ABB vs Siemens Contactor: Three Questions When the Load Doubles

You have a motor starter running at 4 kW, 400 V—AC-3 duty. Next quarter, the line adds a second identical motor on the same contactor. The catalog says both ABB AF09 and Siemens 3RT2016 are rated 9 A / 4 kW at 400 V. So you double the load to 8 kW. Which one survives? The answer isn't in the rating table. It's in the provenance of those numbers—how they were derived, what they assume, and where the assumptions break down. Here are the three questions that matter.

1. The Wide-Range Coil: Why a Single Part Number Covers Your New Voltage—and Why It Might Not Cover Your New Current

The ABB AF09 uses an electronic wide-range coil that accepts 24–500 V AC (50/60 Hz) and 20–500 V DC with one SKU. The Siemens 3RT2016, by contrast, uses a conventional coil that must be ordered at the specific control voltage (e.g., 24 V, 110 V, 230 V). At first glance, the ABB contactor seems easier to stock—and it is, for voltage flexibility. But the electronic coil's advantage is also its hidden constraint: it draws a small, continuous power (The reversal: If you control the enclosure temperature (

2. The Mechanical Life Figure: 1 Million Operations—But at What Load?

ABB lists mechanical life for AF09 as ~1 million operations. Siemens lists similar mechanical life for 3RT2016 (size S00). Those numbers are derived from no-load or low-load tests per IEC 60947-4-1, which allows manufacturers to report mechanical endurance without electrical wear. Electrical life at AC-3 4 kW is typically 0.5–1 million operations for both, but at 8 kW (which exceeds the AC-3 rating), electrical life drops dramatically—often to 0.1–0.2 million operations. The mechanism is arc erosion: when the contactor opens under double load, the arc energy is roughly 4× higher (I²t), and the contact material (silver alloy) erodes faster. ABB's AF line uses silver-tin-oxide (AgSnO₂) contacts, which are better at resisting welding under high inrush but can erode faster under sustained high-current arcs compared to silver-cadmium-oxide (AgCdO) used in some legacy Siemens designs. The worked consequence: If you are cycling the contactor twice per minute in a conveyor system, the ABB's electrical life at double load may be 50,000 cycles, while the Siemens (with a different contact alloy) might reach 70,000 cycles—both are below the catalog mechanical life by a factor of 20. When this flips: If your load doubling is temporary (e.g., a startup peak that drops to 4 kW after 2 seconds), the thermal time constant of the contacts means neither contactor degrades significantly; then the ABB's wider coil range is the deciding factor.

3. The Overload Relay Pairing: Why Cross-Brand Indexing Breaks at 2× Load

Siemens SIRIUS 3RT2 contactors pair with 3RU2 thermal overload relays, which are mechanically indexed to the contactor frame. ABB AF contactors pair with ABB overload relays (e.g., T16 series), also frame-indexed. At 4 kW, both systems offer a coordinated starter with a Class 10 or 20 trip curve. At 8 kW, you are forced to use a larger frame size: neither the ABB AF09 nor the Siemens 3RT2016 is rated for 8 kW AC-3. You must step up to, say, ABB AF16 (rated 7.5 kW) or Siemens 3RT2026 (rated 7.5 kW). The question is not which contactor survives at 2× load—the answer is neither, per IEC 60947-4-1. The provenance of the rating is that AC-3 current values are guaranteed at 690 V max, with a utilization factor that assumes the motor is started and stopped infrequently. When you double the load, you violate the utilization category. The hidden insight: Many engineers think they can "stretch" a 9 A contactor to 12 A with a larger overload relay set to a lower trip current. That does not protect the contactor—the overload relay protects the motor, not the contactor. At 12 A, the contactor's main poles will weld sooner because the contact force is designed for 9 A peak. The Siemens 3RT2016 has a slightly larger contact surface area (45 mm wide vs. ABB AF09's roughly 40 mm wide), which gives it about 10 % more thermal capacity—meaning it might survive at 11 A where the ABB would fail. Rule-of-thumb: If your load is within 85–100 % of a contactor's AC-3 rating, the frame-size margin matters. If you are at 200 %, neither works—you must select the next frame.

The Non-Obvious Insight: The Provenance of the Rating Determines the Real Margin

The IEC 60947-4-1 standard allows manufacturers to rate contactors at 690 V, but most real-world installations are at 400 V. At 690 V, the arc energy is higher, so the AC-3 rating is often lower. Some manufacturers "cheat" by rating at 690 V to show a higher kW figure (since P=√3·V·I·PF). The ABB AF09's AC-3 rating of 4 kW at 400 V is consistent with the 9 A rating. The Siemens 3RT2016's 4 kW at 400 V is also consistent. But if you look at the 690 V column, ABB may list 5.5 kW while Siemens lists 5.5 kW as well—meaning both have the same headroom at higher voltage. The provenance check: if your load doubles to 8 kW at 400 V, you need a contactor with 18 A AC-3 rating, which corresponds to about 7.5 kW at 400 V—that's a size S0 or AF16. The Siemens SIRIUS 3RT2 S0 (e.g., 3RT2026) and ABB AF16 are the correct picks. The decision framework is: never use the catalog AC-3 rating as a continuous load rating; use 80 % of it as a planning factor for thermal margin. That simple rule would have caught the doubling problem before installation.

Failure Mode: The Enclosure Airflow Trap

Assume you pick the larger frame (AF16 or 3RT2026) and run at 8 kW continuous. Both now have ~20% thermal margin. But if the panel is 400 mm wide and the contactor is mounted next to a 500 W power supply, the ambient around the contactor may reach 70 °C. The ABB AF16's electronic coil has a derating to 0.9 × rated current at 70 °C. The Siemens 3RT2026's conventional coil may not derate until 80 °C. In that specific thermal failure mode, the Siemens holds up slightly better—but both need forced ventilation to stay within IEC limits. The reversal: If the panel is well-ventilated (

Decision Rules

• If load ≤ 80 % of contactor AC-3 rating: pick based on logistics—ABB for coil versatility, Siemens for fixed-voltage simplicity.
• If load is 80–100 % of rating: check ambient temperature. If >55 °C, Siemens conventional coil is safer. If
• If load > 100 % of rating: do not stretch. Step up one frame size. Overload relay setting does not protect the contactor.
• If contactor is cycled more than 60 times per hour: electrical life drops by 10× compared to mechanical life. Use the manufacturer's electrical life curve, not the catalog mechanical life.

The load doubling scenario is a trap for engineers who treat catalog numbers as safe continuous ratings. The provenance of the rating—what test conditions, what ambient, what utilization category—tells you the real boundary. Both ABB and Siemens build high-quality contactors, but the decision requires looking past the headline specs at the assumptions behind them.

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