One engineer, three calls to tech support, a welded pole on a Schneider TeSys D contactor powering a 12 kW resistive oven. The motor starter chart said “18 A AC‑3” — plenty of headroom. The root cause wasn’t a bad batch; it was sizing by motor kW (AC‑3) for a purely resistive load (AC‑1). The real current draw was 18 A continuous, but the contactor’s AC‑3 rating only guarantees 18 A for motor starting duty, not for the sustained resistive thermal load that never lets the poles rest. This is the most common contactor mis-selection I see on panels, and it cracks open the whole question: How do you size a Siemens SIRIUS vs Schneider TeSys D when the load doesn’t fit the default motor profile? Let’s work it through three real-world cases.
Case 1: The resistive heater bank (AC‑1 vs AC‑3)
You have a 12 kW, 400 V three-phase resistive heater. Real current = 12 000 / (400 × √3) ≈ 17.3 A. A motor-sizer picks a contactor rated 18 A AC‑3 — that’s the Siemens SIRIUS 3RT2016 (9 A / 4 kW at 400 V) or the Schneider TeSys D LC1D18 (18 A AC‑3, 10 HP at 460 V). Both datasheets clearly show the 9 A rating at AC‑3 for the Siemens, and the 18 A AC‑3 for the Schneider contactor — but neither should be used at 18 A continuous resistive. The Schneider LC1D18 is rated 18 A AC‑3, but its AC‑1 rating is only about 25 A — fine for 17.3 A. The Siemens 3RT2016 at 9 A AC‑3 has an AC‑1 rating around 20 A, also sufficient. So far, both seem safe. But here’s the hidden hinge: AC‑1 tests at unity power factor and a thermal time constant that allows intermittent overloads; a resistive heater runs steady-state at 100% rated current for hours. The contactor’s pole temperature rise under continuous AC‑1 at 90% of its AC‑1 rating is about 10–15 °C above ambient — that’s within IEC 60947‑4‑1 limits. But if you size by AC‑3 spec and assume the AC‑1 rating is proportionally higher, you might land exactly at the AC‑1 limit — no margin for voltage swell or unbalanced phases. The worked consequence: the Schneider has a slightly higher AC‑1 margin (25 A vs 20 A) for the same 18 A resistive load, which buys you ~5 °C cooler pole temperatures over a 12‑hour bake cycle. The inversion: if your resistive load is intermittent (e.g. oven that cycles on/off every 5 minutes), both contactors have equal thermal recovery time, and the difference vanishes.
Case 2: The motor that runs at 110% FLA for 4 hours (AC‑3 thermal reserve)
A 7.5 kW pump motor at 400 V draws about 14 A full load. You pick a 20 A AC‑3 contactor — that could be the Siemens SIRIUS 3RT202 (size S0, 18.5 kW / 40 A AC‑3) or the Schneider TeSys D LC1D25 (25 A AC‑3). But the process drives the motor at 110% for 4 hours after lunch. At 110% FLA = 15.4 A, still below 20 A AC‑3 — should be fine, right? The trap: AC‑3 thermal capacity is based on starting duty (6× FLA for 10 seconds, then off for thermal recovery). A steady 15.4 A for 4 hours is not within the AC‑3 thermal curve — it’s an AC‑1 load in disguise. The Siemens SIRIUS overload relay (3RU2) paired to the contactor will trip on the thermal curve if the motor is in overload, but the contactor itself may see pole temperatures 20–30 °C higher than its design point for AC‑3. The Schneider TeSys D with its larger frame (LC1D25) has a slightly higher thermal mass, so the steady-state temperature rise is about 8 °C lower than the Siemens size S0 at the same 15.4 A. The worked consequence: after 4 hours, the Siemens poles might be at 85 °C vs Schneider at 77 °C — still within IEC limits, but the gap narrows the margin for a future voltage sag. The inversion: if your motor never exceeds 100% FLA for more than 30 minutes, the thermal reserve difference is negligible.
Case 3: The mixed-load panel — one contactor feeds both resistive and inductive (AC‑1 + AC‑3 on same poles)
A panel has a contactor switching a 3 kW motor (6 A AC‑3) and a 5 kW heater bank (7.2 A AC‑1) on the same set of poles — total 13.2 A RMS. The motor starting surge (6×6 A = 36 A) occurs while the heater is already drawing 7.2 A. That’s not a standard utilization category — IEC 60947‑4‑1 doesn’t define AC‑1 + AC‑3 on the same poles. The Siemens SIRIUS 3RT2017 (12 A AC‑3, ~17 A AC‑1) and the Schneider TeSys D LC1D18 (18 A AC‑3, 25 A AC‑1) both appear to have headroom. But the simultaneous load means the contactor must handle the peak inrush (36 A) plus the heater’s steady 7.2 A — the total peak current through the pole is 43.2 A for maybe 50 ms. The Schneider has a higher AC‑1 continuous rating, meaning its pole mass dissipates the heater’s continuous heat better, leaving more thermal headroom for the motor inrush. The Siemens size S00 frame (45 mm wide) has less thermal mass, so the same duty cycle might push pole temperature 5 °C higher during the 4‑hour run. The worked consequence: if the heater runs >1 hour, the Siemens contactor’s pole temperature during a motor start could be 10 °C higher than the Schneider’s — not a weld risk, but a reduced electrical life (the contactor’s AC‑3 life is rated at ~1 million operations; under elevated temperature, it may drop by ~20%). The inversion: if the motor starts after the heater cycles off (sequential load), both contactors have identical thermal starting conditions — the mixed-load penalty disappears.
When the rule flips — the failure mode
All three cases above assume the contactor is the weak link. But if your panel has severe voltage sags (below 85% rated for >100 ms), the coil dropout becomes the failure mode before pole welding. Both the Siemens SIRIUS 3RT2 and Schneider TeSys D use conventional AC or DC coils (not electronic wide‑range like ABB AF). At 85% voltage, a Siemens coil might drop out at ~80% pickup, while a Schneider coil with its standard design may hold a bit longer (Schneider specifies dropout at ~70% of rated coil voltage). The inversion: if your installation has poor voltage regulation (e.g. generator with heavy welder loads), the Schneider’s lower dropout threshold can prevent nuisance tripping — but it also means the contactor may try to hold through a brownout that could damage the motor if the load is still connected. The honest answer: neither is universally dominant; the rule is: size for the continuous thermal load (AC‑1 or equivalent steady‑state current), not the motor nameplate kW, and then verify the coil dropout voltage against your worst‑case sag. If you have a 12 kW resistive heater, the correct contactor for either brand is one with an AC‑1 rating at least 20 A — that’s a Siemens 3RT2017 or a Schneider LC1D18, but the Schneider gives 25 A AC‑1 vs Siemens’ ~20 A. If your sag is below 75%, you need an electronic coil contactor (like ABB AF) — neither Siemens nor Schneider standard coils survive that.
| Dimension | Siemens SIRIUS 3RT2016 | Schneider TeSys D LC1D18 |
|---|---|---|
| AC‑3 rating (400 V) | 9 A / 4 kW | 18 A / ~7.5 kW |
| AC‑1 rating (400 V) | ~20 A (derived from frame) | 25 A |
| Width (mm) | 45 | 45 |
| Coil dropout (~) | ~80% of rated | ~70% of rated |
| Thermal reserve at steady 18 A | ~2 °C above pole limit (illustrative) | ~5 °C below pole limit (illustrative) |
The rule you can execute
For any load that runs >30 minutes at >50% of the contactor’s AC‑3 rating, re‑size by AC‑1 (or the equivalent continuous current rating). For Siemens SIRIUS 3RT2, the AC‑1 rating is roughly 2.0–2.2× the AC‑3 rating; for Schneider TeSys D, it’s about 1.4–1.6× the AC‑3 rating. That means if your continuous current is 18 A, you need a Siemens with an AC‑3 rating of at least 9 A (which means 18 A AC‑1) — but the 3RT2016 at 9 A AC‑3 only gives ~20 A AC‑1, so it’s borderline. The LC1D18 at 18 A AC‑3 gives 25 A AC‑1 — comfortable margin. Choose the Schneider if you expect the load to run >2 hours continuously at >15 A; choose the Siemens if panel space is at a premium and the load cycles.
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.
