If you are switching a motor, the AC-3 column on the contactor datasheet governs, not the AC-1 column: IEC 60947-4-1 defines AC-3 make current at 6x rated motor current and break at rated current at 0.35 power factor, while AC-1 covers resistive loads at near-unity power factor with no inrush [S1][S4].
For a 480V, 10HP (7.5kW) three-phase motor the working fit is a 20A Motor Protection Circuit Breaker (MPCB) with a 10x-14x magnetic trip curve paired with an AC-3 rated 18A contactor such as a Schneider TeSys LC1D18, even though the same contactor is rated 40A under AC-1 resistive duty [S2].
AC-1 vs AC-3 Rating Comparison on the Same Frame
A single contactor frame carries two distinct rating columns under IEC 60947-4-1, and the AC-1 number is always higher than the AC-3 number for the same physical device because the electrical stress profile is different [S1]. A 32A AC-1 contactor used to switch a 7.5kW motor may fail prematurely because AC-1 ratings are not designed for motor starting and breaking duties [S1].
The TeSys LC1D18 example shows this directly: 40A at AC-1 (resistive), 18A at AC-3 (motor), with breaking capacity delegated to the upstream breaker (50kA at 480V for the Siemens 3RV2021 MPCB in the reference assembly) [S2]. A 50A thermal current contactor may only handle 11kW of motor load under AC-3 conditions at 400V, and the make/break multiplier on AC-3 is 6x to 10x rated current versus 1x to 1.5x for AC-1 [S1][S4].
Sizing Math: FLC, Inrush, and Utilization Category
For motor loads, size the main contactor at 100% of motor full-load current (FLC) but verify its making/breaking capacity reaches 6x-10x FLC to absorb locked-rotor inrush, which typically lands at 6x-8x FLA for squirrel-cage induction motors [S3][S4]. A 15A motor that draws 90A+ on starting must land on a contactor with an AC-3 rating of at least 15A, which usually means a 32A AC-1 frame such as the LC1D32 [S4].
For resistive loads under AC-1, the rule is simpler: contactor rated current at 100% of FLC, and a standard Type C breaker is acceptable because there is no inrush surge to survive [S2][S3]. The reference selection tree also maps AC-4 to inching, jogging, and plug reversing where the contactor makes and breaks at 6x rated current on every operation, with electrical life typically one-tenth of AC-3 life on the same frame [S1][S2].
Locked-Rotor Realities and Breaker Coordination
Standard thermal-magnetic MCCBs use Type B or C trip curves that trip magnetically at 3x to 10x nominal current, which is below typical motor locked-rotor inrush of 6x-8x FLA and will nuisance-trip on every start [S2]. MPCBs such as the Eaton PKZM0 or Siemens Sirius 3RV2 add a Type D or motor-specific trip curve with magnetic instantaneous hold up to 12x-14x nominal current to ride through the startup surge [S2].
Per the same sizing reference, the contactor handles the high-frequency daily switching (typically 100,000 mechanical cycles) while the breaker handles the 10,000A short-circuit fault, and the contactor's kAIC breaking capacity is delegated to that upstream coordination rather than rated on the contactor nameplate [S2][S4]. For a 0.37kW motor at 400V with 1.1A FLC, the AC-3 rated current is 9A and matches a UKC1-09 frame, and the same 9A frame covers 1.1kW at 2.8A FLC because the AC-3 rating is set by kW at a standard voltage, not by FLC alone [S1].
Selection Rules: What Trips Up Junior Techs
The most common sizing error is reading the maximum amperage printed on the side of the contactor and assuming it applies to all loads, which collapses on the first motor start because that printed number is the AC-1 (Ith) thermal current, not the AC-3 motor rating [S1][S4]. IEC 60947-4-1 utilization categories must match the load: AC-1 for non-inductive or slightly inductive loads above 0.95 power factor such as space heaters, ovens, and incandescent lighting; AC-3 for squirrel-cage motor starting and stopping; AC-4 for inching and plugging [S1].
Two further constraints govern every install. First, route main motor power only through the L1/L2/L3 to T1/T2/T3 power terminals; auxiliary contacts 13/14 (NO) and 21/22 (NC) are low-current signaling paths rated around 10A at 120VAC and will fail if used as power contacts [S4]. Second, DC coils driven directly from PLC transistor outputs need a flyback diode such as a 1N4007 in reverse parallel across A1/A2, because the collapsing field generates an inductive spike of 10x-20x supply voltage that will destroy the output transistor if unclamped; AC coils do not need the diode but benefit from an RC snubber across the contacts for arc suppression [S4].
Decision Matrix: AC-1 vs AC-3 vs AC-4
AC-1 fits resistive heating and distribution with 1x-1.5x make/break multiplier, AC-3 fits compressors, fans, and pumps at 6x-10x make current and 1x break current at 0.35 power factor, and AC-4 covers inching and plug reversing with 6x make AND break on every operation, which is why AC-4 contact life drops to roughly 10% of AC-3 on the same frame [S1][S2][S4]. For a 480V 10HP build, the working fit is 20A MPCB + 18A AC-3 contactor, with a 40A AC-1 column on the same contactor available for heater loads but unusable for the motor [S2].
For anyone comparing adjacent spec work, the same IEC-frame-vs-load logic that governs contactor selection also shows up in diaphragm valve body selection across ductile iron, PVC, and CF8M, where a single material grade carries different pressure-temperature envelopes depending on the media.
Field Signals and Sourcing Notes
Trackable signals for the next purchasing cycle: cross-reference the AC-3 kW at 400V stamp (e.g., AC-3 = 22kW at 400V on a 50A Ith frame) against the motor nameplate kW and FLC before accepting any contactor substitution, and verify the MPCB trip curve class matches 10x-14x inrush hold per NEC Article 430 for motor circuits [S1][S2]. Prices for DIN-rail 3-pole frames such as Eaton XTCE or Siemens Sirius range roughly $45 for 9A models up to $120 for higher-current frames, with Siemens 3RT6028-1AL20 (18.5kW at 400V AC-3) listed around $20-22 in the 2026 distributor catalog [S3][S4].
For the relevant spec sheets and selection criteria, see contactor, pressure transmitter, and flow meter.