A thermal overload relay sized strictly to motor nameplate full-load current (FLC) is the baseline rule used across three-phase induction motor control, with 115% of FLA as the standard setting for normal motors and 125% for 1.15 service-factor units per common motor-control sizing practice [S4].
The relay sits downstream of the contactor in a Direct-On-Line (DOL) starter and is the only component whose trip curve is matched to the motor's own thermal limit, not the supply fault level. That distinction governs every sizing decision in the rest of this guide [S1].
Step-by-Step Sizing: From Nameplate to Heater Current
Step 1 is mechanical: read the motor's rated HP, voltage, FLC, and service factor directly off the nameplate, since FLC is the only number the relay actually cares about [S1]. For a 6 HP, 415 V three-phase motor at 0.8 power factor, FLC works out to 4476 W / (1.732 × 415 V × 0.8) = 7.78 A, and the same FLC value is then carried into the relay-sizing branch of the DOL workflow [S1].
Step 2 applies the relay setting rule: relay set current = FLC × service factor. A 30 A FLA motor with a 1.15 service factor lands at 34.5 A, which is the dial setting on a thermal or electronic overload, not the wire-heater rating [S4]. For bimetallic relays, Step 3 is the heater-element lookup: the catalog lists interchangeable thermal units per ampere range (for example, 7.78 A FLC maps to a specific heater code inside the 6-10 A bracket), and that code is what gets physically clipped into the relay [S2].
Thermal vs. Electronic: Two Sensing Families Compared
Bimetallic thermal overload relays use two bonded metals with different coefficients of expansion (typically a steel core paired with a copper-nickel or nickel-chromium alloy) and a calibrated heater coil; current-induced heat deflects the strip, and the strip's retained thermal memory makes repeated start-stop duty more conservative trip-by-trip, suiting the 1-800 HP three-phase general-purpose band [S5]. Electronic overload relays replace the bimetal and heater with a current transformer and microcontroller, delivering adjustable trip current, selectable trip class, and phase-loss / phase-imbalance detection in a single device, at higher unit cost [S4].
Selection logic by criterion: (1) Trip-point accuracy, electronic wins and runs roughly ±2-5% across the dial versus bimetal's wider ambient-sensitive band; (2) Cost per amp, bimetallic wins, roughly half the price in most 5-100 A frames; (3) Variable-load / VFD-driven service, electronic wins because bimetal's thermal memory fights legitimate rapid load swings; (4) Maintenance simplicity in a single-vendor starter, bimetallic wins because heater elements are field-replaceable without recalibration tools [S4][S5].
Trip Class, Reset Mode, and the Bimetallic Eutectic Split
Trip class is the second selector after FLC: Class 10 trips within 10 s at 600% of FLA and matches standard TEFC motors; Class 20 fits high-inertia fans, mixers, and small conveyors; Class 30 is reserved for extra-long starting loads where acceleration can exceed 30 s without damage [S5]. Picking Class 10 on a Class 20 duty produces nuisance trips, while Class 20 on a Class 10 motor leaves windings exposed during a locked-rotor event.
Reset mode is the operational gate: manual reset forces an operator to investigate before restart and is mandatory on conveyors, crushers, and any machine where an unattended re-start is a safety hazard, while automatic reset suits HVAC fans and pump rooms where remote restart is required and the underlying fault is non-persistent [S5]. Eutectic alloy relays, a sub-family of thermal overloads using a low-melting-point alloy that liquefies on overload and rotates a ratchet, only ship with manual reset, and that constraint alone often drives the buy decision between bimetallic and eutectic in identical three-phase panels [S5].
Who Should NOT Use a Plain Bimetallic Thermal Relay
Three duty profiles break a standard bimetallic relay and should be diverted to electronic or specialised protection: (1) severe chemical exposure, where the standard enclosure and bimetal elements will fail prematurely, often specified together with motor protection coatings and enclosures for chemical-plant service; (2) VFD-driven motors with high switching frequency, where the relay should be mounted downstream of the inverter output to avoid nuisance trips from reflected harmonics; (3) mining conveyor and crusher duty, where dust ingress, vibration, and high start inertia make Class 20+ electronic units the more reliable choice, as mapped in the mining thermal-relay spec guide [S4][S5].
Ambient temperature is the quiet killer: a bimetallic relay calibrated for 40 °C loses roughly 1% trip current for every 5 °C above that, so an uncompensated bimetal in a 55 °C panel can under-protect a motor already running at its thermal ceiling [S5]. When the panel is outdoor, in a boiler room, or on a mill floor, specify ambient-compensated bimetallic units or switch the entire element to electronic.
Sourcing, Standards, and Vendor Documents
Selection catalogs from major motor-control vendors publish per-frame ampere ranges, heater-element codes, and dimensional drawings that convert the FLA result into a specific order code, and they remain the cleanest source for the final part number [S2]. Vendor guidance also confirms the field practice that relays must be sized to the motor's current, not the contactor's, and that the relay, contactor, and circuit breaker each have separate sizing paths even though they share the FLC input [S3].
Two cross-discipline references worth pulling before a final pick: a thermal relay selection overview for the principles of bimetallic and eutectic operation, and a linear guide selection reference when the driven load is a positioning axis whose inertia profile changes the trip class needed. Final signal: lock the trip class and reset mode before pricing, since a Class 10 / manual-reset bimetallic at 34.5 A set current and a Class 20 / automatic-reset electronic at the same set current are not interchangeable, and the order code is meaningless without both fields filled.
For component-level specifications, see crossed roller guide.