An MPCB or motor-rated branch-circuit OCPD is sized 150% to 300% of the motor full-load current per NEC Table 430.52(C)(1), while the separate overload relay protecting the motor itself is sized at 115% to 125% of FLA [S1][S4].
The two numbers are not the same device, and confusing them is the most common spec error: a 250% inverse-time breaker is short-circuit and ground-fault protection only, the thermal overload relay is what protects the windings from sustained overcurrent [S3][S4].
FLC, FLA, and Locked-Rotor Current: the Three Numbers Behind Every Sizing
Every motor protector selection starts with three currents off the nameplate: full-load current (FLC or FLA, the running current at rated horsepower, voltage, and frequency), locked-rotor current (LRC, also called starting current, typically 6 to 8 times FLC for standard induction motors), and service factor (1.0 for most integral horsepower, 1.15 for some NEMA Design B machines) [S4].
For a 3-phase, 10 HP, 460 V motor, NEC Table 430.250 lists FLC at 14 A; the branch-circuit OCPD then lands at 35 A on an inverse-time breaker (14 A × 250%) or 24.5 A rounded up to 25 A on a dual-element fuse (14 A × 175%) [S4]. The motor protector discussion that follows uses this number chain as the backbone, because every other selection question (trip class, adjustable vs. fixed, short-circuit coordination) cascades from it.
NEC Table 430.52(C)(1): the Multiplier Matrix That Decides OCPD Type
Per NEC Table 430.52(C)(1), the maximum rating or setting for motor branch-circuit short-circuit and ground-fault protective devices is locked to the protective device type: non-time-delay fuse at 300% of FLC, dual-element (time-delay) fuse at 175% of FLC, instantaneous-trip circuit breaker at 800% of FLC for standard motors (1100% for Design B energy-efficient and Design B premium-efficiency motors), and inverse-time circuit breaker at 250% of FLC [S4].
DC motors run lower: 150% of FLC for fuses, 150% for inverse-time breakers, 250% for instantaneous-trip, because DC locked-rotor current is closer to running current and the same multiplier logic does not apply [S4]. Synchronous motors of the low-torque, low-speed type (typically below 450 RPM, used for compressors and large pumps) need special consideration per the table notes, and the engineer should cross-check 430.54 for exceptions before locking the OCPD rating [S4].
Inverse-Time Breaker vs. Dual-Element Fuse vs. Instantaneous-Trip: a Criteria Comparison
For a 3-phase, non-wound-rotor induction motor on a 480 V system, the four realistic OCPD options line up against three decision criteria: maximum multiplier of FLC, ability to carry motor starting inrush without nuisance trip, and selectivity with upstream feeder protection. The numeric thresholds come from NEC Table 430.52(C)(1) [S4].
Inverse-time circuit breaker at 250% FLC is the most common industrial pick: thermal trip handles running overload region, magnetic trip handles short-circuit, and the breaker can be raised to a higher rating per 430.52(C)(1) Exception 1 if the motor will not start [S3][S4]. Dual-element (time-delay) fuse at 175% FLC gives the smallest OCPD, which improves selectivity with upstream devices, but fuse replacement after a fault is slower than resetting a breaker [S4]. Non-time-delay fuse at 300% FLC is rarely used on motor circuits because the 300% multiplier is so close to LRC that nuisance trips are common, with the table allowing only 300% to 400% of FLC for this fuse class on standard motors [S4]. Instantaneous-trip circuit breaker at 800% FLC (or 1100% on Design B energy-efficient) is a short-circuit-only device and must be paired with a separate overload relay, which is the classic NEMA motor-starter architecture [S4].
Overload Relay Sizing at 115% to 125% FLA: the Other Half of the Spec
Per NEC 430.32, the motor overload protective device (the thermal or electronic overload relay in the starter, not the branch-circuit OCPD) is sized at no more than 125% of the motor FLA for motors with a 1.15 service factor or marked 40°C rise, and at no more than 115% of FLA for all other motors [S1][S3].
For a motor with FLA of 14 A, the overload relay trip range is 16.1 A to 17.5 A maximum, which is well below the 35 A inverse-time breaker OCPD that sits upstream; the two devices are coordinated on purpose, the OCPD handles short-circuit, the overload relay handles sustained overcurrent [S1][S3][S4]. Adjustable electronic overload relays typically offer a 3:1 to 5:1 current setting range on a single frame, with selectable trip classes (Class 10, 20, 30) that change the cold-start trip time at 600% of FLA: Class 10 trips within 10 seconds, Class 20 within 20 seconds, Class 30 within 30 seconds [S2].
Trip Class and Starting Duty: Picking 10, 20, or 30
Trip class selection is driven by motor locked-rotor withstand time and load starting inertia. Class 10 is the default for standard IEC and NEMA motors on normal-inertia loads such as fans, centrifugal pumps, and conveyors, where the motor reaches rated speed in under 5 seconds [S2]. Class 20 is specified for high-inertia loads such as mixers, crushers, and some reciprocating compressors, where starting time stretches to 5 to 15 seconds and a Class 10 relay would nuisance-trip on cold start. Class 30 covers very high inertia or long-acceleration applications including large centrifugal compressors and heavy hammer mills, with cold-start trip times up to 30 seconds at 600% FLA [S2].
Standard induction motors typically tolerate locked-rotor for 10 to 20 seconds when cold and 5 to 15 seconds when hot, so the trip class envelope must be coordinated with the motor thermal limit curve; otherwise the relay will let the motor fail long before it trips, defeating the purpose of the motor protector [S2].
Thermal Magnetic, Electronic, and Adjustable MPCBs: Which Type Fits Which Duty
Thermal-magnetic MPCBs combine a bimetallic strip for overload and a solenoid for short-circuit, and they are the most common type specified for general-purpose industrial motors [S2]. Electronic MPCBs use a current transformer or Rogowski coil feeding a microprocessor, with adjustable trip current, trip class, and ground-fault pickup; they are the right pick when diagnostic data (last trip current, time-to-trip, thermal memory) is needed or when the motor is on a soft-starter or VFD with non-standard current waveforms [S2]. Adjustable MPCBs allow the installer to set the overload current within a 3:1 to 5:1 range on a single frame, which reduces spare parts inventory; fixed MPCBs are lower cost and are the right pick on motors with steady duty and no need to retune the trip [S2].
Phase-failure protection (loss of one phase on a 3-phase motor) and phase-sequence protection are not included in every MPCB, and on critical pumps, compressors, and HVAC fans the spec should explicitly call for phase-loss and phase-reversal trip, not just overcurrent [S2].
Conductor Sizing at 125% FLC: the Partner Calculation
Per NEC 430.22, motor branch-circuit conductors are sized at 125% of the motor FLC, not 125% of the OCPD rating, which is why a 14 A motor gets #14 AWG copper (15 A ampacity per NEC 310.16, derated as needed) and not 6 AWG copper even though the inverse-time breaker is 35 A [S3][S4].
The 125% multiplier on the conductor and the 250% multiplier on the breaker coexist on purpose: the conductor must carry FLC continuously without exceeding its insulation temperature rating, while the breaker only sees the motor starting inrush for a few seconds and can be sized to ride through it [S3][S4]. For multiple motors on a single feeder, the feeder conductor is sized at 125% of the largest motor FLC plus the full FLC of every other motor, per NEC 430.24 [S3].
Who Should NOT Pick the Mainstream 250% Inverse-Time Breaker
Specifiers should look past the inverse-time circuit breaker as the default in three cases: (1) Design B energy-efficient motors, where NEC 430.52(C)(1) allows the inverse-time breaker to stay at 250% FLC, but the instantaneous-trip breaker max rises to 1100% FLC because the motor's lower LRC leaves headroom; (2) very large motors above 100 HP, where 250% of FLC can exceed the available fault current rating of standard molded-case breakers, and the engineer may need to specify a coordinated high-interrupting-capacity breaker or a fuse with current-limiting let-through; (3) VFD-driven motors, where the OCPD must coordinate with the drive's own semiconductor protection and thermal model, and the upstream breaker is often sized at 125% to 150% of drive input current rather than 250% of motor FLC. [S3]
For the VFD case, the brake resistor sizing logic for data centers discussion covers a related protection challenge, because dynamic-brake resistors are sized off the same motor thermal limit curve that the overload relay sees. Likewise, linear guide selection on automated machinery and crossed-roller guide duty cycles matter when the motor starting duty is high, because the mechanical load inertia is what the overload relay must ride through.
Standards, Sourcing, and Shortlist Logic
Per NEC Article 430 Part IV, the OCPD for motor branch-circuit short-circuit and ground-fault protection is governed by Table 430.52(C)(1), with the conductor governed by 430.22 (125% FLC) and the overload relay by 430.32 (115% to 125% FLA) [S3][S4]. IEC 60947-4-1 governs MPCB construction and test, with utilization category AC-3 for standard induction motor starting and AC-4 for inching or plug-reversing duty.
Shortlist logic for the buyer: (1) pull FLC and LRC off the motor nameplate and NEC Table 430.250 (3-phase) or 430.248 (single-phase); (2) pick OCPD type from Table 430.52(C)(1), default inverse-time breaker at 250% FLC; (3) size branch conductor at 125% FLC per 430.22; (4) size overload relay at 115% to 125% FLA per 430.32; (5) pick trip class (10, 20, or 30) from motor LRC withstand and load inertia; (6) verify OCPD interrupting rating (AIC) exceeds available fault current at the line terminals, with 35 kAIC standard on molded-case breakers and 65 to 100 kAIC available for high-fault applications; (7) if the motor is on a VFD or soft-starter, re-coordinate the upstream OCPD with the drive manufacturer's protection curve rather than defaulting to 250% FLC [S2][S4].
Trackable signals for the next spec cycle: NEMA MG-1 motor efficiency revisions, IEEE 841 petrochemical motor updates, and any change to NEC 430.52(C)(1) values for Design B premium-efficiency motors in the 2026 NEC cycle.