Contactors look simple — a coil pulls in a set of contacts. The selection mistakes engineers make come from not understanding the utilisation categories. A contactor “rated 25A” might fail at 12A on a motor load. Heres how to read the spec sheets and pick correctly.
Utilisation categories (IEC 60947-4-1)
- AC-1: Resistive / lightly inductive loads (heaters, lamps, transformers at idle). Easy switching — full nameplate current applies.
- AC-2: Slip-ring motor starting (rare in modern practice).
- AC-3: Squirrel-cage induction motor starting & switching. This is what you want for any standard 3-phase motor. The AC-3 rating is typically lower than the AC-1 rating — derating is normal.
- AC-4: Plugging / inching duty (frequent reversing or jogging). Significant further derate.
- AC-15: Switching electromagnets (e0 g. solenoid valves, contactor coils via auxiliary contacts). Low rated current but high pickup current.
Picking by motor kW
The fastest rule of thumb: contactor AC-3 rating in kW @ 400V should match or exceed the motor kW. So a 5.5 kW motor needs an AC-3 ≥ 5.5 kW contactor. Common sizes:
| Motor (kW @ 400V) | AC-3 (A) | ABB AF range | Schneider TeSys | Siemens SIRIUS 3RT |
|---|---|---|---|---|
| 2.2 | 9 | AF09 | LC1D09 | 3RT2015 |
| 4.0 | 12 | AF12 | LC1D12 | 3RT2017 |
| 5.5 | 16 | AF16 | LC1D18 | 3RT2018 |
| 7.5 | 22 | AF22 / AF26 | LC1D25 | 3RT2025 |
| 11 | 26 – 32 | AF26 / AF30 | LC1D32 | 3RT2027 |
| 15 | 40 | AF40 | LC1D40A | 3RT2035 |
| 22 | 50 – 65 | AF50 / AF65 | LC1D50A | 3RT2037 |
Coil voltage — dont guess
Always specify the coil voltage explicitly. ABB AF-series contactors use wide-band AC/DC coils (e0 g. 100–250 V AC/DC) which simplifies stocking. Schneider TeSys D and Siemens 3RT have fixed-voltage coils — order the exact V you have.
Auxiliary contacts
Each contactor typically has 1NO and 1NC built in. For control circuits, add front-mount or side-mount aux blocks: 2NO+2NC is the typical addition. Verify the brands auxiliary part numbers match the main contactor frame (ABB CA5, Schneider LADN, Siemens 3RH).
Can I substitute brands mid-project?
Yes within frame-size class. ABB AF22 ↔ Schneider LC1D25 ↔ Siemens 3RT2025 are technically interchangeable for AC-3 motor switching at the rated kW — same trip times, same heat dissipation. Wiring is identical (A1-A2 coil, 1-2, 3-4, 5-6 contacts). Brand mixing within a single panel is OK provided the auxiliary blocks match each main.
Sourcing
EPFINDER carries the AF (ABB), TeSys D & F (Schneider), SIRIUS 3RT (Siemens), and DILM (Eaton) ranges in stock. Browse contactors or submit a project BOM.
