Schneider Electric LRD21 — Thermal Overload Relay Buying Guide

By Abdullah Zahid
16 min read
Schneider Electric LRD21 TeSys thermal overload relay 12-18A Class 10A for LC1D motor starter applications

Schneider Electric LRD21 Thermal Overload Relay — Class 10A, 12–18A Adjustable Range, TeSys D Compatible: Complete Buyer's Guide

When a conveyor goes down, a pump stops responding, or an HVAC starter fails inspection, the maintenance engineer or controls specialist searching for an LRD21 thermal overload relay already knows what they need — they just need to confirm the current range, verify the contactor match, and get the part moving. The Schneider Electric LRD21 is a 3-pole, Class 10A bimetallic thermal overload relay with a 12–18A adjustable range, designed to mount directly under TeSys D contactors including the LC1D18 and LC1D25. It delivers automatic phase loss detection, covers ambient temperatures from -20 to 60°C without derating, and carries UL, CSA, and IEC 60947-4-1 certification — making it one of the most specified motor protection components in standard industrial motor starter assemblies worldwide.

If you have already confirmed the LRD21 is the right part for your application, check current pricing and availability at LeadTime.ca — we ship worldwide.

Who Should Buy the LRD21 — and Who Should Order a Different Model

The Schneider Electric LRD21 is the right choice when all of the following apply to your application:

  • Your motor's full-load amp rating (FLA from the nameplate) falls between 12A and 18A — not estimated, confirmed from the motor label
  • Your existing or specified contactor is an LC1D18, LC1D25, LC1D32, or LC1D38 from the TeSys D family
  • Your site voltage is 600V (CSA/UL version) or 690V (IEC version) — confirm before ordering, as versions are not field-convertible
  • Your application is three-phase AC — LRD21 is not suitable for single-phase motor circuits
  • You require Class 10A trip classification (trips within 10 seconds at 2x set current) — not Class 20A
  • Your application does not require remote monitoring, diagnostics, or network connectivity

If your motor FLA exceeds 18A, the correct choice is the LRD22 (18–25A range) or the LRD32 (25–32A and above). If you require remote fault logging, Ethernet connectivity, or adjustable trip class via software, an electronic overload relay is the appropriate product — not the LRD21.

On this page:

What the LRD21 Actually Does in a Motor Starter Circuit

The Schneider Electric LRD21 is a 3-pole bimetallic thermal overload relay. Inside the relay, current flowing to the motor passes through bimetallic sensing elements that physically deflect under heat generated by elevated current. When motor current exceeds the set threshold for a sustained period, that deflection trips the relay mechanism, sending a signal to open the contactor's control coil and de-energize the motor. This thermal mechanism is what defines Class 10A behavior: at exactly twice the set current, the LRD21 will trip within 10 seconds per IEC 60947-4-1.

The LRD21 also incorporates automatic phase loss detection. If one supply phase drops to zero while the other two phases are carrying approximately 130% of the set current (Ir), the relay detects the imbalance and trips immediately — protecting the motor from the sustained single-phase overheating that destroys windings in minutes. This detection is built-in and requires no adjustment or separate phase monitor device.

One point that every buyer must understand before installation: the LRD21 does not provide short-circuit protection. It is an overload and phase loss device only. Short-circuit protection — the instantaneous response to a fault current spike — must be provided upstream by a dedicated circuit breaker or fuse. The LRD21 and the upstream protective device work together; neither is sufficient alone. The relay's adjustment range of 12–18A covers the rotary dial, which should be set to 90–110% of the motor nameplate FLA — for a 16A FLA motor, a setting between approximately 15.5A and 17.5A is appropriate.

Manual controls on the relay face include a red stop button for manual trip and a blue button for reset after a trip event. After an overload trip, the bimetallic element requires approximately 5 minutes of cooling before the relay is ready to reset — pressing the blue button before this cooling period may result in immediate re-trip if the underlying overload condition has not been cleared.

Typical System Architecture for TeSys D Motor Starters

The LRD21 sits at the output stage of a motor starter assembly — between the contactor's switching contacts and the motor terminal box — where it monitors all three phase currents simultaneously.

  • Upstream protection: circuit breaker or fuse providing short-circuit protection feeds the motor circuit
  • TeSys D contactor (LC1D18, LC1D25, LC1D32, or LC1D38) mounted in the control panel — main switching element
  • LRD21 mounted directly under the contactor using factory mounting clips, receiving motor supply through its power input terminals
  • Motor terminal box connects to the LRD21 power output terminals (T1-T2-T3), completing the three-phase power path
  • LRD21 control circuit wired in series with the contactor's control coil — a trip signal from the relay opens the coil circuit and drops the contactor, removing power from the motor

Typical Applications and Deployment Scenarios

The LRD21's 12–18A range targets motors in the approximately 5.5 kW to 7.5 kW range at 400V three-phase — a motor size class that appears across virtually every industrial sector. In manufacturing environments, this includes punch presses, plastic injection molding equipment, stamping machinery, and assembly line drives where predictable overload protection with fast Class 10A response is essential.

HVAC system integrators specify the LRD21 extensively for rooftop unit motor starters, chilled water pump starters, and fan coil unit controls. The -20 to 60°C operating range without derating is particularly relevant here — rooftop mechanical rooms and outdoor equipment enclosures routinely see temperature extremes that would derate or compromise lesser components.

In water treatment and pumping applications, the LRD21's automatic phase loss detection addresses a specific and costly failure mode: utility phase loss events that cause single-phase motor burnout. For emergency backup pump systems where unmonitored failure is unacceptable, the built-in phase loss function provides a critical layer of protection without requiring a separate phase monitor relay.

OEM panel builders include the LRD21 as a standard bill-of-materials component in pre-assembled TeSys motor starter kits. The direct mount under TeSys D contactors eliminates separate mounting hardware and reduces panel assembly time — a practical benefit that makes the LRD21 a default selection for volume panel builds in the 5.5–7.5 kW motor range.

Application Typical Deployment
Conveyor and packaging line motor starters Replacement relay after thermal element failure; LC1D25 contactor with 15–17A motor FLA
HVAC rooftop unit and chilled water pump New build or aging relay replacement; wide temperature range covers outdoor enclosures
Water treatment pump motor protection Emergency duty starter; phase loss detection critical for unattended operation
Machine tool motor circuits (lathes, milling, drilling) New panel build; integrator selects LRD21 for standard 5.5–7.5 kW motor protection
OEM control panel motor starter kits Standard BOM component; direct mount under LC1D18 or LC1D25 in pre-assembled starter
Material handling and warehouse automation Conveyor drive starters; Class 10A response suited to constant-duty conveyor motors

LRD21 Specifications, Variant Comparison, and Where It Fits in the TeSys LRD Family

Specification LRD21 Value
Thermal Class 10A per IEC 60947-4-1 (trips within 10 seconds at 2x set current)
Current Adjustment Range 12–18A (rotary dial; set to 90–110% of motor FLA)
Tripping Threshold 1.14 ±0.06 Ir per IEC 60947-4-1
Phase Loss Sensitivity Automatic; trips when one phase = 0 and two phases ≈ 130% Ir
Voltage Ratings 600V (CSA/UL) or 690V (IEC) — order matching version; not field-convertible
Frequency 0–400 Hz AC; also rated for DC networks
Ambient Temperature Range -20 to 60°C without derating
IP Rating IP20 per IEC 60529
Terminal Type Screw clamp; accepts 1.0–2.5 mm² wire (AWG 18–12 equivalent)
Certifications UL, CSA, IEC 60947-4-1, ATEX, CCC, EAC, DNV-GL, UKCA, BV, RINA, LROS

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

Feature LRD21 (Thermal) LRD22 (Thermal) Electronic Overload Relay
Current Range 12–18A 18–25A 1–150+ A (varies by model)
Thermal Class 10A standard 10A standard Adjustable or selectable
Phase Loss Detection Built-in, automatic Built-in, automatic Advanced, with diagnostics
Temperature Compensation Automatic, -20 to 60°C Automatic, -20 to 60°C Adjustable; temperature sensor input
Contactor Mount Direct under LC1D18/25/32/38 Direct under LC1D25/32/38/63 No; separate box mount required
Remote Monitoring No No Yes — Ethernet, Profibus, CANopen, analog outputs
Manual Adjustment Rotary dial, 12–18A Rotary dial, 18–25A Digital keypad or software
Cost Range Low Low Moderate to High
Recommended Motor Size 5.5–7.5 kW @ 400V 7.5–11 kW @ 400V 7.5 kW to 200+ kW depending on model
Maintenance None; replace if failed None; replace if failed Calibration and diagnostics possible

If your motor FLA exceeds 18A or your contactor is an LC1D32 or LC1D38 requiring broader protection coverage, the LRD22 is the correct next step — confirm the right model and check availability at LeadTime.ca.

Expert Verdict: Is the Schneider Electric LRD21 Right for Your Project?

For maintenance engineers replacing a failed relay on a conveyor, pump, or HVAC starter, and for panel builders specifying standard motor protection in the 5.5–7.5 kW range at 400V three-phase, the LRD21 is the definitive choice within the TeSys D ecosystem. The combination of automatic phase loss detection, a -20 to 60°C operating range without derating, and direct mount compatibility with the LC1D18 and LC1D25 contactors eliminates the integration guesswork that slows down urgent replacements and new builds. The 12–18A rotary dial covers the most common small-to-medium industrial motor FLA ratings in a single adjustment, and the IEC 60947-4-1 Class 10A trip characteristic delivers fast, reliable overload protection for constant-duty applications like conveyors and pumps where even brief sustained overload causes real damage.

The LRD21 has real limits, and buyers should be honest with themselves about them. If your motor FLA is above 18A — even by 1A — the LRD22 is the correct order, not a compromise setting on the LRD21 dial. For LC1D32 or LC1D38 contactors paired with larger motors, the LRD22 (18–25A) or LRD32 (25–32A+) will serve the application correctly where the LRD21 falls short. And if your application requires remote fault logging, network-connected diagnostics, or MES integration, a thermal relay at this price point is simply the wrong product category — an electronic overload relay is what you need, regardless of cost difference. The LRD21 does what it does exceptionally well; the mistake is asking it to do something outside its design scope.

From a procurement standpoint, the LRD21 is a standard stock item at most authorized automation distributors, with typical in-stock lead times of 1–5 business days for the standard 600V CSA/UL version. The 690V IEC version may carry slightly longer lead times depending on your region and distributor. For volume orders, project-specific builds, or international shipments, confirming availability before committing to a build schedule is always worthwhile. Check current stock and pricing for the LRD21 at LeadTime.ca — we source and ship worldwide, including to locations where local distributor stock runs thin.

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

What Engineers Need to Know Before Ordering the LRD21

The LRD21 is a well-understood part in the industrial automation community, but three ordering mistakes appear with enough consistency that any buyer at the purchase stage should treat them as active risks, not theoretical ones. The first and most costly is setting the current adjustment dial too high to prevent nuisance trips during commissioning. When a motor trips on startup and a technician dials the relay up to the 18A maximum to stop the trips — even if the motor FLA is only 12A or 14A — the motor loses adequate protection. Sustained overload conditions that should trip the relay in 1–3 minutes at correct settings will instead run the motor into thermal damage before the relay responds. The correct response to nuisance trips is always to investigate the root cause: mechanical binding, power supply imbalance, or motor undersizing — not to raise the protection threshold.

The second recurring problem is ordering the LRD21 without confirming the existing contactor model. Maintenance teams working from memory or informal verbal requests order the LRD21 and arrive on-site to find an LC1D40 or a non-TeSys contactor that the relay is physically or electrically incompatible with. The LRD21 is engineered specifically for the TeSys D line — LC1D18, LC1D25, LC1D32, and LC1D38. For any contactor outside this list, the correct approach is to identify the actual contactor model from the nameplate and cross-reference before ordering. When in doubt, bringing both the contactor model number and the motor FLA to a specialist distributor takes minutes and prevents a costly wrong-part return.

The third issue is less common but more damaging when it occurs: assuming phase loss protection is active without commissioning verification. The LRD21's phase loss detection is built-in and automatic, but it only functions if the relay is correctly wired in a three-phase configuration with all three input terminals connected and torqued. A commissioning step that simulates a phase loss event — de-energizing one supply phase at no-load and confirming the contactor drops out — takes under two minutes and should be documented in the startup checklist for any critical-duty application such as emergency pumps or process cooling systems. For applications where motor failure carries safety or environmental consequences, a separate phase loss monitor as redundant protection is worth considering alongside the LRD21.

Wiring and Installation Overview

  • Power circuit terminals accept solid or stranded wire sized for the motor and upstream protection; tighten to 2.5 N.m per datasheet — do not over-torque screw clamp terminals
  • Control circuit terminals accept 1.0–2.5 mm² flexible or solid copper wire; tighten to 1.7 N.m; connect in series with the contactor control coil and any manual stop or enable switches
  • For direct mount under LC1D contactors: align relay mounting pins or clips under the contactor baseplate, seat until clips engage, verify relay is level and parallel before tightening fasteners
  • Strip wire insulation 5–7 mm before insertion; use crimp ferrule end sleeves on stranded wire if site standards require it; avoid nicked insulation
  • After installation, verify all three phase input terminals are connected — all three must be present for phase loss detection to function; apply lockout/tagout before connecting or removing the relay

For full wiring diagrams and step-by-step installation procedures, refer to the Schneider Electric product documentation available from the manufacturer.

Compatible Contactors and TeSys D System Integration

The LRD21 is a TeSys D ecosystem component and is rated for direct mounting under the following TeSys D contactors. Compatibility with non-TeSys contactor systems is not supported.

Contactor Model Contactor Nominal Amps Protected Motor (Approx. @ 400V 3-Phase) LRD21 Setting Recommendation
LC1D18 18A 5.5 kW 12–18A (typical: 15–16A) Primary choice; direct mount, optimal fit
LC1D25 25A 7.5 kW 12–18A (typical: 16–18A) Primary choice; direct mount, optimal fit
LC1D32 32A 11 kW 12–18A (maximum 18A) LRD21 covers only up to 18A; consider LRD22 (18–25A) for better coverage
LC1D38 38A 15 kW 12–18A (maximum 18A) LRD21 not recommended; use LRD22 or LRD32
Non-TeSys contactor Varies Varies Not rated Do not use LRD21; order appropriate relay for system

Mounting accessories including DIN rail clips, plate-mount kits, and contactor coupling hardware are available separately from Schneider Electric. For hazardous area installations, verify whether an ATEX-certified variant is required — specialized certification variants exist within the TeSys LRD family and must be confirmed at purchase.

Wrong-Part Prevention Checklist — Confirm All 8 Points Before Ordering

Confirm every item on this checklist before placing your order. A single missed check is the most common cause of wrong-part returns and project delays with this component.

  1. Verify motor nameplate FLA (full-load amps) is between 12–18A. Do not estimate; read the motor label.
  2. Confirm contactor model is LC1D18, LC1D25, LC1D32, or LC1D38. Three-phase starters use these standard models.
  3. Check site voltage matches selected relay (600V or 690V). Do not assume; verify incoming power specifications.
  4. Confirm three-phase AC application. LRD21 is not for single-phase motors or DC systems.
  5. Select correct mounting method (under contactor, plate, or DIN rail). Installation method determines delivery — verify before ordering.
  6. Verify thermal class requirement is 10A standard (most common). If Class 20A is needed, order different model.
  7. Confirm no requirement for remote monitoring or diagnostics — if needed, electronic overload relay is required.
  8. Check wire gauge and terminal type (screw clamp, 1–2.5 mm²) matches your installation standards.

If any item on this checklist is unresolved, contact the LeadTime.ca team — our team can cross-reference your contactor model and motor FLA against the TeSys LRD family and confirm the correct part number before you order.

Frequently Asked Questions

Can I use the LRD21 on an LC1D32 or LC1D38 contactor, or do I need the LRD22?

The LRD21 will physically mount under an LC1D32 or LC1D38, but its maximum current setting of 18A creates a protection gap for motors sized to match those larger contactors. If your motor FLA is at or below 18A and you are using an LC1D32 for other design reasons, the LRD21 can technically be set to its maximum. However, if the motor FLA is between 18A and 25A — which is typical for motors matched to the LC1D32 or LC1D38 — the LRD22 (18–25A range) or LRD32 (25–32A+) is the correct choice. Do not force a mismatch between motor FLA and relay range.

What is the difference between Class 10A and Class 20A, and how do I know which the LRD21 is?

Class 10A means the relay trips within 10 seconds at exactly twice the set current (2x Ir) per IEC 60947-4-1 — this is faster, more aggressive protection suited to motors that cannot tolerate sustained overload, such as conveyor drives and pump starters. Class 20A allows up to 20 seconds at 2x Ir, which gives motors with higher inrush current or occasional overload cycling more tolerance before tripping. The LRD21 is Class 10A standard. If your application requires Class 20A — for example, a motor with a known high-inrush characteristic — a different model in the TeSys LRD family is required.

Does the LRD21 provide short-circuit protection, or do I still need a circuit breaker?

The LRD21 provides overload protection and phase loss detection only — it does not respond to instantaneous short-circuit fault currents. A separate circuit breaker or fuse upstream of the motor starter is always required to handle true fault conditions. The LRD21 and the upstream circuit breaker work as a complementary pair: the circuit breaker handles faults, the LRD21 handles sustained overload. Neither device alone provides complete motor protection.

How do I verify that the phase loss detection is functioning after installation?

During commissioning, the recommended test is to de-energize one supply phase while the motor is running at no-load and confirm that the contactor drops out promptly. If the contactor does not drop out, investigate the relay wiring — verify all three power input terminals are connected and tightened to 2.5 N.m. Phase loss detection is automatic and built-in, but it only functions in a correctly wired three-phase configuration. Document the test result in your commissioning checklist and do not sign off on critical-duty applications without a successful trip demonstration.

Can the LRD21 be used in a circuit with a VFD or soft-starter?

The LRD21 can be installed downstream of a VFD or soft-starter, but the relay monitors the actual motor current output from the drive — at reduced speed or during a soft-start ramp, motor current is proportionally lower and the relay will not trip until sustained high current is reached. Many VFDs include internal electronic overload protection; confirm with the drive manufacturer whether an external thermal relay is acceptable, required, or redundant for your specific drive-motor combination. For VFD applications requiring external overload protection, some engineers prefer an electronic overload relay with more precise trip curve control.

Is the LRD21 a direct drop-in replacement for an older TeSys LRD relay in the same current range?

The LRD21 is part of the current TeSys LRD product family and is designed to mount directly under the same TeSys D contactors as earlier LRD-series relays in the same current bracket. Verify that your existing contactor is one of the compatible LC1D models (LC1D18, LC1D25, LC1D32, or LC1D38) and that the replacement current range matches the motor FLA. If the existing relay was a different current range variant, order the LRD model matching the motor nameplate FLA, not necessarily the same model number as the relay being replaced.

Why Order the LRD21 From LeadTime.ca

  • Global shipping — LeadTime.ca ships the LRD21 to customers worldwide, including regions where local distributor stock is limited or lead times are extended
  • Specialist sourcing — hard-to-source voltage variants and certified versions (including 690V IEC and ATEX) can be confirmed through direct inquiry before order
  • Volume and project pricing — contact the team directly for volume orders or BOM-level project quotes; bulk orders for OEM panel builds handled promptly
  • Lead time transparency — availability confirmed before you commit to a build schedule; no surprises on delivery
  • Fast response — controls engineers and procurement specialists are served by a team with industrial automation sourcing expertise, not a general call center

LRD21 At-a-Glance: Key Facts for Purchase Decisions

  • 3-pole thermal overload relay, Class 10A per IEC 60947-4-1 — trips within 10 seconds at 2x set current
  • Current adjustment range: 12–18A rotary dial; set to 90–110% of motor nameplate FLA
  • Tripping threshold: 1.14 ±0.06 Ir per IEC 60947-4-1 standard
  • Automatic phase loss detection: trips when one phase = 0 and two phases reach approximately 130% Ir — no adjustment required
  • Compatible contactors: LC1D18, LC1D25, LC1D32, LC1D38 (TeSys D family only)
  • Voltage: 600V CSA/UL version or 690V IEC version — confirm at order; not field-convertible
  • Operating temperature: -20 to 60°C without derating; storage -60 to 70°C
  • Impulse withstand: 6 kV — surge immunity for VFD and soft-starter installations
  • IP20 per IEC 60529; dimensions 45 mm W x 70 mm D x approximately 40 mm H; weight 0.124 kg
  • Terminal type: screw clamp, 1.0–2.5 mm² wire; control circuit torque 1.7 N.m, power circuit torque 2.5 N.m
  • Certifications: UL, CSA, IEC 60947-4-1, ATEX, CCC, EAC, DNV-GL, UKCA, BV, RINA, LROS
  • Typical protected motor range: 5.5–7.5 kW at 400V three-phase
  • Motor FLA above 18A: order LRD22 (18–25A) or LRD32 (25–32A+) instead
  • Does not provide short-circuit protection — upstream circuit breaker or fuse always required

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