What is the difference between AC-1 and AC-3 utilization categories in contactors?
Compare AC-1 (resistive/non-inductive loads), AC-3 (squirrel-cage induction motors) and AC-4 (plugging/inching) contactor utilization categories under IEC 60947-4-1.
What is the main difference between AC-1 and AC-3 utilization categories in contactors? Under IEC/EN 60947-4-1, AC-1 applies to resistive or slightly inductive loads with a power factor of cos φ ≥ 0.95 (such as electric heaters and ovens), whereas AC-3 applies to starting squirrel-cage induction motors (which draw 5 to 8 × rated current at locked-rotor start) and switching them off after they have reached full speed. Because motor starting stresses the main contacts far more than resistive switching, the same contactor has a much higher thermal current rating (Ith) in AC-1 than its motor operational current (Ie) in AC-3 (for example, a contactor rated 18 A / 7.5 kW in AC-3 is typically rated 32 A in AC-1).
AC-1, AC-3 and AC-4 contactor utilization category comparison table
| Criterion | AC-1 Utilization Category | AC-3 Utilization Category | AC-4 Utilization Category |
|---|---|---|---|
| Target Load Type | Resistive (ohmic) or non-inductive loads | Squirrel-cage induction motors (normal starting and stopping) | Squirrel-cage motors (inching/jogging, plugging, reversing under load) |
| Power Factor (cos φ) | cos φ ≥ 0.95 | cos φ ≈ 0.65 – 0.85 (~0.35 at start) | cos φ ≈ 0.35 – 0.65 |
| Making Current | 1 × Ie (no inrush surge) | 5 – 8 × Ie (locked-rotor motor inrush) | 6 – 10 × Ie |
| Breaking Current | 1 × Ie (low arc energy) | 1 × Ie (broken at nominal running speed) | 6 – 8 × Ie (broken while the motor is still accelerating) |
| Example Contactor Rating | Ith = 25 A (AC-1) | Ie = 9 A / 4 kW (400 V AC-3) | Ie ≈ 4.5 – 6 A (derated due to severe arc erosion) |
| Typical Application | Resistance heaters, industrial ovens, lighting banks, distribution feeders | Fans, pumps, compressors, conveyors and standard geared motors | Crane hoists/travel drives, spot welders, frequent jogging/inching machinery |
| Popular AC-3 Current Ratings | 25 A · 32 A · 40 A · 65 A | 9 A · 12 A · 18 A · 32 A | 18 A · 32 A · 50 A |
| Popular Coil Voltages | 220 V AC · 24 V DC | 220 V AC · 24 V DC · 24 V AC | 220 V AC · 24 V DC |
| Category and Guide | Contactors · Lighting Contactors | Contactors · Selection Guide | Contactors · Contactor Accessories |
How do you size a contactor for motor vs heater loads?
Selecting a contactor by the wrong utilization category leads either to premature contact welding or to unnecessary oversizing:
- Always size induction motor contactors by the 400 V AC-3 rating (kW and Ie): A 400 V, 11 kW (~22 A) motor cannot be driven by a 9 A contactor whose nameplate states "Ith = 25 A (AC-1)"; you must select a power contactor rated at least 25 A or 32 A in AC-3.
- Upsize for AC-4 inching/hoist duties and add mechanical interlocks: When a motor is stopped or reversed before reaching full speed, breaking arc energy increases sixfold compared to AC-3. Upsize the contactor frame according to the manufacturer's AC-4 life curves and install a mechanical interlock between reversing pairs.
- Use dedicated AC-6b capacitor contactors for power factor correction: Switching power capacitors generates inrush peaks up to 100–150 × rated current; always use dedicated power factor correction contactors equipped with early-make damping resistors.
Frequently asked questions
Can a contactor marked Ith = 25 A run a 25 A motor?
The "Ith = 25 A" value printed on a small contactor refers to its AC-1 thermal current for resistive heater loads; its 400 V AC-3 motor rating is typically only 9 A (4 kW) or 12 A (5.5 kW). Driving a 25 A (11 kW) induction motor requires a contactor rated 25 A or 32 A under AC-3 (which has an AC-1 rating of 40–50 A).
What is the AC-3e utilization category and how does it differ from AC-3?
Introduced in IEC 60947-4-1, AC-3e covers high-efficiency IE3 and IE4 induction motors. While IE3/IE4 motors draw lower steady-state current, their lower stator winding resistance causes locked-rotor starting peaks to be 15–25 % higher than IE2 motors; AC-3e contactors are verified to make these higher starting surges without contact welding.
How are main, delta and star contactors sized in a star-delta starter?
In a star-delta starter, the main and delta contactors each carry 58 % of the motor's rated current (In / √3 ≈ 0.58 × In) in AC-3 duty, whereas the star contactor carries 33 % of rated current (0.33 × In) only during the starting transition.