ABB vs Siemens Contactor: When the Load Doubles, Which One Still Opens?

Here’s a question that looks academic until it costs you a production shift: Your motor starter is sized for 4 kW at 400 V AC-3. Then the process engineer adds a second pump, same drive, on the same contactor — load doubles to 8 kW. Does your existing ABB AF09 still make every break? Does the Siemens 3RT2016? I ran the numbers through two real-world cases — 4 kW→8 kW and 18.5 kW→37 kW — and the answer is not symmetrical.

The Ground Rules

Both the ABB AF09 and the Siemens SIRIUS 3RT2016 are rated 9 A / 4 kW at 400 V AC-3 per IEC 60947-4-1. That’s the standard for motor starting. We’re comparing only the contactor — not overload relays or the entire starter — because the question is whether the main power poles can handle the thermal and mechanical stress when the current doubles. I’ll use a worked scenario: Load A = 4 kW (nominal), Load B = 8 kW (double), both AC-3, both 400 V. Then a second scenario at 18.5 kW / 40 A to test the bigger frame.

Dimension 1: Continuous Thermal Current (I_th) vs. AC-3 Switching

The ABB AF09 has an AC-1 rating of 25 A and an AC-3 rating of 9 A. The Siemens 3RT2016 (size S00) is also 9 A AC-3, but its AC-1 rating is typically 20 A. At first glance, the ABB contactor has 25% more current-carrying capacity in resistive/continuous duty.

How that changes the outcome: AC-3 duty involves breaking a motor that’s already started — the contactor opens under load, but at a lower inrush than a locked-rotor condition. The real wear mechanism is arcing, which scales non-linearly with current and with the number of operations. At 9 A AC-3, both are fine. At 18 A (double), the ABB’s AC-1 margin means the contacts have a larger conductive cross-section (the manufacturer builds the same physical contact for 25 A continuous, then derates for AC-3). The Siemens contactor, with a smaller AC-1 headroom, will run the contacts closer to their thermal limit during the make/break arc. Worked consequence: If you double the load to 18 A, the ABB AF09 will see contact temperature rise ~15% above its AC-3 rating (illustrative, based on contact geometry ~2 mm² difference), whereas the Siemens 3RT2016 will see ~30% over — the arc energy per break is higher relative to the contact mass. That means accelerated pitting.

Reverse case: If your load is purely resistive (AC-1) or you operate fewer than 5,000 cycles per year, the thermal headroom doesn’t matter — both will survive years. The Siemens is more compact (45 mm wide vs. ABB’s 45 mm also), so if panel space is the constraint, the 3RT2016 fits the same footprint.

Dimension 2: Coil Power Consumption and Voltage Tolerance Under Brownout

The ABB AF contactor uses an electronic wide-range coil: 100–250 V AC/DC or even 24–500 V AC / 20–500 V DC on the AF09. The Siemens 3RT2 uses a conventional electromagnetic coil with a narrow tolerance (±10% on nominal voltage).

How that changes the outcome: When the load doubles, the motor inrush current also doubles (still ~6× FLA for a standard squirrel-cage), which causes a voltage sag on the line. If the sag drops below the Siemens coil’s pick-up voltage, the contactor drops out — and you get an unintended stop. The ABB’s electronic coil can hold in down to ~70% of rated voltage, and even if the control voltage sags to 120 V on a 240 V system, it stays closed. Worked scenario: An 8 kW motor at 400 V draws about 18 A running, but starting inrush hits ~108 A. A 100 kVA transformer feeding the panel will see a voltage dip of ~8% on a cold start. That 8% dip (from 400 V to 368 V) is still within Siemens’ ±10% tolerance, so no drop. But if the transformer is undersized or the line is long (e.g., 200 m of 4 mm² cable adds ~0.2 Ω per phase, giving another ~22 V drop at inrush), the total dip ≈ 13–15%. Now the Siemens coil sees ~340 V — below its 380 V pick-up threshold for a 400 V coil. That’s a drop-out. The ABB AF09 coil, even with a 100–250 V wide-range variant, stays pulled in.

Reverse case: If your power supply is rock-solid (dedicated transformer, no shared loads), the coil type is irrelevant. And the Siemens coil consumes less power when closed (about 5 VA vs. ABB’s ~8 VA), which can matter in PLC-based systems with many contactors.

Dimension 3: Mechanical Life and Overload Relay Integration

The ABB AF09 specifies mechanical life ~1 million operations. Siemens rates the 3RT2016 for about 10 million mechanical operations. That’s a 10:1 difference — but the catch is that mechanical life is measured with no electrical load (no arc wear). Under AC-3 load, the electrical life is the bottleneck, and both are roughly similar: ~100,000 operations at 9 A AC-3.

How that changes the outcome: When the load doubles to 8 kW / 18 A, the ABB’s electrical life drops by a factor of about 6 (from 100k to ~15k cycles, illustrative based on empirical AC-3 decay curves), because arc energy scales with I²t. The Siemens’ electrical life drops similarly — the contact material is comparable (AgSnO₂). But here’s the non-obvious part: the ABB AF series has an electronic coil that can be wired in parallel with the overload relay’s trip contact without an extra interposing relay, because the coil power is low enough to be switched by the overload’s auxiliary contacts directly. The Siemens 3RU2 overload relay has a higher burden, often requiring a separate control relay or a contactor with a higher-rated auxiliary. In a doubled-load scenario, you might need to add an auxiliary contact anyway (the AF09 comes with 1 NO built-in, the 3RT2016 also 1 NO), but the ABB’s coil drive is simpler.

Worked consequence: If you’re doubling the load on an existing starter panel, and you need to replace the contactor, the ABB AF09 will fit the same rail and you can reuse the same overload relay (provided it’s ABB’s own range) — the wiring is drop-in. The Siemens 3RT2 requires matching the 3RU2 overload, which is a different footprint and current range. That’s a redesign.

Reverse case: If you’re designing a new panel from scratch and you prefer to buy the whole SIRIUS family (contactor + overload + bus system), the Siemens ecosystem is more cohesive and has a longer mechanical life for high-cycle applications (e.g., 30,000 operations/year for 30 years — but that’s mechanical, not electrical).

Dimension 4: The Bigger Frame — 18.5 kW → 37 kW

Now scale up: the Siemens 3RT20 range includes a 40 A frame rated 18.5 kW at 400 V AC-3. The ABB AF range goes to larger frames (AF40, AF80) with the same electronic coil concept. Double the load to 37 kW / about 72 A. The Siemens 3RT20 40 A can’t handle that — you need to jump to a size S2 or S3 (75 A frame). The ABB AF40 is rated 40 A AC-3 (18.5 kW), but the AF80 is 80 A / 37 kW — same electronic coil platform. The jump in physical size is proportional. No surprise here: both scale. But the ABB’s electronic coil means you don’t need a different control transformer for the larger contactor; the wide-range coil accepts 100–250 V or 24–500 V regardless of frame size. Siemens’ 3RT2 larger frames still need a specific coil voltage — you stock five coil voltages per size. That’s a stock-keeping cost.

Worked consequence: For a plant standardizing on a single control voltage (e.g., 230 V), both work. But if you have mixed voltages (120 V in one panel, 230 V in another), ABB’s range covers both with one SKU per frame size. Siemens needs two SKUs. Over 50 contactors, that’s 50 fewer part numbers.

Reverse case: If your plant has a strict 24 V DC control system, ABB’s AF09 covers 20–500 V DC, so it works. Siemens offers 24 V DC coils, but not across all frame sizes — you may need a transformer.

Ranked Picks Table (Decision Framework)

Non-Obvious Insight: The Coil Is the First Failure Mode, Not the Contacts

Most engineers check contact pitting. But in the load-doubling scenario I just walked through, the Siemens 3RT2 coil will drop out first (brownout), causing a nuisance trip. The ABB coil holds. If the trip causes a pump to stop and a tank to overflow, the cost is not a contactor replacement — it’s a spill cleanup. The coil’s voltage tolerance is the hidden spec that changes the decision.

Failure Mode / Reverse Case

If your doubled load is not motor (AC-3) but resistive (AC-1) — for example, doubling the load on a contactor controlling an electric heater — both contactors can handle 2× the AC-3 rating comfortably, because the AC-1 rating is 25 A for ABB and 20 A for Siemens. In that case, the decision is price: Siemens 3RT2016 lists at ~$45, ABB AF09 ~$55 (illustrative, market prices vary). The Siemens wins on cost. But the original question “when the load doubles” almost always means motor load, because resistive loads are static.

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.