When Load Doubles, Which Contactor Provenance Actually Protects You? 3 Decision Rules

The number. The Siemens SIRIUS 3RT2016 is listed at 9 A AC‑3 (400 V), corresponding to 4 kW. The Schneider TeSys D LC1D18 is rated 18 A AC‑3 (400 V), i.e. roughly 7.5 kW (illustrative, based on 380–415 V power factor ~0.85). Both are IEC 60947‑4‑1 compliant.

Mechanism. IEC 60947‑4‑1 defines AC‑3 ratings for starting squirrel‑cage motors: make current ~6× rated current, break at rated current. A 9 A rating means the contactor is design‑tested to close into 54 A inrush and interrupt 9 A at 400 V, up to 6 kV impulse withstand. The Siemens 3RT2016 is a Size S00 frame — 45 mm wide, limited arc chamber volume. The TeSys D LC1D18 uses a wider frame (45 mm as well but deeper arc‑splitter stack) — the manufacturer engineered 18 A AC‑3 from the same physical envelope; the provenance is internal arc‑energy management, not just contact material.

Worked consequence. If your load doubles from 18.5 kW to 37 kW (say two motors on one contactor, or a single motor with high‑inertia fan start), a single 3RT2016 at 9 A AC‑3 cannot make the inrush: 37 kW at 400 V ~ 74 A FLC, inrush ~444 A. The contactor would weld on the first close. You need at least a 3RT2 size S2 (50 A AC‑3). The TeSys D LC1D18, at 18 A AC‑3, also fails — you need a LC1D50 (50 A AC‑3). The provenance tells you: never scale a single contactor from the table without cross‑checking the frame size and arc‑chamber ratings. The decision rule: when load doubles, ignore the AC‑1 rating (resistive) — the AC‑3 rating from the same IEC table is your binding constraint. If you cannot find a single‑frame contactor at that AC‑3 level, you must parallel or upgrade frame; paralleling is rarely allowed in the standard (only specially tested pairs). So the rule is: pick a frame whose AC‑3 rating ≥ 2× original motor FLC × 1.15 safety factor.

When this reverses. If the load is purely resistive (AC‑1), the doubling is trivial: both contactors handle 25 A (3RT2016) and 25 A (LC1D18) respectively — no issue. Or if you use a soft‑starter to limit inrush to 150 %, the AC‑3 rating becomes less binding.

The number. The Siemens SIRIUS 3RT range uses conventional coils: you must order the exact coil voltage (e.g. 24 V AC, 230 V AC, 24 V DC — each a separate part number). The Schneider TeSys D EverLink offers coil options from 24 V to 480 V AC and 24 V DC, but each is a discrete coil — there is no single wide‑range coil in the TeSys D line. However, the ABB AF contactor (a key alternative) has an electronic wide‑range coil covering 100–250 V AC/DC in one SKU. The Siemens contactor line does not offer a wide‑range coil as standard; each 3RT is a discrete coil.

Mechanism. When load doubles, you often add a second motor downstream, possibly on a different transformer tap or in a facility where control voltage sags during startup. A conventional coil (Siemens) will drop out if voltage dips below ~85 % of nominal — e.g. a 230 V AC coil drops out at ~195 V. An electronic wide‑range coil (ABB) maintains pick‑up down to ~70 % of the lower range limit. The provenance here is the coil's voltage‑withstand curve: conventional coils rely on copper wire turns and magnetic flux; wide‑range coils use a switch‑mode supply internally, holding the contactor closed even during deep sags.

Worked consequence. Imagine the doubled load causes a 12 % voltage drop on a 240 V line (down to 211 V). A Siemens 3RT with a 230 V coil will see 211 V → ~92 % nominal, still above dropout, but if the sag dips further (e.g. 20 % due to transformer impedance), it hits 192 V → dropout, the contactor opens under load, arc may cause contact welding. The TeSys D with a 240 V coil also drops out at ~204 V (85 %). The decision rule: if your plant has weak utility or generator power with ±15 % voltage swings, a wide‑range coil (like ABB AF) is the only provenance that guarantees hold‑in during a double‑load startup. When does the rule reverse? If your facility has a dedicated, stiff transformer (impedance

The number. Siemens 3RT contactors pair exclusively with 3RU2 (thermal) or 3RB2 (solid‑state) overload relays from the SIRIUS family. Schneider TeSys D contactors pair with LR2D and LRD overloads, also brand‑locked. Neither overload is interchangeable across brands — the mounting rail and bimetal curve are proprietary.

Mechanism. When load doubles, you must change the overload heater (or recalibrate an electronic unit). The provenance of the overload relay is its trip curve: IEC 60947‑4‑1 Class 10 or Class 20. A Class 10 relay trips in ≤10 s at 7.2× FLC. If the overload is matched to the original motor (say 18 A heater), and you now run a 37 A motor on the same contactor, the heater is undersized — nuisance tripping on startup. You swap to a 40 A heater. But the contactor frame itself must accommodate the larger heater: a Siemens 3RT2016 (Size S00) can only accept 3RU2 heaters up to 32 A. Above that, you need a larger frame (S0, S2). The decision rule: before doubling the load, check the maximum heater range of the contactor frame, not just the contactor ampere rating. The provenance is the frame size's physical capacity to accept a higher‑rated overload block.

Worked consequence. A 3RT2016 with a 3RU2 overload tops out at 32 A thermal. If your doubled load is 37 A FLC, you cannot stay in the same frame — you must replace the entire contactor and overload pair. The TeSys D LC1D18 with a LR2D overload also tops out at 25 A. So both force a frame jump. The non‑obvious insight: the overload relay provenance (frame size limit) is often the binding constraint before the AC‑3 rating. Many engineers check the contactor A rating and miss the heater limit. This is the hidden failure mode: you install a 40 A contactor but the heater maxes at 32 A, so you either misuse a larger heater (wrong curve) or overload the heater (thermal damage).

When this reverses. If your doubled load still falls within the same heater range (e.g. original 10 A, new 18 A, both within a 3RU2 range 10–32 A), no frame change needed — only the heater module swap. Then provenance is irrelevant; any brand works.