Motor contactor sizing starts with the motor's nameplate full-load amperage (FLA), not the nameplate kilowatts, because FLA already embeds voltage, power factor, and efficiency for the specific connection the contactor will switch [S1][S2]. Converting kW to FLA with I = kW × 1000 / (√3 × V × η × cosφ) gives the steady-state current the contactor must carry, while the locked-rotor inrush of 5–8× FLA during a direct-on-line start is what dictates the utilization category [S4].
The right contactor for that FLA is selected on its AC-3 current rating at the motor's line voltage, because AC-3 covers squirrel-cage motor starting and stopping while running, the typical DOL duty profile [S1]. A 1.25× multiplier over FLA is the standard safety margin for standard cycling duty, with a larger margin, or a different control device, called for once the application approaches AC-4 inching or repeated plugging [S2].
Convert kW to FLA Before Touching a Contactor Catalog
FLA is the only number that maps to a contactor's published AC-3 ampere rating, and a generic kW-to-amps shortcut that ignores power factor and efficiency is not safe for selection [S1]. A 415 V, 5 HP, three-phase DOL motor with code A, 80% efficiency, and 0.8 power factor works out to FLA = (5 × 0.746) × 1000 / (1.732 × 415) = 6 A, with locked-rotor current in the 7–22 A range for that same code band [S4].
US-style 460 V induction motors are normally read off the nameplate; a 15 HP, 460 V, three-phase nameplate around 21 A is the figure to enter the contactor selection table, not the 15 HP figure [S1]. Whenever a panel builder estimates FLA from kW alone, they should apply the standard textbook formula I = kW / (√3 × V × cosφ × η) and add a check against the actual nameplate before ordering, because the difference between an undersized and a right-sized contactor is roughly the difference between FLA and AC-3 rating [S1][S4].
Pick the Utilization Category Before the Ampere Frame
IEC utilization categories are the language a contactor is tested under, and they determine what "40 A" actually means on the data sheet: a 40 A AC-1 resistive rating typically drops to 18–25 A AC-3 motor duty at the same voltage, and AC-4 inching or plugging ratings are lower still [S1]. Standard DOL start/stop falls under AC-3, while any application that plugs, jogs, or reverses under power is AC-4 and is severe enough that simply "adding a little margin" is not enough, a larger frame or a soft starter/VFD is often the correct answer [S1][S3].
NEMA sizes (00, 0, 1, 2, 3, …) bundle conservative motor-duty assumptions into each size step, so a NEMA-sized contactor is implicitly sized for inductive service rather than resistive [S1]. For most pump, fan, and compressor duties the AC-3 column on the IEC sheet is the correct reference, with AC-4 only consulted for reversing starters and inching [S1][S5]. Field experience confirms this: 208 V, 17 A FLA, 7.5 HP coolant pumps cycling roughly once per minute have burned through AC-3 contactors rated for that horsepower inside a year, because the duty cycle exceeded what AC-3 was designed for [S3].
Apply the 1.25× Margin, Then Step Up for Cycling and Inrush
The widely cited starting formula for a motor contactor is Contactor Size = FLA × 1.25 to 1.5, with 1.25× as the baseline for standard start/stop service and the higher end of that band for harsh environments or frequent starts [S2]. A worked example from the field: 52 A FLA motor × 1.25 = 65 A, so the spec calls for a contactor with an AC-3 rating of 65 A or higher at the motor's line voltage [S2].
Inrush is the physical reason that margin exists. Motors draw 5–8× FLA at locked rotor, and DOL starting current is normally 6–7× FLA on a 5 HP code-A machine, so the contactor must absorb that surge without welding [S1][S4]. Above the standard margin, two situations force a larger contactor or a different starting method: AC-4 jogging, where a 5–8× inrush happens during a break-and-make event, and high-cycling loads such as compressor contactors, which call for a deliberately oversized frame because the electrical life, not the steady-state current, is the limiting factor [S1][S2][S3].
Contactor, Overload Relay, Fuse, and Coil Voltage Form a Bundle
A motor contactor is one part of a DOL starter; the fuse, circuit breaker, thermal overload relay, and coil voltage have to be matched to the same FLA for the system to actually protect the motor [S4]. The contactor ampere rating is the smallest of the switching parts in a DOL bundle: a 5 HP, 415 V DOL motor with 6 A FLA and 45 A starting current typically needs a contactor rated 24 A or larger even though the motor is well under 5 kW, because short-time withstand, not running current, governs the frame [S4].
Coil voltage is selected separately from the power circuit, with 24 V, 120 V, and 240 V being the common control-side options that must match the control transformer or PLC output [S2][S5]. The overload relay downstream of the contactor is set at or just above FLA, so the contactor's AC-3 rating and the relay's current setting both need to clear the same locked-rotor event without nuisance trips [S4][S5]. Reviewing manufacturer datasheets for the specific contactor frame, not the generic kW chart, is the last sanity check before pulling the trigger on a part number [S2].
When a Bigger Contactor Is Not the Answer
Once a contactor is failing in under a year on a small motor, the diagnosis is rarely "go one frame up" and is more often "change the control method" [S3]. On 7.5 HP coolant pumps cycling every minute, a soft starter or VFD is the standard remedy because it cuts the inrush event that an AC-3 contactor is being asked to interrupt; in a 2023 forum discussion the same motor, the same AC-3 contactor, kept failing until a VFD (around $500 for that size) replaced the across-the-line contactor [S3].
For jog, plug, or rapid reversing duty, AC-4 ratings on the same physical contactor can be less than half of the AC-3 rating, so a different control approach (VFD, soft start, or a dedicated AC-4 frame) is usually cheaper than upsizing contactors every time a duty cycle intensifies [S1][S3]. The decision threshold reads cleanly: standard start/stop, AC-3 contactor at 1.25× FLA; high cycling or jogging, oversize the frame or replace the starter with a VFD; reversing under load, accept that AC-4 governs and pick accordingly [S1][S3][S5].
Field-Spec Comparison: AC-3 Frame vs NEMA Size vs Soft Start
For a 5 HP, 415 V three-phase motor, three realistic options line up against the same FLA = 6 A, locked-rotor inrush 7×: an IEC AC-3 contactor around 9 A minimum with 1.25× margin pushing to roughly 8 A and a practical 9–12 A frame, sized for 24 A short-time withstand per DOL starter practice [S4]; a NEMA size 00 or 0 contactor, which folds the inductive margin into the size step rather than publishing AC-3 amps; and a soft starter or VFD, which removes the inrush event at the cost of higher unit price and added setup [S1][S3][S4]. On a 52 A FLA motor, the 1.25× rule lands the spec at a 65 A AC-3 contactor, while a compressor duty on the same FLA steps up to a deliberately larger frame because electrical life, not running current, is the constraint [S2].
The selection signal is that AC-3 + 1.25× FLA is the correct path for standard cycling, NEMA size is the right call when the panel builder wants the conservative inductive rating baked in, and any application approaching once-per-minute starts is past the contactor's economic operating envelope and belongs on a soft starter or VFD [S1][S2][S3]. Two trackable follow-ups on this rule: confirm whether the contactor manufacturer's data sheet quotes AC-3 amps at the same line voltage the motor will actually run at (220 V vs 380 V vs 460 V changes the rating), and check the contactor's electrical-life curve at the actual cycling rate rather than at nameplate AC-3 amps [S1][S2][S5].
For the relevant spec sheets and selection criteria, see contactor, ac motor, and drive motor.
Background reading: Hydraulic Power Unit Cooler Sizing: kW-per-kW Heat Rejection.