AC/DC range (or 24–500 V for the wider variants). So the “actual kept efficiency” of the ABB is spec-stable; the Siemens drifts with line voltage.

Worked consequences — what it means for a decision: If your facility has a regulated, clean 400 V supply staying within ±5%, the Siemens coil’s 10 W vs ABB’s 3 W difference is a small line item. But if you’re on a generator or a long feeder that sees 360 V under load (common in rural plants), the Siemens coil still pulls ~8 W, while the ABB still pulls ~3 W. The gap widens at high line: at 480 V, Siemens may exceed 12 W; ABB stays at 3–4 W. The marginal cost of the extra heat (and the fan to move it) can tip the panel lifecycle cost.

When this dimension flips (the reversal): If you control voltage tightly with a line conditioner, or your panel has only 5–10 contactors, the coil power difference is negligible. Also, the ABB electronic coil has a small inrush surge (typically

2. The eligibility gate: does your control voltage even let you use the “efficient” coil?

The numbers: The ABB AF09’s electronic coil comes in four wide ranges covering 24–500 V AC and 20–500 V DC. That means one coil SKU covers 24 V, 48 V, 110 V, 120 V, 208 V, 220 V, 240 V, 277 V, 380 V, 400 V, 415 V, 480 V — both AC and DC. The Siemens SIRIUS 3RT2016 coil is voltage-specific: you order a 24 V AC coil, a 110 V AC coil, a 230 V AC coil, etc., with separate DC coils. For a panel that uses mixed control voltages, you stock multiple Siemens coils.

Mechanism — why this changes the decision: The eligibility gate is simple: if your facility uses a non-standard control voltage (say 347 V for lighting circuits, or 48 V DC for a battery-backed system), the Siemens coil for that voltage may not be a standard catalog item — lead time or special order. The ABB AF range covers 24–500 V AC/DC with one or two part numbers. So the “efficiency” of the ABB coil is available to you; the Siemens coil may not even be orderable in the right voltage without a premium.

Worked consequences: Imagine you’re designing a panel for a solar + battery plant where control power is 48 V DC. The ABB AF09 with 24–500 V DC coil works directly. For Siemens, you’d need a 48 V DC coil (which exists, but may be less common, with longer lead time). If you accidentally order a 24 V DC coil, it burns out on 48 V. The ABB wide-range coil eliminates that ordering error — you can’t put the wrong voltage coil in because one coil covers all. The “efficiency you can actually keep” becomes operational: you keep your sanity, and you keep the panel running on day one.

Reversal: If your facility has a single, stable control voltage (e.g. 120 V AC from a utility transformer), the Siemens coil is just as available and cheaper per unit (the electronic coil adds ~$10–20 over a conventional coil). The stocking advantage of the ABB only pays off when you have multiple control voltages or frequent panel variants. Also, some engineers prefer conventional coils because they are simpler to diagnose with a multimeter—an electronic coil can fail in ways that look like a shorted transistor, which is harder to trace on site.

3. Mechanical life — the spec that’s almost never the limit (and why that matters)

The numbers (from datasheets): The ABB AF09 is rated for ~1 million mechanical operations. The Siemens 3RT2016 is also rated for ~1–3 million depending on model. On paper, they’re comparable. But the non-obvious insight is that in most motor-starting applications (AC-3), the contact life is limited by electrical erosion, not mechanical wear — and electrical life is heavily duty-cycle dependent. At 4 kW / 400 V, the ABB AF09 AC-3 electrical life is about 0.5–1 million operations; the Siemens 3RT2016 is similar. So the mechanical life spec is a red herring for 90% of users.

Mechanism — the real gating factor: The coil power discussion matters here because on a high-duty-cycle application (e.g., a conveyor that cycles every 10 seconds), the coil of the Siemens dissipates 12 W of heat inside the contactor enclosure. That heat raises the internal temperature, which reduces the thermal capacity of the arc chamber and can accelerate contact wear. The ABB, with its low coil dissipation, runs cooler inside. In a tightly packed panel, the difference in ambient temperature near the contacts can be 5–10 °C — enough to shift the electrical life curve downward.

Worked consequence: For a conveyor with 2 million operations per year, the Siemens might need contact replacement at 1.5 million due to accelerated wear from internal heat; the ABB might go to 2 million. The difference in replacement labour and downtime can dwarf the coil cost. But note: this is illustrative — the exact effect depends on panel airflow, mounting density, and load current. The point is that the coil’s thermal signature can change the electrical life, not just the electric bill.

Reversal: If the application is a rarely-cycled pump (a few starts per day), the internal temperature is irrelevant — both contactors will outlast the machine. And if you’re mounting contactors on a well-ventilated 35 mm DIN rail with 10 mm spacing, the heat from the Siemens coil is dissipated harmlessly. The “heat-shortens-life” argument only applies in high-density, high-cycle scenarios.

Decision rule — a threshold you can use

Like-for-like comparison — Siemens 3RT2016 vs ABB AF09 (size S00 / 9 A AC-3)

All ratings per IEC 60947-4-1. Derived/illustrative values labelled as such. See source notes below.

⚠️ A real failure mode: the undersized control transformer

Here’s a case that actually happens. A panel has 30 ABB AF contactors, each with a 3 W coil — total 90 W. The designer picks a 100 VA transformer. Fine, right? But the inrush of 30 electronic coils is simultaneous (each pulls ~5 A for 20 ms) – total peak inrush ~150 A for 20 ms. A 100 VA transformer might sag to 70 V during that spike, causing the PLC to brown out. The Siemens conventional coils have a gentler inrush profile (higher per-coil, but spread across a longer time). The lesson: the “efficient” coil can still cause a system-level failure if the transformer isn’t sized for the electronic inrush. This is a classic trade-off — the low steady-state power of the ABB coil masks a transient demand that a conventional coil doesn’t have.

The rule you can carry to your next panel

If your control voltage is standard and stable, and your panel density is low, the Siemens SIRIUS contactor’s conventional coil is just fine — and cheaper. But if you face variable voltage, mixed DC/AC control, or you want to reduce heat and coil SKU complexity, the ABB AF’s electronic coil delivers efficiency that actually holds across line conditions. The “efficiency you can keep” is the one that doesn’t disappear when the generator kicks in or when you open the panel to four different coil voltage spares. That’s the ABB AF, provided you don’t forget the inrush transient.

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