MPCB Trip Classes 10, 20 and 30 Explained
What do MPCB trip classes 10, 20 and 30 mean? A trip class, defined in IEC 60947-4-1, states the maximum time an MPCB's thermal overload element allows before tripping at 7.2x the current setting from a cold start — Class 10 trips within roughly 10 seconds, Class 20 within 20 seconds, Class 30 within 30 seconds. Pick the wrong class and a high-inertia load nuisance-trips on every start, or a genuinely stalled motor overheats before the breaker reacts. This article covers the IEC test point itself, why Class 10 covers most direct-on-line motors, when Class 20 and Class 30 are needed, how to match a motor's start time to the class, what a mismatch costs, and how trip class relates to contactor coordination.
What Sets a Trip Class Apart in an MPCB
The thermal element in an MPCB is a bimetal strip, dial-set to the motor's full-load current (FLC). Trip class describes how that bimetal responds to sustained overcurrent — it says nothing about the instantaneous short-circuit trip, which sits fixed at roughly 12-13x In regardless of class. Trip class only governs the thermal curve: how long the breaker tolerates a current above setting before opening. For a full overview of construction and ratings, see the MPCB engineering guide.
IEC 60947-4-1 defines four standard classes: 10A, 10, 20, and 30. The number is the maximum trip time in seconds at 7.2x the current setting, tested from a cold, unenergized state. A Class 10 device might trip in 4-9 seconds at that test point; a Class 30 device is allowed up to 30 seconds. Same test current, different tolerance, because the thermal curve is shaped differently inside the device.
Class 10 and Class 10A: The Default for Direct-on-Line Motors
Class 10 is the standard trip class for most MPCBs sold for direct-on-line (DOL) starting. It suits motors that reach full speed in under 10 seconds, the majority of pump, fan, and general machine-tool motors under roughly 30 kW. Class 10A is a tighter variant, tripping faster still, typically under 4 seconds at the 7.2x test point. It suits equipment or upstream protection schemes that need a quicker thermal response than standard Class 10 gives.
Schneider TeSys GV2 and GV3, ABB MS132 and MS165, and Siemens SIRIUS 3RV2 all ship Class 10 as the base thermal-magnetic offering. That is not a coincidence. It matches the start time of the largest share of DOL-started motors in general industry, so most catalogs list Class 10 as standard and treat other classes as an order option. Datasheets list the class next to the current range rather than as a separate spec sheet; when two frame sizes share a current range but different classes, the class is often the deciding factor for which one ships as standard versus special-order.
Class 20 and Class 30: Room for High-Inertia Starts
A motor driving a large flywheel, centrifuge, or crusher can take 15-25 seconds to accelerate to full speed. Starting current during that window sits at 5-8x FLC, well above the thermal element's trip threshold under a standard class. A Class 10 MPCB reads that extended high current as an overload and trips before the motor finishes starting. Class 20 (up to 20 s) or Class 30 (up to 30 s) gives the bimetal more thermal margin, so the same motor starts without a nuisance trip.
What we see in the field: panel builders sometimes default to Class 10 across an entire panel for simplicity, then chase repeated nuisance trips on the one high-inertia drive that actually needed Class 20. Checking the motor's real start time against the class before ordering avoids that rework later. Frame size does not predict trip class on its own — a compact frame and a larger one can both ship as Class 10; class comes from the thermal element's design, not the device's physical size.
The IEC Test Point: How Manufacturers Rate a Trip Class
The class number is not marketing language. It is a pass/fail test defined in IEC 60947-4-1: energize the thermal element from cold at 7.2x the current setting and measure the trip time. The result must fall inside the class's window, so a device that trips in 15 seconds at this test point cannot be labeled Class 10, no matter what the datasheet claims elsewhere. Manufacturers publish trip-time curves as time-current graphs, extrapolating this single test point across the full overload range from roughly 1.1x to well above 7.2x.
Formula: Trip-class test point — Source: IEC 60947-4-1, clause 7.2.1.2
ttrip ≤ tclass at I = 7.2 × Iset (from cold)
| Symbol | Description | Unit |
|---|---|---|
| ttrip | Measured time for the thermal element to open the contacts | s |
| tclass | Maximum allowed trip time for the declared class (10A, 10, 20, 30) | s |
| Iset | Current dial setting, at or near the motor's FLC | A |
Matching Trip Class to the Motor's Actual Start Time
Selecting a class starts with the motor's start-up curve, available from the motor manufacturer or measured on commissioning. Compare the time the motor spends above roughly 5x FLC, typical DOL inrush, against the class's ceiling at 7.2x. A 6-second start fits comfortably under Class 10. An 18-second start on a high-inertia fan needs Class 20; guessing wrong means the breaker trips mid-start, not because anything is faulty, but because the thermal element read a long high-current period as an overload.
Locked-rotor current and locked-rotor time, both on the motor nameplate or datasheet, give a faster proxy when a full start-up curve is not available. If locked-rotor time exceeds the class's ceiling at 7.2x, move up a class before ordering rather than after the first nuisance trip. Star-delta and soft-start methods reduce the current the MPCB sees during acceleration, which is one reason some panel builders stay on Class 10 for loads that would otherwise need Class 20 direct-on-line; that trade-off is covered in MPCB sizing for star-delta and soft starting. Setting the dial itself follows a separate rule, covered in how to select and set an MPCB for a motor.
What a Mismatched Trip Class Costs
Undersized class, Class 10 on a load needing Class 20, means repeated nuisance trips at start, wasted commissioning time, and pressure to set the dial higher than the motor's FLC, which then under-protects the motor against a genuine overload. Oversized class, Class 30 on a load that starts in 6 seconds, means the thermal element tolerates overcurrent longer than necessary, so a real locked-rotor or stalled condition runs longer before the breaker opens, raising winding temperature and shortening insulation life.
Neither error shows up on a dial or a nameplate at a glance. It shows up as a trip log, or a burned-out motor months later. There is also a coordination angle: manufacturers publish their Type 1 and Type 2 tables against the declared class and current setting, so an MPCB running outside that declared match no longer has a documented basis for the coordination rating printed in the catalog. Check the class marking stamped near the rating on the MPCB body against the load's start behavior before commissioning, not after.
Trip Class Comparison: 10A, 10, 20, 30
The four classes sit on one time-current scale, but the load types they serve rarely overlap.
| Criteria | Class 10A | Class 10 | Class 20 | Class 30 |
|---|---|---|---|---|
| Max trip time at 7.2x In (cold) | ~4 s | ~10 s | ~20 s | ~30 s |
| Typical use | Fast-response or sensitive loads | General DOL motors, most pumps and fans | High-inertia loads (large fans, centrifuges) | Very high-inertia or frequent-start loads (crushers, mills) |
| Motor start time accommodated | Under 4 s | Under 10 s | 10-20 s | 20-30 s |
| Availability | Select models | Standard on most families | Available on select frame sizes | Less common; electronic overload relays often used instead |
Trip Class and Coordination With the Contactor
Trip class does not change the Type 1 or Type 2 coordination rating published for an MPCB and its paired contactors. Coordination is about short-circuit let-through energy, not thermal overload behavior, so the two ratings are independent: an MPCB can be Class 20 and still carry a Type 2 coordination rating with the matching contactor, provided the manufacturer's table lists that combination. See MPCB Type 1 vs Type 2 coordination explained for how that rating is built and published. Browse the current range of motor protection circuit breakers and manual motor starters by class and frame size.
Frequently Asked Questions
What does trip class 10 mean on an MPCB?
Class 10 means the thermal overload element trips within roughly 10 seconds when tested at 7.2x the current setting from a cold start, per IEC 60947-4-1. It is the default class for most direct-on-line motors with start times under 10 seconds.
What is the difference between Class 10 and Class 10A?
Class 10A trips faster than Class 10 at the same 7.2x test point, typically within 4 seconds versus up to 10 seconds. Class 10A suits loads or upstream protection schemes that need a quicker thermal response.
When do I need Class 20 or Class 30 instead of Class 10?
Use Class 20 or Class 30 when the motor's start time exceeds what Class 10 tolerates, typically high-inertia loads like large fans, centrifuges, or crushers that take 15-30 seconds to reach full speed. Check the motor's actual start-up curve rather than assuming.
Can I change the trip class of an MPCB after purchase?
No. Trip class is built into the thermal element's design and is not field-adjustable; only the current setting dial is. A mismatched class means replacing the MPCB with one built for that class, or moving to a magnetic-only MPCB with a separate, adjustable electronic overload relay.
Does trip class affect short-circuit protection?
No. Short-circuit protection comes from the fixed magnetic trip, roughly 12-13x In, which is independent of thermal trip class. Trip class only governs how long the thermal element tolerates sustained overcurrent during starting or a stall.
Where is the trip class marked on an MPCB?
It is stamped on the device body near the current rating, shown as '10', '10A', '20', or '30'. If it is not marked, check the datasheet for the family; most European MPCBs default to Class 10 unless a different class variant is specified at order.
Conclusion
Trip class is a fixed, tested property of the thermal element: seconds allowed at 7.2x the current setting from cold. Class 10 covers most direct-on-line motors. Class 20 and Class 30 exist for loads that genuinely take longer to start, not as a generic upgrade for "more protection." Match the class to the motor's actual start-up time, confirm it against the manufacturer's coordination table with the paired contactor, and remember the dial setting follows a separate rule entirely: set to FLC, not to the breaker's maximum.
