Buying guide
Contactor Size for a Motor: kW to Amps Chart and Selection Guide
An undersized contactor welds its contacts; an oversized one wastes money and panel space. Use this guide to match a contactor and overload relay to a three-phase motor on Pakistan’s 400 V, 50 Hz supply — for direct-on-line (DOL) and star-delta starters.
Rule 1: size by the AC-3 rating, not the AC-1 rating
Every contactor datasheet lists several ratings. For squirrel-cage induction motors that are started and stopped normally, the one that matters is AC-3 (in amps or kW at 400 V). The much higher AC-1 figure is for resistive loads such as heaters and must not be used for motors. If the motor is inched, plugged or reversed while running (AC-4 duty), choose a larger contactor as the manufacturer’s AC-4 table specifies.
Motor kW to amps chart (400 V, 3-phase, 50 Hz)
The table shows typical full-load current (FLC) for 4-pole IEC motors. Always use the current on your motor’s nameplate when it is available — efficiency class and pole count change the figure.
| Motor kW | HP (approx.) | Typical FLC (A) | Contactor AC-3 rating | Overload relay range |
|---|---|---|---|---|
| 0.75 | 1 | 1.8 | 9 A | 1.6–2.5 A |
| 1.5 | 2 | 3.5 | 9 A | 2.5–4 A |
| 2.2 | 3 | 4.9 | 9 A | 4–6 A |
| 3 | 4 | 6.5 | 9 A | 5.5–8 A |
| 4 | 5.5 | 8.5 | 9 A | 7–10 A |
| 5.5 | 7.5 | 11.5 | 12 A | 9–13 A |
| 7.5 | 10 | 15.5 | 18 A | 12–18 A |
| 11 | 15 | 22 | 25 A | 16–24 A |
| 15 | 20 | 29 | 32 A | 23–32 A |
| 18.5 | 25 | 35 | 38 A | 30–38 A |
| 22 | 30 | 41 | 50 A | 37–50 A |
| 30 | 40 | 55 | 65 A | 48–65 A |
| 37 | 50 | 66 | 80 A | 55–70 A |
| 45 | 60 | 80 | 95 A | 63–80 A |
| 55 | 75 | 97 | 115 A | 80–104 A |
| 75 | 100 | 132 | 150 A | 110–140 A |
Star-delta starter sizing
In the usual star-delta starter, the line and delta contactors are each connected in series with a motor winding, so every contactor carries only part of the line current and can be smaller than a DOL contactor for the same motor:
- Main (line) contactor ≥ 0.58 × FLC
- Delta contactor ≥ 0.58 × FLC
- Star contactor ≥ 0.33 × FLC
- Overload relay (in the motor winding circuit) set to 0.58 × FLC
Example: a 37 kW motor with 66 A FLC needs main and delta contactors of at least 38 A, a star contactor of at least 22 A (use 25 A), and an overload relay set to about 38 A. The star and delta contactors must be mechanically and electrically interlocked, with a timer (typically 5–10 s) controlling the changeover. These factors assume that arrangement. If your starter feeds the delta contactor from the winding terminals instead, the line contactor carries the full line current and the overload relay is set to the full nameplate current, so always follow the starter’s wiring diagram. Star-delta also needs a six-terminal motor wound for 400 V delta.
Choosing the coil voltage
- 230 V AC – the most common control voltage in Pakistani panels, taken phase-to-neutral.
- 24 V DC – when a PLC output drives the coil directly; safer for operators and immune to supply dips.
- 110 V AC – via a control transformer, common in older or imported machinery.
- 400 V AC – line-to-line control without a neutral; avoid where possible for safety.
Voltage fluctuation and load-shedding are hard on AC coils. If your site sees frequent dips, a DC or wide-range electronic coil reduces chattering and coil burn-out.
Auxiliary contacts, interlocks and accessories
- Auxiliary contacts (NO/NC) for self-hold (latching) circuits, indication and PLC feedback.
- Mechanical interlock for reversing and star-delta pairs so both contactors can never close together.
- Surge suppressor (RC or varistor) across the coil when it is switched by a PLC or electronics.
- Start/stop push buttons and pilot lights — see push buttons & indicators.
Quick selection checklist
- Read the motor nameplate FLC at 400 V.
- Choose a contactor whose AC-3 current is at or above the FLC (DOL) or 0.58 × FLC (star-delta).
- Upsize one frame for frequent starts, high ambient temperature or AC-4 duty.
- Pick an overload relay whose range brackets the FLC (or 0.58 × FLC for star-delta).
- Match the coil voltage to your control circuit.
- Add auxiliary contacts and interlocks as the circuit requires.
Browse contactors & motor starters from Schneider Electric, Siemens, ABB and Chint, or send us the motor nameplate photo for an exact recommendation.
Frequently Asked Questions
What size contactor do I need for a 7.5 kW (10 HP) motor?
A 7.5 kW, 400 V motor draws about 15.5 A, so an 18 A (AC-3) contactor with a 12–18 A overload relay is the usual direct-on-line choice.
What is the difference between AC-1 and AC-3?
AC-1 is the rating for resistive or slightly inductive loads such as heaters. AC-3 is for starting and stopping squirrel-cage motors and is always lower. Size motor contactors by AC-3.
At what size should I use star-delta instead of DOL?
Many installations switch to star-delta or a soft starter above roughly 7.5–11 kW to limit starting current, but the right point depends on the supply capacity and the load. Check with your utility and the driven machine’s requirements.
Why does my contactor hum or chatter?
Low or fluctuating coil voltage, a dirty or damaged magnet face, or the wrong coil voltage are the usual causes. Check the control supply during starting and replace the coil or contactor if the pole faces are worn.