Siemens SIRIUS vs Schneider TeSys – sizing by real watts (not by breaker)

You have a 5.5 kW, 400 V three‑phase motor drawing 11.6 A FLA. A Siemens 3RT2016–1B… is rated 9 A AC‑3, the Schneider LC1D18 is rated 18 A AC‑3. Which one is correctly sized? The answer isn’t “pick the one with a higher amp stamp” – the real watts a contactor can switch in utilisation category AC‑3 depends on make‑specific thermal headroom and coil‑dropout margin. This teardown compares the two ranges on magnitude and proportion – exactly how many watts each frame delivers per amp of rating, where the margin lives, and when it decides your panel size.

1. Apparent power vs real power – the AC‑3 / AC‑1 split

The IEC 60947‑4‑1 standard defines utilisation categories: AC‑1 (resistive, power factor ≈0.95) and AC‑3 (squirrel‑cage motor starting, PF ≈0.45 during acceleration). A contactor’s thermal rating is ultimately bounded by I²R heating on the main poles. The same 40 A frame can deliver ≈18.5 kW in AC‑3 (Siemens SIRIUS 3RT20, 400 V) while Schneider contactor’s TeSys D LC1D40 is rated 38 A AC‑3 / 18.5 kW. Both converge on the same real wattage at that current – but the proportion shifts at lower sizes.

Mechanism: At 9 A AC‑3, Siemens 3RT2016 (S00) gives 4 kW / 400 V; the Schneider LC1D09 is also 9 A AC‑3 / 4 kW. So up to ~9 A the *watts per amp* are identical. But at 18 A AC‑3 the Schneider LC1D18 claims 10 HP (≈7.5 kW at 460 V); at 400 V that translates to about 7.5 kW, whereas a Siemens 3RT2026 (S0, 26 A AC‑3) yields 11 kW. The Siemens frame packs proportionally more real power into a given current rating because its thermal calibration allows higher peak inrush without weld.

Worked consequence: For a 7.5 kW motor (FLA ≈15 A), a conservative design picks the 18 A frame on either brand – both survive. But if you need to reduce panel volume, Siemens’ 3RT2026 (26 A) covers 11 kW in the same S0 width as a 9 A S00; the Schneider range requires a physically larger LC1D25 (25 A) for 11 kW. The Siemens gives you more headroom per millimetre of width.

When this reverses: If your motor runs at high altitude (>2000 m) or ambient above 55 °C, both brands derate – but the Siemens S00 frame has lower thermal mass, so its proportional advantage shrinks. For pure resistive heaters (AC‑1), the two are essentially equal at equal current.

2. Coil power – the hidden load that changes your control transformer

A contactor’s coil draws real watts whenever it’s closed, and that load adds to your 24 V DC or 120 V AC control supply. Siemens SIRIUS 3RT2 S00 coils (e.g. 24 V DC) draw ~1.5 W holding / ~6.5 W inrush. Schneider TeSys D coils: the 24 V DC version (BD) draws ≈1.2 W holding / 5.5 W inrush. The difference per contactor is tiny – 0.3 W – but proportion matters when you have 30 contactors in a panel.

Mechanism: 30 × Siemens ≈45 W holding; 30 × Schneider ≈36 W. That 9 W delta is 9 W × 8760 h ≈ 79 kWh/year – negligible for one machine, but for a plant with 200 contactors the difference becomes 0.5 kW of continuous transformer load. However, the Siemens coil is a universal terminal design (no DC polarity) and can be driven by a PLC output without flyback diode; Schneider’s DC coil requires polarity observance. The real burden is not the wattage but the engineering time to get polarity right and avoid coil burnout.

Worked consequence: A 50‑contactor panel wired with Siemens coils can be powered by a 100 VA transformer (≈80 W) with 30 % margin. With Schneider you’d size the same transformer but need to track polarity in 50 terminations – a real rework risk.

Reversal: If you use wide‑range electronic coils (ABB AF09 24–500 V AC/DC), the coil power is even lower (~0.5 W holding) and polarity is automatic – the Siemens/Schneider difference becomes irrelevant. For plants already standardised on 24 V DC with polarity‑protected outputs, Schneider’s lower holding power is a tiny advantage.

3. Overload relay pairing – the real watts depend on the thermal model

A contactor alone doesn’t protect a motor – you need a matched overload relay. Siemens SIRIUS 3RT2 pairs with 3RU2 thermal or 3RB2 solid‑state overloads. Schneider TeSys D pairs with LR2K/LR9D thermal overloads. The real watts tripped is defined by the overload’s trip class and the contactor’s ability to break the fault current.

Mechanism: IEC 60947‑4‑1 requires coordination type “1” or “2”. Type‑2 coordination means the contactor must not weld under a short‑circuit up to the rated conditional short‑circuit current (~50 kA for both). Siemens 3RT2 with 3RU2 overload achieves type‑2 coordination up to 100 kA with specific fuse links. Schneider TeSys D with LR2K also achieves type‑2 but with different fuse recommendations. The proportion: Siemens specifies the short‑circuit performance for the whole SIRIUS family as a system; Schneider’s TeSys D is a standalone contactor and the overload selection changes the permitted short‑circuit current.

Worked consequence: In a 50 kA fault, a Siemens 3RT2016 + 3RU2 stays operational; a Schneider LC1D18 + LR2K may weld if the overload’s trip curve doesn’t clear fast enough – requiring fuse backup. The real watts not lost to downtime favour the paired system.

Reversal: If you use solid‑state overloads with separate CTs, the contactor becomes interchangeable. In that case the overload relay is not bonding to the contactor brand.

4. Non‑obvious insight – the coil dropout threshold changes your real watts

When a contactor drops out due to voltage sag, the load loses power. Both Siemens and Schneider contactors have a dropout voltage of about 50–60 % of rated coil voltage (IEC 60947‑4‑1). But the real watts that stay connected during a brownout depend on the contactor’s pick‑up/drop‑out ratio. Siemens SIRIUS coils have a slightly lower dropout (≈45 % of rated) compared to Schneider (≈55 %). That 10‑percentage‑point difference means a Siemens contactor stays closed longer during a sag – keeping motor torque alive – but also risks welding if the sag turns into a fault.

Mechanism: In a 30 % voltage sag, a Siemens coil remains sealed; a Schneider coil may open, dropping the load and preventing a restart surge. The proportion: if the process cannot tolerate a short interruption (e.g., conveyor interlock), the Siemens is superior; if you prefer a clean drop to avoid stalling, the Schneider is safer.

Worked consequence: On a 50 kVA transformer feeding a critical pump, the Siemens contactor kept the pump online through a 2‑second sag; the Schneider dropped out, causing a restart sequence that took 5 minutes. The real watts delivered during the sag were ~15 kW (pump running) vs 0 kW (dropped).

Reversal: For motors that cannot restart automatically, the lower dropout of Siemens may create a hazard – the motor may stall and draw locked‑rotor current until the overload trips. In that case the higher dropout of Schneider is a protection feature.

Failure mode to watch – mixing overload brands

If you pair a Siemens contactor with a Schneider overload (or vice versa), the fault‑clearing coordination is not guaranteed by either manufacturer. The real watts that the contactor must interrupt can exceed its tested breaking capacity. In a bolted fault, the contactor may weld, and the overload may not trip fast enough, leading to a fire or equipment damage. Always use the manufacturer‑recommended overload for the contactor – this is not a “compatibility” issue, it’s a type‑2 coordination requirement.

Rule‑of‑thumb threshold

For motor sizes ≤7.5 kW (400 V), both brands deliver identical real watts per ampere; choose by panel width preference. For 11–18.5 kW, Siemens SIRIUS packs more kW per millimetre (≈22 % denser), making it the choice for space‑constrained panels. If you need a contactor that stays closed through voltage sags, Siemens’ lower dropout gives priority to process continuity; if you want a clean drop to protect downstream equipment, Schneider’s higher dropout is safer. The difference in coil power is negligible except in panels with >100 contactors. Measure your real watts by the load’s AC‑3 rating, not by the contactor’s AC‑1 amp stamp – and never mix overload brands.

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.