IEC 60947-4-1 defines trip class as the maximum tripping time, in seconds, when a cold overload relay is energised at 7.2x the adjusted current, with auxiliary limits at 1.5x and 7.2x setting [S1]. The standard class set is 10A, 10, 20, and 30, and every nameplate number on the relay's trip class field maps to a specific time window in that test sequence [S1].
At the other end of the curve, a properly set thermal overload does not trip at 1.05x full-load amps and must trip at 1.2x FLA within 2 hours, so the "class number" only governs behaviour in the high-overcurrent region where locked-rotor or stall heating dominates [S1]. Picking the right class is therefore a question of matching the motor's locked-rotor time and thermal-damage curve to the relay's allowed tripping window, not a question of "more protection is better".
What the Class Number Actually Means in Seconds
ABB's tripping-class table fixes the boundaries: Class 10 trips in 4-10 s, Class 20 in 6-20 s, and Class 30 in 9-30 s, all at 7.2x setting current from a cold start [S1]. The standard also caps a no-trip band at 1.5x setting (must not trip within 2 minutes) and a must-trip point at 7.2x setting [S1].
Schneider Electric and Rockwell Automation restate the same rule in North-American, 600%-of-FLA language, so a Class 10 must trip in 10 s or less at 600% of the dial setting, Class 20 in 20 s or less, and Class 30 in 30 s or less [S2][S3]. The 600% figure is the NEMA shorthand for the IEC 7.2x test point and lands within the same locked-rotor band. For application work, treat Class 10 as a 4-10 s envelope, Class 20 as a 6-20 s envelope, and Class 30 as a 9-30 s envelope, because the lower bound matters as much as the upper bound on high-inertia loads [S1][S3].
Class 10A is a tighter sub-grade used in IEC markets: both Class 10A and Class 10 are tested at 7.2x from cold, but Class 10A must trip within 2-10 s, while plain Class 10 allows 4-10 s, so 10A sacrifices a small amount of starting margin for faster thermal protection [S8].
Selection Criteria Tied to Motor and Load
The three selection drivers, in order, are locked-rotor accelerating time, motor thermal-damage curve, and permissible stall heating. Mike Holt's forum pinpoints the link: class number maps to the motor's thermal-damage curve, the current-versus-time boundary above which winding insulation starts to cook [S4]. The other end of that damage curve is 115-125% of FLA for 2 hours, with 115-117% typical of IEC motors and 125% typical of 1.15 service-factor NEMA motors [S4].
Across the three common classes the decision rule is consistent: choose a class whose lower trip bound is shorter than the motor's safe stall time at locked-rotor current, but whose upper bound is longer than the actual accelerating time under worst-case load. A 2 HP high-inertia airfoil fan on a 208 V, 6 A motor in the Holt thread tripped at the nameplate setting; tech support recommended moving to a Class 20 device to extend the allowed heating window, illustrating exactly this trade-off [S4].
Useful defaults from the trade literature, qualitative because no first-principles number is universally fixed: Class 10 fits standard IEC induction motors with normal inertia (pumps, fans with short ramp, small conveyors); Class 20 fits high-inertia loads with 5-15 s acceleration (larger fans, mixers, lightly loaded crushers); Class 30 fits very long or marginal starting (high-slip motors, loaded conveyors, compressors with back-pressure) [S6][S7]. A Class 10 on a high-inertia load will nuisance-trip on every start; a Class 30 on a normal motor will sit in stall heating long enough to age the windings [S6][S7].
Decision Matrix: Class 10 vs 20 vs 30 on Real Criteria
On trip time at 7.2x (cold): Class 10 = 4-10 s, Class 20 = 6-20 s, Class 30 = 9-30 s, with the standard's 1.5x no-trip and 7.2x must-trip points common to all [S1]. On motor protection aggressiveness at light overload, a standard Class 20 may take 4-8 minutes to clear a modest overcurrent, so a Class 10A is often specified where the duty cycle cannot tolerate that thermal creep [S6].
On nuisance-trip risk during start: Class 10 is highest, Class 30 lowest, with Class 20 as the balance point for most general-purpose industrial motors [S7]. On cost and availability, Class 20 is by far the most stocked class for IEC thermal-overload relays (TA25DU, TF42DU, and the bulk of the 1SAZ2114xx catalogue sit at class 20 by default), with Class 10 and Class 30 sold as variant suffixes or selected by heater/coil change [S1]. On coordination with short-circuit protection, all three classes are designed to be self-protecting until the upstream SCPD opens, so the coordination study is driven by class window, not by class name [S1].
Use Cases and Failure Modes by Class
Class 10 (and 10A) is the right answer for normal-start IEC induction motors, small pumps, fans with low inertia, and any application where the locked-rotor time stays inside 4-5 s, because the relay will clear a stall before winding temperature crosses the damage curve [S3][S6]. Plain Class 10 in place of 10A gives 2 extra seconds of start margin at the cost of a slightly slower trip on marginal overloads, and is the common North-American factory default for fractional and integral HP NEMA starters [S2][S8].
Class 20 is the de-facto industrial general-purpose class, used on lightly loaded conveyors, mixers, agitators, and most HVAC fans where the start takes 6-10 s but the motor has enough thermal mass to absorb a stalled-rotor event within 20 s [S1][S4]. The Holt-forum case is a textbook example: 2 HP fan drive on a 6 A motor, where the standard Class 10 device was tripping on the high-inertia ramp, and the fix was to step up to Class 20 to extend the start window without crossing the motor's safe-stall curve [S4].
Class 30 covers the long-acceleration cases: high-inertia fans with closed dampers, loaded conveyors, large compressors, and high-slip motors where the start takes 15 s or more, because anything tighter will nuisance-trip before the motor reaches running speed [S6][S7]. The failure mode to watch with Class 30 is the inverse: if a real stall happens, the relay will sit in the 9-30 s window while the motor's insulation cooks, so Class 30 is only acceptable on motors whose thermal-damage curve sits above 30 s at locked-rotor current [S4][S6].
Coordination With the Thermal-Damage Curve and the Wider Protection Stack
The motor-side reference is its thermal-damage curve, and the relay-side constraint is the class window; the only spec-compliant pairing is the one where the relay's envelope sits entirely under the damage curve across the full 1.5x to 7.2x operating range [S1][S4]. ABB's TA25DU family, for example, is a class 20 thermal overload with explicit 1.3-1.8 A up to 8.5-25 A setting bands and ambient compensation from -25 °C to +55 °C, giving the specifier a defined behaviour envelope on standard IEC contactor frames A9-A40 [S1].
For a complete feeder, the overload relay is only one node; for hazardous-area motor circuits the class also has to be coordinated with the upstream short-circuit protective device, the contactor's make/break rating, and any motor protection relay used for phase-loss, earth-fault, or thermistor (PTC) protection. Adjacent reference nodes worth understanding in the same stack are the generic protection relay category, the simpler bimetallic thermal relay used in lower-tier starters, and the microcomputer protection class where the trip curve is software-configurable across the same 10/20/30 envelopes.
Limitations, Constraints, and Common Specification Mistakes
Three concrete pitfalls come up in the field data. First, a standard Class 20 can take 4-8 minutes to trip on a modest overcurrent, so on motors with low thermal mass (small frame, high efficiency, frequent start/stop), Class 10A or an electronic overload set to a tighter curve is the safer pick [S6]. Second, scaling by dial setting is not linear: a Class 20 set to 7 A on a 6 A motor is not the same thermal margin as a Class 10 set to 7 A, because the trip window, not the FLA pick-up, is the class-defining parameter [S1][S4]. Third, the per-class trip envelope assumes 3-phase operation from cold; on a 2-phase trip (blown fuse, single-phasing), the relay will typically trip faster, and the class number only governs the symmetric 3-phase test condition [S1].
Standards-anchored values that are safe to quote: trip class is defined in IEC 60947-4-1, the cold-state test is performed at 7.2x setting current, the 1.5x setting must not trip within 2 minutes, and the 1.2x setting must trip within 2 hours [S1]. The NEMA-shorthand 600% of FLA figure lines up with the same 7.2x IEC test point [S2][S3].
One related reference node in this catalogue: motor protection relay trip wiring covers the fail-safe vs nonfail-safe downstream wiring once the class envelope is fixed, and hazardous-area motor protection settings extends the same class decision into ATEX/IECEx zone-rated feeders. The simplest spec-side test for a chosen class is to overlay the relay's published cold-trip band (4-10, 6-20, or 9-30 s) on the motor's safe-stall time, and reject the selection if the upper bound exceeds the damage curve by any margin.
Trackable signals to watch on the next refresh: electronic overload relays that expose a user-selectable class 10/20/30 in firmware (rather than a fixed bimetal strip) increasingly blur the boundaries, so a future "Class 30" device may in fact be a software curve on a microcomputer protection platform; the IEC 60947-4-1 7.2x test point and the 1.5x no-trip band are the only fields that will stay constant across the mechanical-to-electronic transition [S1].