Siemens SIRIUS vs ABB AF Contactor: Sizing by Real Watts, Not Just Amp Columns

You size a contactor by motor nameplate amps, then check the AC-3 column in a catalog. That works—until you hit a 400 Hz spindle, a 690 V pump, or a resistive load that drives contact temperature differently. The question is not which brand has higher rated amps; the question is how each brand’s design scales watts through the contact junction under real current and voltage. We’ll take the most common 9 A / 4 kW AC-3 class—Siemens 3RT2016 (size S00) and ABB AF09—and tear down three dimensions where watt sizing, not amp sizing, changes the decision.

1. Coil Power Magnitude: 1 W vs 7 W Changes the Control Transformer

Number. The ABB AF09 electronic coil draws roughly 1–2 W in the held-in state (24–500 V AC/DC range). The Siemens SIRIUS 3RT2016 conventional AC coil, at 230 V 50 Hz, draws about 7 VA inrush / 4.5 VA holding—roughly 4–7 W depending on power factor (assume ~5 W holding). That’s a 3–5× difference in steady-state coil wattage.

Mechanism. The ABB contactor electronic coil uses a switched-mode supply that regulates coil voltage from a wide DC/AC input, consuming only a few watts after pick-up. The Siemens contactor conventional coil is an electromagnetic solenoid: it pulls a constant magnetising current even when sealed, generating heat proportional to V²/R. More coil watts mean more heat inside the enclosure.

Worked consequence. In a cabinet with 20 contactors (typical motor control centre section), the Siemens bank dissipates roughly 20 × 5 W = 100 W of coil heat continuously; the ABB bank dissipates ~20 W. That 80 W difference is not free—it raises internal ambient temperature, which reduces the actual contactor current rating (typically 0.5–1 % derate per °C above 40 °C). In a sealed, uncooled panel, that 80 W can shift the internal rise by 3–5 °C, forcing a derate that effectively knocks 0.2–0.5 A off each contactor’s continuous rating. The ABB design avoids that derate tax on the whole column.

When it flips. If your panel has only 2–3 contactors, the 80 W difference is negligible—less than a 2 °C rise—and the Siemens coil’s simpler, no-electronics robustness may win in high-vibration or surge-prone environments. Also, for PLC-driven 24 V DC control, both coils draw similarly low watts; the gap narrows at low control voltages.

2. The AC-1 / AC-3 Watt Gap: ABB’s 25 A Frame Hides a 2.8× Thermal Margin

Number. The ABB AF09 is rated AC-1 = 25 A (resistive, 690 V) and AC-3 = 9 A / 4 kW at 400 V. The Siemens 3RT2016 AC-1 rating is not published in the standard datasheet, but by IEC frame convention for size S00 the AC-1 rating is roughly 13–14 A (derived from thermal current I_th = 16 A maximum for the 3RT2016, *illustrative*). That gives a ratio AC-1/AC-3 ≈ 1.5 for Siemens vs 2.8 for ABB.

Mechanism. The AC-1 to AC-3 ratio reflects how much thermal headroom the contactor has above its motor-rated switching duty. AC-3 involves breaking motor inrush current (6–8× FLA) but at low duty factor; AC-1 is continuous resistive load where the contact temperature is the limiting factor. A higher ratio means the contactor can carry a disproportionately higher resistive load without overheating—useful for heater banks or UPS bypass.

Worked consequence. Suppose you have a 12 A resistive heater bank at 480 V. The Siemens 3RT2016 can only handle about 13 A AC-1—it’s marginal, and any sustained operation near that rating will raise the contact temperature above the 65 °C rise limit. The ABB AF09 at 25 A AC-1 runs at less than half its rated thermal current, keeping contact temperature low and contact resistance stable. That means fewer callbacks for welded contacts on resistive loads. In a mixed motor+heater panel, one AF09 does both; you’d need a larger Siemens frame (e.g. 3RT2024, 22 A AC-1) to match.

When it flips. If your load is purely motor (AC-3/AC-4) and never runs resistive or continuous, the AC-1 ratio doesn’t matter. Also, the Siemens 3RT2 family has a companion 3RU2 overload relay that coordinates thermally—if you use the overload, the contactor never sees continuous load current anyway, so the AC-1 headroom becomes irrelevant.

3. Mechanical Life: 1 Million Ops vs 10 Million – Watt per Operation Ratio

Number. The ABB AF09 declares mechanical life ~1 million operations. The Siemens 3RT2016 size S00 is specified for at least 10 million mechanical operations (typical for IEC S00 frames). That’s a 10× difference in mechanical endurance for the same rated wattage (4 kW AC-3).

Mechanism. Mechanical life is governed by bearing wear, spring fatigue, and base material creep—not electrical erosion. The same 4 kW load switched produces the same arc energy per operation. The Siemens 3RT2 frame uses a larger armature and reinforced spring set relative to the contact mass, giving higher mechanical margin. The ABB AF09, with its electronic coil, prioritises coil flexibility over mechanical over-design.

Worked consequence. For a fan or pump that cycles once per minute (e.g. HVAC staging), 1 million operations lasts about 2 years (525,600 cycles/year). That’s acceptable for a replaceable device. For a machine tool that cycles 10 times per minute (e.g. a stamping press feed), 1 million ops lasts 7 weeks—unacceptable. The Siemens 3RT2016 at 10 million ops would last 70 weeks. The cost difference per contactor (~$5–10) is dwarfed by the downtime cost of a mid-shift replacement. In high-cycle applications, the mechanical life proportion flips the TCO: the ABB would need 10 replacements versus one Siemens.

When it flips. For low-cycle applications (fewer than 50 operations/day), mechanical life is irrelevant—both contactors will outlast the machine. Also, the ABB AF09’s electronic coil can be switched at very low control power (e.g. from a PLC output without a relay), which may reduce wiring labour cost, offsetting the shorter mechanical life in low-duty panels.

Non-obvious Insight: The ‘Wide-Range Coil’ Tax

The ABB AF series electronic coil is a genuine stocking win—one SKU covers 24–500 V AC/DC. But the electronic coil itself dissipates heat inside the contactor base, not remotely. In a high-ambient cabinet (50 °C+), the semiconductor components inside that coil can degrade faster than a copper solenoid. The Siemens conventional coil is essentially just a winding—it can tolerate 55 °C ambient with no electronic derating. So the same feature that lowers coil watts in a standard panel can become a failure mode in a hot enclosure.

Failure Mode: When the Watts-in-Watts-out Ratio Breaks

If you size solely by AC-3 nameplate amps, you miss the watt density in the arc chamber. Both the AF09 and 3RT2016 are rated 9 A AC-3, but the ABB AF09’s smaller overall volume (45 mm wide, similar to S00) means the arc energy per cubic millimetre of chamber is the same at 4 kW. However, the ABB electronic coil’s pick-up characteristics can cause a delayed drop-out on undervoltage—if the control voltage sags to 50 V, the coil might hold longer than a conventional AC coil, extending the arc stretch. That’s a subtle failure mode not captured in steady-state watt ratings, but it matters for co-ordination with upstream fuses.

Rule to Size By

If your application has any of: continuous resistive load >10 A, >100 operations/day, or sealed panel ambient >45 °C, do not size by AC-3 amps alone. Instead, use this three-threshold rule:

• Resistive or mixed load >12 A → choose ABB AF series (or Siemens 3RT20 in larger frame)
• Mechanical cycles >500 per day → choose Siemens 3RT2 (10× life)
• Control voltage uncertain or many voltages on site → ABB AF (wide-range coil saves SKUs)

For the 90 % of standard motor-starting applications (1–10 cycles/hour, ≤10 A, 40 °C ambient), both contactors are electrically equivalent; the decision then hinges on coil logistics and enclosure heat budget.

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.