Short answer: size a contactor on the motor’s full-load current at the voltage it will actually run on, in AC-3 duty, not on the kW on the nameplate. Kilowatts are a convenient way to look up a first candidate, but the same 7.5 kW motor draws about 15.5 A at 400 V, about 14 A at 440 V and a different figure again at 60 Hz — and the contactor is chosen for the current it must break, not for the shaft power it describes.
We supply contactors, overload relays and starters as an independent trading company — not a manufacturer, and not an appointed distributor of any brand. This is the sizing method we use when a buyer sends a motor list with kW only, which is most of the time.
The table everyone uses, and what it really means
Four-pole motors at 400 V, standard efficiency, give a working rule of roughly 2 A per kW. That is close enough for a first pass and not close enough for a purchase order, because it ignores efficiency, power factor, pole count and the actual supply voltage.
| Motor kW (4-pole, 400 V) | Typical full-load current | Contactor frame to start from (AC-3) | Overload range to order |
|---|---|---|---|
| 0.75 | 2.1 A | 9 A | 1.6–2.5 A |
| 1.5 | 3.5 A | 9 A | 2.5–4 A |
| 2.2 | 5.0 A | 9 A or 12 A | 4–6 A |
| 3.0 | 6.6 A | 12 A | 5.5–8 A |
| 4.0 | 8.5 A | 12 A | 7–10 A |
| 5.5 | 11.5 A | 18 A | 9–13 A |
| 7.5 | 15.5 A | 18 A or 25 A | 12–18 A |
| 11 | 22 A | 25 A or 32 A | 17–25 A |
| 15 | 29 A | 32 A or 40 A | 23–32 A |
| 18.5 | 35 A | 40 A | 28–40 A |
| 22 | 41 A | 50 A | 34–45 A |
| 30 | 56 A | 65 A | 45–63 A |
| 37 | 68 A | 80 A | 54–75 A |
| 45 | 82 A | 95 A | 70–85 A |
Read the third column as a floor, not an answer. “9 A or 12 A” appears where the frame boundary sits near the motor current, and the choice depends on the duty and the ambient rather than on the kW.
Why the same kW gives different amps
Three things move the current away from the 2 A/kW rule. Voltage is the largest: power is roughly voltage times current times power factor times efficiency times the square root of three, so current scales inversely with voltage. A 7.5 kW motor at 440 V draws about 14 A, not 15.5 A. A 380 V motor draws slightly more than the 400 V figure. A 60 Hz motor of the same kW usually runs faster and can draw slightly more or less depending on the design, which is why the nameplate is the only safe source.
Pole count matters too: a 4-pole 7.5 kW motor runs near 1,450 rpm at 50 Hz, an 8-pole near 720 rpm, and the slower motor is physically larger for the same power with a higher current. Efficiency class moves the figure by a few per cent — an IE3 motor draws less than an IE1 of the same rating, which sounds like good news until a panel is designed around the older, higher current and the “improved” motor is what arrives.
Three checks, not one
Thermal check (AC-3). The contactor’s AC-3 rated current must be at least the motor’s full-load current, derated for the ambient inside the enclosure. A 40 °C panel interior is normal in the Gulf and in parts of Africa, and contactor ratings are usually published at 40 °C ambient already — read the footnote rather than assuming.
Duty check (AC-4). AC-3 assumes the contactor breaks a running motor. Inching, plugging, reversing under load and crane or hoist duty are AC-4, where the contactor makes and breaks starting current. The same frame carries far less in AC-4, often around a quarter to a third of its AC-3 current, and a starter that reverses every ten seconds must be sized on the AC-4 figure or it will be replaced within months.
Coordination check. The contactor has to ride through the starting current and the fault current until the protective device clears. That is a type-2 coordination question answered by the manufacturer’s coordination table for the specific contactor, overload relay and short-circuit protective device combination — not by adding up current ratings.
| Duty | What the contactor does | Sizing basis | Typical use |
|---|---|---|---|
| AC-1 | Non-inductive or slightly inductive load | Rated thermal current | Heaters, resistive loads |
| AC-3 | Makes starting current, breaks running current | Motor full-load current | Standard motor starters, pumps, fans |
| AC-4 | Makes and breaks starting current | Manufacturer’s AC-4 table, often 25–35 % of AC-3 | Inching, plugging, reversing, hoists |
Star-delta and other reduced-current starts
In a star-delta starter the main and delta contactors each carry the motor’s full-load current divided by the square root of three, about 58 %, and the star contactor carries about a third. A 15 kW motor at 29 A therefore uses main and delta contactors sized around 17 A and a star contactor around 10 A in principle — but most designers size the delta contactor for the full motor current because it also breaks the motor circuit on changeover, and because standardising on one frame is cheaper than stocking three. Follow the starter manufacturer’s table, and do not apply the 58 % rule to the delta contactor without checking what it is asked to break.
Other voltages on the same motor list
A motor list written for a 400 V project and re-used for a 440 V 60 Hz destination is the classic export error. The kW stays, the current falls, and the contactor could be smaller — but the coil must change to the 60 Hz voltage, the overload relay must be re-set to the new nameplate current, and the short-circuit coordination must be checked against the new fault level. On 220–240 V three-phase systems the current for the same kW roughly doubles against 400 V, which moves the starter up two or three frames.
What to put on the purchase order
Motor kW, poles, nameplate current, voltage and frequency; the duty (AC-3, or AC-4 with the operations per hour); the ambient inside the enclosure; whether the starter is direct-on-line, star-delta, soft start or reversing; the coil voltage and frequency; the overload relay range; the short-circuit protective device if coordination type 2 is required; and the enclosure type with its IP rating. If the motor list gives kW only, ask for the nameplate current before confirming the order — it is the one number that turns a table into a starter that works.
FAQ
Can I size a contactor from kW alone?
For a first quotation, yes, using about 2 A per kW at 400 V. For the order, no: the nameplate current at the actual voltage and frequency is the basis, and the overload relay setting is taken from it.
Why does a 7.5 kW motor sometimes need a 25 A contactor instead of 18 A?
Because the frame boundary sits near the motor current, and the higher frame is chosen for ambient, duty or standardisation across a panel. Also because a motor with a higher nameplate current than the typical table value — an older or slower design — may exceed the smaller frame’s AC-3 rating.
Is a bigger contactor always safer?
Electrically it has more margin, but it costs more, takes more rail space, and changes the coil and auxiliary contact accessories. Oversizing by one frame for ambient or duty is normal; oversizing by three because the current was never checked is a habit worth losing.
Does the overload relay get sized to the contactor or to the motor?
To the motor. The relay’s setting range must contain the motor’s full-load current, and it is set to that current — or to the service-factor value where the motor has one. The contactor rating does not enter the setting.
Related: Contactor coil voltage codes · AC-1 vs AC-3 vs AC-4 contactor duty · Setting an NR2 thermal overload relay · Star-delta starter wiring · Reversing contactor interlock wiring · Motor protection circuit breakers · Cross-reference an existing device · Request a quotation