Class 10 vs Class 20 vs Class 30: Matching Trip Class to Start Time

What trip class matches a motor's actual start time? IEC 60947-4-1 tests every overload relay at 7.2 times its current setting from a cold start: Class 10A trips in 2-10 s, Class 10 in 4-10 s, Class 20 in 6-20 s, and Class 30 in 9-30 s, so the class number tracks the upper trip-time limit in seconds. Pick a class shorter than the motor's real run-up time and the relay opens on every start; pick one far longer than the load needs and a genuinely stalled rotor sits at locked-rotor current longer than it should before the contact drops. This piece covers what the class test measures, why run-up time — not FLC — sets the class, where Class 10/10A stops and Class 20 starts, when only Class 30 survives a start, and how coordination shifts once you move up a class.

What the Trip Class Number Actually Tests

The class test is narrow by design. A relay is set to a known current, cold-soaked to ambient, then loaded at 7.2x that setting, and a stopwatch runs until the contact opens. Nothing about the motor is in the test — no torque curve, no inertia, no duty cycle. The class number is a property of the relay's thermal element, not of the load it will protect.

That single number has to stand in for the whole motor start, though, because the relay cannot tell the difference between a motor drawing 7.2x FLC because it is starting and a motor drawing 7.2x FLC because the shaft has jammed. The only variable the relay engineer controls is how long the element tolerates that current before opening. Match that duration to the motor's run-up and the relay ignores a normal start; leave a locked rotor at the same current and it still trips, just later than a faster class would.

Why Run-Up Time, Not FLC, Decides the Class

Two motors can share the same FLC and still need different classes. A 15 kW pump with a light impeller reaches full speed in under two seconds. A 15 kW fan with a large, heavy blade can take ten seconds or more to spin up the same air column to speed, drawing close to locked-rotor current the entire time. The dial setting — which current the relay treats as 1x — is set to FLC on both. The class, which decides how long the relay tolerates a multiple of that setting, has to track the slower motor's run-up, not its FLC.

Torque and inertia set that interval, not power rating. A load with a quadratic torque-speed curve (a centrifugal fan or pump) accelerates faster once it starts moving, because the opposing torque is low at low speed. A load with high inertia and comparatively flat motor torque — a flywheel, a large crusher, an extruder screw full of material — takes much longer to reach speed, and the current stays high for that whole stretch.

Class 10 and 10A: the Default for Standard Loads

Most catalog motors run up in one to three seconds against a light-inertia load, well inside the 2-10 s window of Class 10A and the 4-10 s window of Class 10. Standard pumps, small fans, compressors, and general machine-tool motors fall here. This is the class stocked as the default on most bimetallic relays under contactors sized for the same frame — Schneider's thermal overload relays in the TeSys LRD family ship Class 10A as standard, and the same default applies across the ABB TA and Siemens SIRIUS 3RU21 bimetal lines.

Class 10A and Class 10 are close enough that many specifications treat them as interchangeable, but they are not identical: 10A caps out at 10 s and floors at 2 s, while 10 floors at 4 s. A relay rated 10A trips faster on a marginal overload than a Class 10 unit at the same current, which matters on loads where a few seconds of extra thermal margin during a slow start would otherwise cause nuisance tripping.

Class 20: the Middle Ground

Class 20 covers motors that need more than a Class 10 relay tolerates but do not carry enough inertia to justify Class 30. Larger centrifugal pumps on long pipe runs, big HVAC supply fans, and some belt-driven conveyor drives with moderate inertia run up in the 6-15 s range. Set a Class 10 relay on one of these and it opens partway through every start, before the load reaches speed — the relay is doing its job correctly by the test definition, it is just matched to the wrong load.

What we see in the field: a machine builder standardizes on Class 10A across a panel to keep spares simple, then one motor — usually the largest fan or the one driving the heaviest flywheel — nuisance-trips on every cold start while the rest run fine. Swapping only that one relay to Class 20 or 30, rather than reworking the whole panel, is usually the fix.

Class 30: High-Inertia and Long-Start Loads

Class 30 exists for loads whose run-up genuinely takes 15-30 s: large induction draft fans, ID/FD fans on boilers, crushers, hammer mills, centrifuges, and extruders starting under load. These motors are frequently started direct-on-line against a flywheel effect or a process load that will not move until the motor is close to full speed, so the current stays near locked-rotor value for most of the start. A Class 10 or 20 relay on one of these opens before the motor gets anywhere near running speed, every single time.

Electronic overload relays widen the choice instead of forcing a single bimetal class. ABB's E-series, Siemens 3RB30/3RB31, and Schneider's LR9/TeSys T electronic line all offer selectable Class 5, 10, 20, or 30 on the same unit, set by a rotary switch or parameter rather than by ordering a different physical part. That flexibility is worth the extra cost on a motor whose duty changes — a fan retrofit with a heavier blade, for instance — because the class can be adjusted without replacing the relay.

Reading the Run-Up Curve Against the Trip Curve

The correct way to pick a class is to compare two curves, not two numbers. Motor manufacturers publish a speed-versus-time or current-versus-time run-up curve for the specific motor and load combination; relay manufacturers publish a family of time-current trip curves, one per class, for the specific relay. Overlay the motor's actual current-versus-time trace on the relay's trip curve for a candidate class. If the motor's curve crosses the relay's trip curve before the motor reaches full speed, that class is too fast for the load — move up. If there is comfortable margin between the motor reaching speed and the relay's trip curve, the class has room, and a faster (lower) class may be safer without causing nuisance trips.

This depends on how good the manufacturer's run-up data actually is; a lot of general-purpose motor documentation gives locked-rotor current and torque but not a full run-up curve for a specific coupled load. In that case, a measured start — clamping current during commissioning — is more reliable than any published curve, because the actual load inertia, not the motor's rated inertia alone, decides how long the start takes.

What Changes When You Move Up a Class

Trip class interacts with contactor and short-circuit-protective-device coordination under IEC 60947-4-1. Manufacturers publish Type 1 and Type 2 coordination tables per relay class, per contactor, per SCPD combination, and a higher class is not automatically covered by the same table entry as a lower one — the extra seconds a Class 30 relay tolerates during a start also apply during a genuine overload before a fault, which changes the energy the contactor and SCPD have to absorb if something goes wrong downstream. Check the Type 1 vs Type 2 coordination tables for the specific class before finalizing a starter, not just the overload relay in isolation.

Some builders dial straight to Class 30 to make a nuisance-tripping problem go away without checking the run-up curve first, but that trades a startup nuisance for a slower response on a real stall, and it can invalidate the coordination table the panel was built against. A relay set to the wrong class is one of the most common causes of overload relay tripping that has nothing to do with an actual overload on the motor.

Conclusion

Trip class is a property of the relay, tested under one specific condition — 7.2x setting from cold. The number that matters to a specific motor is not the class rating on its own but how that trip-time band compares to the motor's actual run-up current and duration. Class 10/10A covers most standard, light-inertia loads; Class 20 covers the middle ground of larger pumps and fans; Class 30 is for loads that genuinely take 15-30 s to reach speed. Get the run-up curve, not just the FLC, before picking a class, and recheck the contactor and SCPD coordination table whenever the class changes. For the underlying mechanics of how the bimetal or electronic element reaches that trip point, see the thermal overload relay engineering guide, and for the full trip-class definitions across 10A, 10, 20, and 30 in one place, see the dedicated overload relay trip class reference. Overload relays and matching contactors sized to these classes are stocked for direct comparison before ordering.