Overload Relay Trip Classes 10A, 10, 20 and 30 Explained
What do overload relay trip classes mean? A trip class number, defined in IEC 60947-4-1, states the maximum seconds a thermal overload relay may take to trip when carrying 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. Pick the wrong class and the relay either nuisance-trips on every motor start or leaves the winding exposed too long during a stalled run-up. This article covers the IEC test method, why four classes exist, which motors fit each one, how electronic relays add Class 5, the shape of the trip-time curve, and how to read a class number off a relay nameplate.
What Trip Class Actually Measures
IEC 60947-4-1 defines trip class with a single repeatable test: apply 7.2x the relay's current setting to a cold relay — one that has not carried current recently and sits at ambient temperature — and measure the seconds until it trips. The result must fall inside the class's published window. A relay stamped Class 20 has to trip somewhere between 6 and 20 s at that test current; a relay that trips in 3 s at 7.2x is not Class 20, whatever the label says.
7.2x is not arbitrary. It approximates a stalled motor's locked-rotor current, the worst case the overload relay has to survive without tripping prematurely during a normal start, yet still catch within a bounded time if the rotor never turns.
Formula: Trip Class Test Point — Source: IEC 60947-4-1
ttrip ≤ tclass,max at Itest = 7.2 × Iset, cold start
| Symbol | Description | Unit |
|---|---|---|
| t_trip | Measured time to trip at the test current | s |
| t_class,max | Upper trip-time limit for the stamped class (10 s for 10A/10, 20 s for Class 20, 30 s for Class 30) | s |
| I_test | Test current applied to a cold relay | A |
| I_set | Dial current setting, normally the motor's nameplate FLC | A |
The Four Trip Classes: 10A, 10, 20 and 30
Four windows cover essentially every industrial motor:
- Class 10A: 2-10 s at 7.2x setting. The tightest window, used on small motors and applications where fast fault clearing matters more than start tolerance.
- Class 10: 4-10 s at 7.2x setting. The default for standard pumps, fans, and general-purpose motors with a run-up under a few seconds.
- Class 20: 6-20 s at 7.2x setting. For loads that take longer to reach speed — larger fans, some compressors.
- Class 30: 9-30 s at 7.2x setting. Reserved for high-inertia loads: large centrifuges, crushers, big fans with heavy wheels, anything whose rotor keeps accelerating well past the point a Class 10 relay would already have tripped.
Class 10A and Class 10 dominate catalog volume because most motor loads start in a few seconds. Class 20 and 30 exist specifically for the minority of applications that do not.
Why the Trip-Time Curve Is Inverse, Not Fixed
The 7.2x figure is one point on a curve, not the whole story. A bimetallic element heats according to I²t: double the current and the heating rate roughly quadruples, so trip time collapses fast as overload current rises. A small overload — say 110% of FLC — can take minutes to trip; a stalled-rotor current at 500-700% FLC trips in seconds. That inverse shape is deliberate: it mirrors how the motor winding itself heats, so the relay tracks the winding's actual thermal risk instead of tripping on a fixed timer.
This is also what lets a motor start at all. Inrush current on a direct-on-line start typically runs 6-8x FLC for a second or two. A relay with a flat, fast trip point would open on every start. The inverse curve tolerates that brief spike while still catching a genuine stall inside the seconds the trip class guarantees.
Matching Trip Class to Motor Start Time
The selection rule is simple to state and easy to get wrong in practice: the motor's run-up current must drop below the relay's trip curve before the class's time limit is reached. If a fan takes 12 s to reach full speed while drawing 5-6x FLC, a Class 10 relay (max 10 s at 7.2x, less at lower multiples) is likely to trip mid-start. Step up to Class 20 or 30 and the same relay tolerates the longer run-up without giving up meaningful protection once the motor is running.
What we see in the field: a Class 10 relay fitted under every motor because that's what's in stock is the single most common cause of nuisance tripping on large fans and centrifuges. The relay is not defective — it is doing exactly what its class promises. The fix is a class change, not a relay swap to a different brand.
Electronic Relays and Selectable Trip Class
Electronic (solid-state) overload relays read current through internal current transformers or shunts and model the motor's heating in a microcontroller rather than a bending bimetal strip. That lets several relay families offer selectable class on a single unit — Class 10, 20, and 30 as a dial or menu setting, with some adding Class 5 for fast-starting, low-inertia loads. One relay body covers a range of applications that would otherwise need separate bimetallic parts stocked for each class.
The trade-off runs the other way for simple, low-duty motors: a fixed-class bimetallic relay is cheaper and needs no configuration. Electronic relays earn their premium on loads that also need phase-loss sensitivity, ground-fault detection, or stall protection alongside the class flexibility — see our guide on thermal vs electronic overload relays for the full trade-off.
Trip Class Across Brands: Schneider, ABB, Siemens
All three major brands anchor their bimetallic line at Class 10A or Class 10 and move to electronic relays for wider class selection:
| Criteria | Schneider TeSys LRD | ABB TA / E-series | Siemens SIRIUS 3RU2 / 3RB3 |
|---|---|---|---|
| Bimetallic class | Class 10A | TA line: Class 10A | 3RU21: Class 10 |
| Electronic class options | LR9 / TeSys T: selectable class | E-series (EF19-EF460): selectable Class 10/20/30 | 3RB30/3RB31: selectable Class 5/10/20/30 |
| Setting ratio, electronic | Wider than bimetal | Roughly 1:3-1:4 | Roughly 1:4 |
| Extra electronic protections | Wide range, selectable class | Phase-loss, thermistor input on some models | Phase-loss; ground-fault on 3RB31 |
The practical difference between brands at Class 10A/10 is small — all three test to the same IEC 60947-4-1 window. The real decision point is whether the application needs the wider class range and extra protections of the electronic line, and that decision is more about the motor's start behavior than the brand badge on the relay.
Reading Trip Class Off the Relay
The class is stamped on the relay body or printed on the dial face, usually near the current-setting scale — "10A," "CLASS 10," "CLASS 20," or "CLASS 30" depending on manufacturer. On electronic relays with selectable class, the current setting shows on a rotary or digital display and the class is a separate switch or menu item; check both, since a relay set to the right amperage but the wrong class still fails to protect a high-inertia load correctly. When replacing a relay, match trip class as carefully as current range — a same-amperage swap at the wrong class either under-protects a stalled motor or nuisance-trips a normal one.
Coordination with the upstream motor protection circuit breakers or fuse and the contactors also depends on trip class: a Type 1 or Type 2 coordination table published by the manufacturer assumes a specific class, so swapping class without checking the table can invalidate the coordination rating documented for that starter combination.
Frequently Asked Questions
What does Class 10 mean on an overload relay?
Class 10 means the relay trips between 4 and 10 seconds when carrying 7.2x its current setting from a cold start, per IEC 60947-4-1. It is the standard class for general-purpose motors with a short run-up.
What is the difference between Class 10 and Class 10A?
Both are tested at 7.2x setting from cold, but Class 10A must trip within 2-10 seconds while Class 10 allows 4-10 seconds. Class 10A is the tighter, faster window and is the standard bimetallic class for Schneider TeSys LRD and ABB TA relays.
When should I use Class 20 or Class 30 instead of Class 10?
Use Class 20 or 30 when the motor's run-up time, at the current it draws while accelerating, exceeds what a Class 10 relay tolerates before tripping. This applies to high-inertia loads such as large fans, centrifuges, and crushers with a run-up of several seconds or more.
Can one relay be set to different trip classes?
Bimetallic relays have a fixed class set at manufacture. Electronic overload relays, such as ABB's E-series or Siemens 3RB30/3RB31, offer selectable class — often 5, 10, 20, and 30 — as a switch or configuration setting on one relay body.
What happens if I use the wrong trip class?
Too low a class for the load's run-up causes nuisance tripping every time the motor starts. Too high a class for a fast-starting motor delays fault clearing during a genuine stall, leaving the winding exposed to overheating for longer than needed.
Is trip class the same as the current setting?
No. The current setting (dial value, normally the motor's nameplate FLC) determines when the relay considers the motor overloaded. Trip class determines how fast it reacts once that overload current is present, tested specifically at 7.2x the setting.
Conclusion
Trip class is a timing spec, not a quality grade — Class 10A, 10, 20, and 30 each define a bounded trip-time window at 7.2x current setting from cold, per IEC 60947-4-1, and the right choice depends entirely on how long the motor takes to reach full speed. Standard pumps and fans sit comfortably on Class 10 or 10A; high-inertia loads need Class 20 or 30 to survive their own start-up current. Electronic relays fold multiple classes into one selectable unit, but the underlying test and the selection logic stay the same regardless of brand. For the selection and setting process, see our guide to selecting and setting an overload relay, and for the full protection picture across a starter, the thermal overload relay engineering guide.
