Siemens 3RV2021-4DA10 — Class 10 Motor Protector Buying Guide

By Abdullah Zahid
13 min read
Siemens 3RV2021-4DA10 size S0 Class 10 thermal-magnetic motor protection circuit breaker for three-phase industrial motor panels

Siemens 3RV2021-4DA10 Motor Protection Circuit Breaker: Specs, Capacity & Selection Guide

When a maintenance technician or panel builder searches for the Siemens 3RV2021-4DA10, they typically already know the motor size and voltage — what they need confirmed is whether this exact catalog number covers their full load ampere range, whether the breaking capacity holds up at their plant's available fault current, and whether it will arrive before their scheduled startup. This is a Class 10, size S0 thermal-magnetic motor protector with an adjustable thermal element from 18 to 25 A, a fixed 325 A magnetic release, and a 690 V AC maximum rating. If your motor runs between 5 hp at 208 V and 15 hp at 480 V, and your plant's available short-circuit current does not exceed 55 kA at 400 V, this is likely the correct part.

If you have already confirmed this is the right part, check current pricing and availability at LeadTime.ca — ships worldwide.

Who Should Buy the 3RV2021-4DA10 — and Who Shouldn't

This motor protection circuit breaker is the right choice for engineers and technicians protecting three-phase AC motors where motor FLA falls within the 18 to 25 A adjustable thermal range and Class 10 trip response is acceptable for the application.

  • Motor full load amps (FLA) confirmed between 18 A and 25 A from the motor nameplate
  • System operating voltage at or below 690 V AC (typical North American installations at 208 V, 240 V, 480 V, or 575 V are well within this limit)
  • Plant available short-circuit current does not exceed 55 kA at 400 V or 4 kA at 690 V
  • Three-phase AC circuit confirmed — this unit is a 3-pole device only
  • DIN rail panel space available — unit occupies 45 mm of rail width
  • No auxiliary contact feedback to PLC or remote monitoring panel required on the base model

If your motor FLA exceeds 25 A, step up to the 3RV2022 series. If your control circuit requires auxiliary contact blocks for PLC feedback or trip indication, select the 3RV2021-4GA10, which includes one NC and one NO auxiliary contact. If ground fault detection is required, a separate GFCI device or external monitoring logic is needed — this model has no built-in ground fault detection.

On this page:

What the 3RV2021-4DA10 Actually Does in a Motor Circuit

The Siemens 3RV2021-4DA10 is a size S0 thermal-magnetic motor protection circuit breaker from the SIRIUS product family. Its job is straightforward and critical: it protects a three-phase AC motor and its branch circuit conductors from two distinct fault conditions simultaneously. The thermal element — a bimetallic strip adjustable via a front-panel rotary dial between 18 and 25 A — responds to sustained overload current over time, tripping the device before the motor windings overheat. The magnetic solenoid release, fixed at 325 A, responds instantaneously to short-circuit faults, disconnecting the circuit in milliseconds before conductors or switching devices are damaged.

The Class 10 designation means the thermal element trips within 10 to 13 seconds when the motor draws 6 times its rated FLA — faster than Class 20 or Class 30 devices. This response speed is appropriate for most general industrial motors with moderate starting inrush. A single unit handles the complete protection function: no separate overload relay, no separate circuit breaker, no separate fuse in the motor branch circuit. The base model carries screw terminals for field wiring and mounts to a standard 35 mm DIN rail in any orientation. It carries a failure rate of 50 FIT at low demand rates per SN 31920, and a mechanical service life rated at 10 million operating cycles — figures that translate directly to the multi-year reliability expectations of industrial maintenance programs.

Typical System Architecture for Motor Protection

The 3RV2021-4DA10 sits at the branch circuit level, directly upstream of the motor contactor and downstream of the main disconnect or distribution bus. It is the first device in the motor branch that provides both overcurrent and short-circuit protection in one package.

  • Main panel distribution bus or MCC section feeds branch circuit conductors at 208 V, 480 V, or other system voltage
  • 3RV2021-4DA10 mounts on 35 mm DIN rail and intercepts all three phases — thermal element monitors sustained overload, magnetic release handles instantaneous faults
  • Downstream load side (T1/T2/T3 terminals) connects to motor contactor coil switching circuit
  • Motor contactor connects to motor terminals — three-phase AC motor rated 5 to 15 hp depending on voltage
  • On a trip event, the 3RV2021-4DA10 opens all three poles simultaneously; manual reset restores the circuit after fault clearance

Where Engineers Deploy the 3RV2021-4DA10

In heavy manufacturing and automotive assembly plants, the 3RV2021-4DA10 protects conveyor drive motors, coolant pumps, and hydraulic power unit motors in the 15 hp class. Its compact 45 mm width makes it practical for dense MCC lineups where panel real estate is constrained.

Food and beverage facilities use this model extensively in pump and compressor circuits where motors run continuously at steady load — precisely the duty profile where Class 10 protection reduces the risk of extended overload damage without nuisance-tripping on startup inrush.

OEM machine builders integrating spindle motors or axis drives into CNC routers and machining centers specify this model for its UL 508 and CSA certifications, which satisfy North American panel listing requirements. The adjustable 18 to 25 A thermal element means a single catalog number covers the motor horsepower range common in mid-size machine tool spindles.

Facilities running standardization initiatives across their SIRIUS motor protection infrastructure — replacing mixed legacy devices with a single vendor platform — find that this model's 10 million cycle service life and broad distributor availability reduce spare parts inventory complexity.

Application Typical Deployment
Three-phase pump or compressor replacement Drop-in swap for worn thermal-magnetic protector, 5–15 hp @ 208–480 V, DIN rail panel
CNC machine tool spindle motor protection New OEM panel build, Class 10 protection, UL 508 / CSA listed enclosure
HVAC fan or air handling unit motor circuit Branch circuit protection for 15 hp @ 480 V three-phase AHU motor, standard MCC section
Food processing conveyor drive Continuous duty motor protection, Class 10, adjustable FLA dial matched to nameplate
SIRIUS platform standardization initiative Multi-unit installation across motor control centers, consistent spare parts and servicing
Emergency motor circuit repair after stall event Technician drop-in replacement matching existing 35 mm DIN rail layout and FLA setting

Key Electrical and Physical Specifications

Parameter Value
Adjustable Thermal Element Range 18–25 A
Fixed Magnetic Release 325 A
Maximum Operating Voltage 690 V AC
Number of Poles 3
Class of Protection (Trip Time at 6x FLA) Class 10 — 10 to 13 seconds
Breaking Capacity (Icu) at 400 V 55 kA
Breaking Capacity (Icu) at 690 V 4 kA
Operating Frequency 50–60 Hz
Operating Temperature Range -20 °C to +60 °C
Width / DIN Rail Mounting 45 mm / 35 mm standard DIN rail, any orientation

Full technical specifications are available on the product page at LeadTime.ca.

3RV2021-4DA10 vs. Related SIRIUS Variants: Which One Do You Need?

Feature 3RV2021-4DA10 3RV2021-4GA10 3RV2022 Series
Thermal Adjustment Range 18–25 A 18–25 A Higher (next frame size S2)
Auxiliary NC Contact No Yes (1) Variant dependent
Auxiliary NO Contact No Yes (1) Variant dependent
Thermal Overload Protection Yes Yes Yes
Magnetic Short-Circuit Protection Yes Yes Yes
Ground Fault Detection No No No
Phase Failure Detection Yes Yes Yes
Remote Trip Feedback Possible No Yes (via contacts) Variant dependent
Best For Standalone motor branch circuit, no PLC feedback needed Integrated control circuits, PLC feedback, trip indication Motors with FLA above 25 A

If your control architecture requires circuit status feedback to a PLC or remote trip indication panel, the 3RV2021-4GA10 is the correct choice — check current availability and pricing at LeadTime.ca for both variants.

Expert Verdict: Honest Assessment for Purchase Decision

The Siemens 3RV2021-4DA10 does exactly what a well-specified motor protection circuit breaker should do: it combines thermal overload and magnetic short-circuit protection in a single compact device, mounts without tools on standard DIN rail, and covers a practical horsepower band — 5 hp at 208 V through 15 hp at 480 V — with a single catalog number. The 55 kA breaking capacity at 400 V is genuinely high for a size S0 frame, and the 50 FIT low-demand failure rate is a meaningful reliability figure for plants calculating MTBF in motor control panels. The Class 10 response at 10 to 13 seconds at 6 times rated current balances motor winding protection against nuisance trips on normal inrush — a real-world concern for technicians who have dealt with chronic false trips from oversensitive overload relays.

Where this model has real limits: the 18 to 25 A thermal range is non-negotiable. If your motor nameplate FLA is 26 A or 17 A, this is not the right part regardless of how close it is — select the appropriate 3RV2022 or lower-range variant. The base model carries no auxiliary contacts, which means no circuit status feedback, no remote trip indication, and no integration into PLC digital input modules without adding an external contact block or upgrading to the 3RV2021-4GA10. Ground fault detection is absent entirely — if phase-to-ground fault monitoring is a plant safety requirement, a separate protection device is mandatory. These are not weaknesses unique to this model; they are deliberate design boundaries of the base configuration.

From a procurement standpoint, this is a standard-stock item at major industrial distributors worldwide, with typical lead times in the 2 to 4 week range for standard orders. Volume requirements of 10 or more units typically unlock pricing below standard distributor list. If you are designing this into an OEM machine for export, confirm regional stock availability before finalizing the BOM — some markets carry limited variant inventory. If this matches your system requirements, view current stock and pricing at LeadTime.ca.

For volume pricing or to confirm lead time before committing to a build, contact the LeadTime.ca team directly — we ship worldwide.

Wiring and Installation Overview

  • Mount on standard 35 mm symmetrical DIN rail (per DIN EN 60715) using snap-on engagement — any mounting orientation is acceptable, including vertical and inverted
  • Main circuit wiring uses screw terminals: line-side conductors L1, L2, L3 connect at the top terminals; load-side conductors T1, T2, T3 connect at the bottom terminals; acceptable conductor sizes are solid 1–2.5 mm² and 2.5–10 mm², and finely stranded conductors with end sleeves
  • Observe voltage-dependent spacing requirements: side spacing between adjacent units is 9 mm at 400 V and 30 mm at 690 V; vertical and lateral clearance to grounded or live parts is 30 mm at 400 V and 50 mm at 690 V
  • Rotary FLA dial is accessible from the front panel after mounting — adjust to the motor nameplate FLA value within the 18 to 25 A range using a flat-head screwdriver; visually confirm alignment before energizing
  • The base model has no auxiliary terminal block — if control circuit feedback is required, either add an external contact block accessory or select the 3RV2021-4GA10 variant; consult the manufacturer installation manual for full wiring diagrams and terminal torque specifications before commissioning

Compatible Accessories and SIRIUS Expansion Options

The 3RV2021-4DA10 base model is designed to accept accessory modules from the SIRIUS product family. The following accessories are relevant to buyers evaluating system integration requirements:

  • 3RV2021-4GA10 — factory-integrated version with one NC and one NO auxiliary contact block, enabling remote trip indication and PLC digital input feedback without external accessories
  • Auxiliary contact blocks (snap-on) — add-on contact modules compatible with the 3RV2021 frame for circuit status monitoring, sold separately for retrofit to existing base model installations
  • 3RV2022 series — next frame size S2 with higher FLA ranges for motors exceeding the 25 A thermal ceiling of the 3RV2021-4DA10
  • 3RV2023 series — frame S3 for large motor circuits requiring 40 A and above thermal protection ranges
  • SIRIUS contactor (3RT series) — designed to mount downstream of the 3RV2021-4DA10 in motor branch circuits for switching control; the combination forms a complete motor starter assembly

Wrong-Part Prevention Checklist Before You Order

Before placing an order for the 3RV2021-4DA10, run through each of the following checks against your actual motor and panel documentation. These are the most common sources of wrong-part orders and field returns for this catalog number:

  1. Confirm your motor's full load amps (FLA) fall within 18–25 A thermal range — field adjustment is manual rotary dial, not factory-set
  2. Verify motor operating voltage is 690 V AC maximum (typical North American installations 208–480 V are below this limit)
  3. Check that your plant's available short-circuit current does not exceed 55 kA at 400 V or 4 kA at 690 V (if it does, this model cannot be used)
  4. Confirm 3-pole requirement — this model is not available in single-pole or two-pole configurations
  5. Do NOT confuse with thermal-only overload relays (no magnetic short-circuit protection) or with molded-case breakers (larger frame, different mounting)
  6. Verify DIN rail mounting space — unit is 45 mm wide; check adjacent devices do not create clearance violations at 400 V (9 mm side spacing) or 690 V (30 mm side spacing)

If any item on this checklist surfaces a mismatch, contact the LeadTime.ca team to confirm the correct SIRIUS catalog number for your application before ordering.

Frequently Asked Questions

Can the 3RV2021-4DA10 protect a 20 hp motor at 480 V?

No. The thermal element adjusts only between 18 and 25 A. A 20 hp motor at 480 V typically draws approximately 27 A FLA, which falls above the 25 A ceiling of this model. The correct selection would be a 3RV2022 series unit with an appropriate higher FLA range. Always verify motor nameplate FLA before selecting a catalog number.

What is the difference between Class 10 and Class 20 protection, and how do I know which one I need?

Class 10 trips within 10 to 13 seconds at 6 times the rated thermal current — faster than Class 20, which trips within 20 seconds at the same overcurrent level. Class 10 is appropriate for most standard industrial AC motors with moderate starting inrush. Class 20 is typically selected for motors with high-inertia loads or extended acceleration times where Class 10 would cause nuisance trips during startup. The 3RV2021-4DA10 is Class 10 only — if Class 20 is required, a different model must be specified.

Does the 3RV2021-4DA10 provide ground fault protection?

No. This model provides thermal overload protection and magnetic short-circuit protection, and it detects phase failure, but it has no integrated ground fault detection capability. If phase-to-ground fault monitoring is required by your application or by local electrical code, a separate GFCI device or external monitoring circuit must be added to the installation.

Is the 3RV2021-4DA10 suitable for use with a variable frequency drive (VFD)?

Thermal-magnetic motor protectors like the 3RV2021-4DA10 are typically installed on the line side of a VFD, not between the VFD output and the motor. Installing a thermal-magnetic device on the VFD output is not recommended because the modulated output waveform can cause nuisance thermal trips. Confirm your specific VFD manufacturer's installation requirements and consult the manufacturer documentation before finalizing placement in the circuit.

What happens if I set the rotary dial above the motor's rated FLA?

Setting the thermal dial above the motor's actual FLA means the overload protection will not trip at the motor's true overload threshold — the motor can sustain current above its nameplate rating for longer than intended before the device responds. This reduces thermal protection effectiveness and can result in motor winding damage or shortened insulation life. The dial must be set to match the motor nameplate FLA, not rounded up to a convenient number.

What certifications does the 3RV2021-4DA10 carry for North American panel installations?

The 3RV2021-4DA10 complies with IEC 60947-4-1, IEC 60947-6-2, UL 508 (industrial control panels in North America), and CSA (Canadian electrical safety). These certifications cover the most common North American and international panel listing requirements. The surge voltage resistance rating is 6 kV, and shock resistance is rated at 25g over 11 ms per IEC 60068-2-27.

Why Source Through LeadTime.ca

  • LeadTime.ca ships SIRIUS motor protection products worldwide — no regional restriction on orders
  • Direct sourcing support for hard-to-locate variants and volume requirements — contact the team to confirm current stock status before finalizing a BOM
  • Volume pricing available for orders of 10 or more units — request a quote for project quantities
  • Fast response on lead time inquiries — useful when motor circuit repair or machine startup schedules cannot wait for standard distributor processing
  • View the 3RV2021-4DA10 product page at LeadTime.ca for current pricing and availability
  • Contact LeadTime.ca directly for volume quotes, lead time confirmation, or variant selection support

At-a-Glance Summary

  • Catalog number: 3RV2021-4DA10 — size S0 thermal-magnetic motor protection circuit breaker, SIRIUS family
  • Thermal element adjustable from 18 A to 25 A via front-panel rotary dial
  • Fixed magnetic release at 325 A for instantaneous short-circuit disconnection
  • Class 10 protection: trips in 10 to 13 seconds at 6 times rated thermal current
  • Maximum operating voltage 690 V AC; operating frequency 50–60 Hz; 3-pole only
  • Breaking capacity: 55 kA at 400 V (Icu), 4 kA at 690 V (Icu)
  • Covers motor ratings from 5 hp at 208 V up to 15 hp at 480 V (three-phase)
  • Width 45 mm, mounts on standard 35 mm DIN rail in any orientation
  • Operating temperature range: -20 °C to +60 °C; mechanical service life: 10 million operating cycles
  • Screw terminals accept solid conductors 1–2.5 mm² and 2.5–10 mm²
  • Certifications: IEC 60947-4-1, IEC 60947-6-2, UL 508, CSA
  • Base model carries no auxiliary contacts — select 3RV2021-4GA10 if PLC feedback or remote trip indication is required
  • No ground fault detection — external protection required if phase-to-ground monitoring is mandated
  • Failure rate: 50 FIT at low demand per SN 31920; B10d value 50,000 operating cycles