FLA on a Motor — Torque, RPM, HP & Amps Reference
FLA on a motor — Full Load Amperes — is the steady-state current the shaft draws at nameplate HP. This page covers the formula linking FLA, torque, HP, and RPM, the NEMA Design A/B/C/D table, the NEC FLA chart for branch-circuit sizing, and the worked example you'll need to size starters and conductors. Reviewed by a licensed PE.
What FLA on a motor actually means
FLA — Full Load Amperes — is the line current a motor draws when delivering its rated horsepower at rated voltage. The number is set during the manufacturer's IEEE 112 test and printed on the nameplate. It is the single most important value when wiring a motor: undersize the conductors and you get insulation failure; oversize the breaker and the overload protection no longer works.
The first practical electric motor was Michael Faraday's 1821 homopolar rotation experiment, which led to the rotating-shaft motor architecture every modern induction, synchronous, brushed-DC, and brushless-DC machine still uses. Two centuries later, FLA, torque, RPM, and HP remain the four nameplate numbers every motor designer balances.
Motor formulas — torque, HP, RPM, FLA
- ·
- T = Shaft torque, lb·ft
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- HP = Nameplate horsepower
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- N = Shaft speed, RPM
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- 5252 = 33 000 ft·lb/min ÷ 2π — converts HP·min/rev to lb·ft
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- T = Shaft torque, N·m
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- P_kW = Shaft power, kW
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- N = Shaft speed, RPM
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- 9549 = 60 000 / 2π — converts kW·min/rev to N·m
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- V = Line-to-line voltage, V
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- I = FLA — Full load amperes, A
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- η = Efficiency at rated load (0.85–0.95)
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- PF = Power factor at rated load (0.80–0.90)
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- N_s = Synchronous speed, RPM
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- f = Supply frequency, Hz
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- P = Number of poles
Standards and motor design classes
| Design | Starting torque | Starting current (LRA) | Slip | Typical use |
|---|---|---|---|---|
| A | ~150 % | 700–1000 % | ≤ 5 % | Specialty, low-inertia |
| B | ~150 % | ~600 % | ≤ 5 % | General-purpose default |
| C | ~225 % | ~600 % | ≤ 5 % | High inertia loads, conveyors |
| D | ~275 % | ~600 % | 5–13 % | Punch presses, oil-well pumps |
| Document | Scope |
|---|---|
| NEC Article 430 | Motor circuits, FLA tables, OCPD |
| NEMA MG 1 | Construction, design A/B/C/D, frame |
| IEEE 112 | Polyphase induction motor test methods |
| IEC 60034-1 | Rotating machines — rating & performance |
| IEC 60034-30-1 | Efficiency classes IE1–IE5 |
| UL 1004-1 | Listing baseline for rotating machines |
Motor FLA chart — NEC Table 430.250 (3-phase induction)
| HP | kW | 208 V | 230 V | 460 V | 575 V |
|---|---|---|---|---|---|
| 1 | 0.75 | 4.2 | 3.6 | 1.8 | 1.4 |
| 3 | 2.2 | 10.6 | 9.6 | 4.8 | 3.9 |
| 5 | 3.7 | 16.7 | 15.2 | 7.6 | 6.1 |
| 10 | 7.5 | 30.8 | 28.0 | 14.0 | 11.0 |
| 20 | 15 | 59.4 | 54.0 | 27.0 | 22.0 |
| 30 | 22 | 88.0 | 80.0 | 40.0 | 32.0 |
| 50 | 37 | 143 | 130 | 65.0 | 52.0 |
| 75 | 55 | 211 | 192 | 96.0 | 77.0 |
| 100 | 75 | 273 | 248 | 124 | 99.0 |
| 150 | 110 | 396 | 360 | 180 | 144 |
| 200 | 150 | 528 | 480 | 240 | 192 |
- Pull the FLA from the nameplate Read FLA / Amps on the motor nameplate. NEMA MG 1 requires it. Note voltage, phases, HP, and RPM at the same time — they are interlinked.
- Cross-check NEC 430.250 For sizing branch-circuit conductors, use the FLA listed in NEC Table 430.250 (3-phase) or 430.248 (1-phase) — not the nameplate. Two motors with identical HP can have different nameplate FLA but the same NEC value.
- Compute rated torque Use T = 5252 × HP / RPM (lb·ft) or T = 9549 × kW / RPM (N·m). A 25 HP motor at 1760 RPM produces 74.6 lb·ft.
- Verify synchronous RPM Nₛ = 120 × f / P. For 60 Hz, 4-pole: 1800 RPM. Loaded slip 0.022 → 1760 RPM. Mismatch by more than 5 % means the motor is overloaded or the supply frequency is off.
- Set protection Conductor 1.25 × FLA, OCPD 2.5 × FLA (NEC 430.22 / 430.52). Overload relay set to 1.15 × nameplate FLA per NEC 430.32(A)(1).
Worked example — 50 HP 460 V 3-phase induction motor
- FLA (NEC 430.250): 65 A.
- Conductor: 1.25 × 65 = 81.25 A → 4 AWG Cu THHN (85 A at 75 °C).
- OCPD (inverse-time breaker): 2.5 × 65 = 162.5 A → 175 A breaker (next std up).
- Overload relay: 1.15 × nameplate FLA. With nameplate 62 A: 71 A trip setpoint.
- Synchronous RPM (4-pole, 60 Hz): 1800; loaded 1760 RPM (slip 2.2 %).
- Rated torque: T = 5252 × 50 / 1760 = 149.2 lb·ft.
- Starting current (LRA, NEMA B): ~6.0 × 65 = 390 A — verify upstream feeder, transformer voltage dip.
AC induction vs. DC vs. brushless DC motor
| Aspect | AC induction (NEMA B) | Brushed DC | Brushless DC (BLDC) |
|---|---|---|---|
| Construction | Stator + cage rotor | Stator + brushes/commutator | Stator + permanent-magnet rotor + electronic commutation |
| Speed control | VFD frequency change | Armature voltage | PWM via electronic controller |
| Efficiency | 87–94 % | 75–85 % | 88–95 % |
| Maintenance | Bearing only | Brushes wear, commutator dressing | Bearing only |
| Cost (10 HP) | $700–1500 | $1500–3000 | $2500–5000 |
| Best for | Pumps, fans, conveyors | Servo, mill drives (legacy) | EVs, robotics, drones |
Related motor topics
Motor torque — torque motor and torque on a motor
"Motor torque", "torque motor", and "torque on a motor" describe related but different things. Motor torque is the rotational force the shaft delivers at any operating point. A torque motor is a specific machine class designed to sit at stall delivering high force without overheating. The torque on a motor — observed externally — is the reaction the housing must resist through its mounts; it is equal in magnitude and opposite in sign to the shaft torque.
Motor RPM, motor speed, and speed of a motor
Synchronous RPM is set by Nₛ = 120 × f / P; loaded RPM is slightly lower because of slip. The "motor speed" stamped on a nameplate is the loaded RPM at rated HP — typically 1740–1780 for 4-pole 60 Hz machines. To change speed, change frequency with a VFD; voltage-only changes only deliver narrow speed adjustment and risk core saturation.
Motor amps, motor amp, motor amperage
All three phrases describe the same nameplate quantity — the FLA. For NEC circuit sizing, however, use the table value (430.250), not the nameplate. A clamp-on ammeter reading at the contactor is the field check: it should sit within ±5 % of FLA at full load and well below 100 % at light load.
Motor HP, motor horsepower, hp motor
HP and horsepower describe the rated mechanical output. 1 HP = 746 W of shaft power, which corresponds to roughly 950 W of input on a NEMA Premium motor (0.79 system efficiency). Motor HP rating is a discrete ladder (1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100…) that matches available NEMA frame sizes.
Motor power, power motor, motor load, motor duty
Motor power is the average mechanical output — its product of torque and angular velocity. Motor load is the fraction of rated HP currently delivered; below 25 % load, NEMA induction motors lose 5–15 percentage points of efficiency. Motor duty (NEMA S1–S10) classifies how the load cycles — S1 continuous, S2 short-time, S3 intermittent — and is essential when sizing motors that don't run flat-out.
Motor size, sizing, and motor starting
Motor size has two meanings: NEMA frame number (mechanical size, e.g. 254T) and HP rating (electrical/output size). Pick HP from the load curve and add a 10–15 % margin; pick frame from the nameplate-to-frame chart in NEMA MG 1. Motor starting introduces inrush at 600 % FLA for NEMA B, which can collapse the upstream voltage; reduced-voltage starters or VFDs cut the surge to 200–300 %.
DC motor and operation of DC motor
A DC motor runs on direct current and uses either brushes/commutator (legacy) or electronic commutation (BLDC). Operation: armature current in a magnetic field produces force per F = BIL; speed is controlled by armature voltage (Va = E + Ia·Ra), and torque scales linearly with armature current. DC motors offer simple speed control and high starting torque but suffer brush wear in the brushed variant.
Motor efficiency and ELV motors
Motor efficiency η = Pₒᵤₜ / Pᵢₙ. Premium NEMA / IE3 motors hit 91–95 % at rated load; IE5 ultra-premium reaches 96 %. ELV (Extra Low Voltage) motors operate below 50 V AC or 120 V DC — used in battery-powered equipment, mobility scooters, and SELV-classified medical devices.
- How to find motor torque (also: how to find torque of a motor / how to measure torque on a motor)?
- Three methods, in order of accuracy. (1) Calculate from nameplate: T = 5252 × HP / RPM (lb·ft) — fastest and accurate to ±5 % at rated load. (2) Use a clamp-on power meter: measure real input watts, multiply by efficiency to get shaft power, then divide by speed. (3) Use a strain-gauge torque transducer or reaction-arm dynamometer — the certified-test method per IEEE 112 and the only one acceptable for warranty disputes.
- How to calculate horsepower of a motor (also: how to calculate motor horsepower)?
- From electrical input on a 3-phase motor: HP = (√3 × V × I × η × PF) / 746. From mechanical output: HP = (T × N) / 5252, with T in lb·ft and N in RPM. Worked example: 460 V, 12 A, η = 0.91, PF = 0.85 → HP = (1.732 × 460 × 12 × 0.91 × 0.85) / 746 = 9.93 → 10 HP class motor.
- What is torque of a motor (also: what is motor torque / what is the torque of a motor / what is torque in motor)?
- Motor torque is the rotational force the shaft can exert, measured in pound-feet (lb·ft) or newton-metres (N·m). It is the immediate output that turns whatever the motor is coupled to — pump impeller, compressor, conveyor pulley. Rated torque is the value at nameplate HP and full speed: T_rated = 5252 × HP / RPM. Starting torque (NEMA Design B is ~150 % of rated), pull-out torque (~200 %), and breakdown torque all scale relative to that baseline.
- What is a torque motor?
- A torque motor is a special design optimized to produce high stall torque continuously without burning out — a regular induction motor would overheat in seconds at zero RPM. Torque motors are used as direct-drive actuators for valves, machine-tool axes, robotics joints, and tape/film tensioners where the load demands force without significant rotation. They are typically permanent-magnet brushless DC machines, water-cooled, with low pole-count and high pole pitch.
- What is FLA on a motor?
- FLA stands for Full Load Amperes — the steady-state line current the motor draws when delivering its nameplate horsepower at rated voltage and frequency. It appears on every NEMA MG 1 nameplate. For NEC sizing, however, the value in Table 430.250 (3-phase) or 430.248 (1-phase) supersedes nameplate FLA; this avoids manufacturer-by-manufacturer variation when sizing conductors and overcurrent protection.
The full-load current of a motor is the current required to produce rated output at rated voltage, frequency, and rated load. For sizing branch-circuit conductors and overcurrent protection, the values in NEC Tables 430.247–430.250 shall be used in lieu of the nameplate value.
Sources
- NFPA 70 — NEC Article 430 (motor circuits, 2023).
- NEMA MG 1-2021 — Motors and Generators.
- IEEE Std 112-2017 — Polyphase induction motor test method.
- IEC 60034-1 / 60034-30-1 — Rotating machines and IE classes.
- U.S. DOE 10 CFR 431 — Motor energy conservation rules.
- NEMA Application Guide for AC Motors (2020).