Trip class is a binding design parameter, not a marketing label: the class number on a motor protection relay is the maximum seconds it will take to open the contactor when the relay sees 600% of its full-load ampere (FLA) setting, with Class 10 at ≤10 s, Class 20 at ≤20 s, and Class 30 at ≤30 s under NEMA ICS 2 and UL 508 test conditions [S3].
The right class is the one that sits between the motor's locked-rotor acceleration time and its thermal damage curve. Under-class and the relay trips every start; over-class and the windings cook before the relay opens [S3][S8].
What the Three Trip Classes Actually Mean
Class 10, Class 20, and Class 30 are defined by the same physical test point (600% of relay setting) and only the trip-time bound changes: 10 s, 20 s, 30 s respectively [S3][S4]. The relay still follows an inverse-time curve, so a locked-rotor fault at 1000% FLA trips much faster than the class number suggests, and a modest overload at 125% of setting must hold for at least 2 hours cold-start per NEMA ICS 2 [S3].
Class 10A, used in IEC markets, tightens the lower end of that curve so the relay must trip within 2 minutes at 150% of setting, while a stock Class 20 may hold 4 to 8 minutes at the same current, which is long enough to damage winding insulation on small IEC motors [S5]. A 2 HP airfoil fan case discussed on Mike Holt's forum (Nov 2018) showed the practical consequence: a starter shipped on Class 10 nuisance-tripped until the user moved to a Class 20 unit, then held cleanly at 5 A running current on a 6 A FLA nameplate [S4].
Normal-Duty Loads: When Class 10 or 20 Is Correct
For general-purpose NEMA Design B motors driving pumps, fans, conveyors, and compressors with normal acceleration, locked-rotor current sits at roughly 600 to 700% of FLA and run-up finishes in 5 to 10 s, so Class 10 or Class 20 lines up with the thermal curve without nuisance trips [S1][S3].
Class 10 is the IEC default for standard industrial motors and for sealed or submersible units where the winding heat sink is poor and any excess I²t matters, while Class 20 is the typical NEMA default for 1.15 service factor motors that can absorb a little more thermal stress [S5]. Choosing the slower class in the normal-duty range also gives margin for voltage dip starts, cold oil, and worn belts that stretch the run-up by a second or two [S1][S3].
High-Inertia Loads: Why Class 30 Exists
High-inertia loads such as large centrifuges, hammer mills, crushers, mixers with heavy product, and large ID/FD fans draw locked-rotor current for a long enough time that a Class 10 or 20 relay will trip before the motor reaches rated speed, even though nothing is electrically wrong [S1][S3][S8]. Siemens support material (ID 30587022) and Rockwell Automation's overload relay KB article both state that Class 30 is usually required for high inertial loads to help prevent nuisance tripping [S1][S8].
Valuadd's May 2026 guidance makes the boundary explicit: very high inertia loads, or any start profile that runs longer than 20 s, push the spec to Class 30 [S3]. Electronic overload relays from Siemens, Eaton, and Rockwell let the same device be re-classed in the field via DIP switch or software, so the panel builder can ship a Class 30 unit and still protect a normal-duty downstream motor after commissioning [S3][S5].
Decision Matrix: Matching Class to Run-Up Time and Load Type
The practical decision is a four-criterion comparison rather than a single threshold. Trip class, typical run-up time at 600% FLA, target load type, and expected motor thermal margin line up as follows, drawn from the trip-class definitions in NEMA ICS 2 / UL 508 and the application guidance across S1, S3, S5, and S8: [S3]
Class 10 / 10A, run-up ≤10 s, targets submersible pumps, hermetically sealed motors, VFD-driven motors with tight IGBT thermal budgets, and small IEC standard motors; the thermal margin on these units is small, so protection has to be fast. Class 20, run-up 10 to 20 s, targets general-purpose NEMA Design B pumps, fans, conveyors, and compressors with 1.15 service factor. Class 30, run-up >20 s or very high WK², targets large centrifuges, hammer mills, crushers, ID/FD fans, and high-inertia mixers; the class number is set to the run-up, not the motor rating [S3][S5][S8].
For deeper pricing and spec-boundary data between MV and LV units, see the MV vs LV motor protection relay selection matrix, and for hazardous-area coordination where the same trip class has to coexist with ATEX/IECEx boundaries, see motor protection relay settings for hazardous area circuits.
Common Failure Modes and Mis-Selection Costs
Under-classing (Class 10 on a Class 30 load) shows up as nuisance trips on every start, with no fault present in the motor or wiring; the relay opens during acceleration because 600% FLA still flows past the 10 s bound [S1][S3]. The field fix is rarely "raise the FLA setting," which just masks the symptom and lets the motor run hot.
Over-classing (Class 30 on a normal-duty motor) is the quieter failure: the relay holds through real overloads long enough to age the winding insulation, and per the 10°C rule, every 10°C of extra thermal exposure halves insulation life [S5]. Viox's Jan 2026 piece notes that a standard Class 20 can hold 4 to 8 minutes at modest overload while Class 10A bounds that to under 2 minutes, which on small IEC motors is the difference between a healthy restart and a rewound stator [S5].
Thermal Memory and Verification After a Trip
Modern electronic overload relays and microcomputer protection relays retain a thermal model through the trip and after a power cycle, so a hot motor that restarts into the same load does not get a fresh "cold" trip curve. This is a deliberate part of how IEC 60947-4-1 high-inertia protection works and is the main reason a Class 30 relay on a high-inertia drive will still protect on the second start, not just the first [S3].
Verification after any change should include reading the actual run-up current trace with a clamp logger or relay event log, not just trusting the nameplate, and confirming the trip time at 600% FLA in the relay's test mode matches the class number. If the run-up regularly lands within 80% of the class time, the next size class is the right move, not the next dial detent.
Watch for: UL 508 / NEMA ICS 2 amendments in the next 12 months that may redefine the 600% test point for VFD-driven motors, and the growing number of electronic overload SKUs that ship defaulted to Class 20, which can under-class a high-inertia retrofit if the installer never re-enters the class setting [S3][S5].
Component reference pages worth checking: vfd duty motor.