How to calculate hp of electric motor?

For a 3-phase motor: HP = (√3 × V × I × η × PF) / 746 — where V is line voltage, I is full-load current, η is efficiency (0.85–0.95 for NEMA Premium), PF is power factor (0.80–0.90 typical). For a single-phase motor drop the √3: HP = (V × I × η × PF) / 746. Example: 460 V, 12 A, η = 0.91, PF = 0.85 → HP = (1.732 × 460 × 12 × 0.91 × 0.85) / 746 = 9.93 → round to 10 HP.

How to calculate horsepower electric motor (alternative formulation)?

If you have shaft torque and speed instead of electrical inputs, use HP = (T × N) / 5252, where T is torque in lb·ft and N is RPM. For metric: kW = (T × N) / 9549, with T in N·m. A 10 HP motor at 1750 RPM produces 30 lb·ft of torque (10 × 5252 / 1750).

How to measure electric motor torque?

Three practical methods. (1) Indirect: T = 5252 × HP / RPM, using nameplate HP and a tachometer reading of actual RPM. (2) In-line torque transducer: a strain-gauge-based shaft coupling that streams instantaneous torque digitally — most accurate, used in test stands. (3) Reaction-arm load cell: bolt the motor on a pivoting bracket, measure the reaction force at a known lever-arm length. Method 1 is fine for everyday troubleshooting; methods 2 and 3 are required for warranty disputes and certified efficiency tests.

How to calculate rpm of electric motor?

Synchronous RPM of an AC induction motor: Nₛ = 120 × f / P, where f is the supply frequency (Hz) and P is the number of poles. A 4-pole 60 Hz motor: Nₛ = 1800 RPM. The actual shaft speed under load is slightly lower because of slip: N = Nₛ × (1 − s), with s ≈ 0.02–0.05 for NEMA B motors. So a "1750 RPM" 4-pole motor runs at 1800 × (1 − 0.0278) = 1750.

What is the electric motor FLA chart?

The FLA chart in NEC Table 430.250 (3-phase induction) lists full-load amperes for standard motor HP ratings at 200, 208, 230, 460, and 575 V. It is the legally required value for sizing motor branch-circuit conductors and overcurrent devices, regardless of the slightly different number printed on the nameplate. Common entries: 1 HP @ 230 V = 3.6 A, 10 HP @ 230 V = 28 A, 50 HP @ 460 V = 65 A, 100 HP @ 460 V = 124 A.

How to calculate hp of electric motor?

For a 3-phase motor: HP = (√3 × V × I × η × PF) / 746 — where V is line voltage, I is full-load current, η is efficiency (0.85–0.95 for NEMA Premium), PF is power factor (0.80–0.90 typical). For a single-phase motor drop the √3: HP = (V × I × η × PF) / 746. Example: 460 V, 12 A, η = 0.91, PF = 0.85 → HP = (1.732 × 460 × 12 × 0.91 × 0.85) / 746 = 9.93 → round to 10 HP.

How to calculate horsepower electric motor (alternative formulation)?

If you have shaft torque and speed instead of electrical inputs, use HP = (T × N) / 5252, where T is torque in lb·ft and N is RPM. For metric: kW = (T × N) / 9549, with T in N·m. A 10 HP motor at 1750 RPM produces 30 lb·ft of torque (10 × 5252 / 1750).

How to measure electric motor torque?

Three practical methods. (1) Indirect: T = 5252 × HP / RPM, using nameplate HP and a tachometer reading of actual RPM. (2) In-line torque transducer: a strain-gauge-based shaft coupling that streams instantaneous torque digitally — most accurate, used in test stands. (3) Reaction-arm load cell: bolt the motor on a pivoting bracket, measure the reaction force at a known lever-arm length. Method 1 is fine for everyday troubleshooting; methods 2 and 3 are required for warranty disputes and certified efficiency tests.

How to calculate rpm of electric motor?

Synchronous RPM of an AC induction motor: Nₛ = 120 × f / P, where f is the supply frequency (Hz) and P is the number of poles. A 4-pole 60 Hz motor: Nₛ = 1800 RPM. The actual shaft speed under load is slightly lower because of slip: N = Nₛ × (1 − s), with s ≈ 0.02–0.05 for NEMA B motors. So a "1750 RPM" 4-pole motor runs at 1800 × (1 − 0.0278) = 1750.

What is the electric motor FLA chart?

The FLA chart in NEC Table 430.250 (3-phase induction) lists full-load amperes for standard motor HP ratings at 200, 208, 230, 460, and 575 V. It is the legally required value for sizing motor branch-circuit conductors and overcurrent devices, regardless of the slightly different number printed on the nameplate. Common entries: 1 HP @ 230 V = 3.6 A, 10 HP @ 230 V = 28 A, 50 HP @ 460 V = 65 A, 100 HP @ 460 V = 124 A.

Reference · Material specs

Electric Motor FLA — Full Load Amps, HP, Torque & RPM

Electric motor FLA (full load amperes) is the steady-state current a motor draws at its nameplate rating — for a 10 HP 230 V three-phase induction motor, that's 28 A from NEC Table 430.250. This page covers the FLA chart for HP from 0.5 to 200, the formulas tying HP, torque, RPM, and current together, and the worked example you need to size conductors and protection. Reviewed by a licensed PE.

What electric motor FLA is and why it matters

Full Load Amperes (FLA) is the current a motor draws while delivering its nameplate rated horsepower at rated voltage and frequency. It is the reference value the National Electrical Code uses to size branch-circuit conductors, overload relays, disconnects, and breakers. Because nameplate FLA varies between manufacturers, NEC 430.6 mandates the use of the standardized FLA values in Tables 430.247 (DC), 430.248 (single-phase AC), and 430.250 (three-phase AC) for sizing — not the nameplate value.

The modern AC induction motor that dominates industrial use was patented by Nikola Tesla in 1888. His three-phase polyphase design — combined with Westinghouse's 60 Hz transmission system — set the architecture every NEMA induction motor still follows.

Electric motor formulas — HP, torque, RPM, FLA

Eq. 01 — HP from 3-phase electrical inputs SI

HP = \frac{\sqrt{3} \cdot V \cdot I \cdot \eta \cdot PF}{746}

·: V = Line-to-line voltage, V
·: I = Line current (FLA), A
·: η = Efficiency (0.85–0.95)
·: PF = Power factor (0.80–0.90)
·: 746 = Watts per HP

Eq. 02 — Torque from HP and RPM SI

T = \frac{5252 \cdot HP}{N}

·: T = Shaft torque, lb·ft
·: HP = Nameplate horsepower
·: N = Shaft speed, RPM
·: 5252 = Conversion constant (33 000 / 2π)

Eq. 03 — Synchronous RPM SI

N_s = \frac{120 \cdot f}{P}

·: N_s = Synchronous speed, RPM
·: f = Supply frequency, Hz
·: P = Number of poles (2, 4, 6, 8…)

Eq. 04 — Slip and actual speed SI

N = N_s (1 - s)

·: N = Loaded shaft speed, RPM
·: s = Slip (0.02–0.05 for NEMA B)

Electric motor FLA chart — NEC Table 430.250

HP	kW	208 V	230 V	460 V	575 V
1	0.75	4.2	3.6	1.8	1.4
1.5	1.1	6.0	5.2	2.6	2.1
2	1.5	7.5	6.8	3.4	2.7
3	2.2	10.6	9.6	4.8	3.9
5	3.7	16.7	15.2	7.6	6.1
7.5	5.5	24.2	22.0	11.0	9.0
10	7.5	30.8	28.0	14.0	11.0
15	11	46.2	42.0	21.0	17.0
20	15	59.4	54.0	27.0	22.0
25	18.5	74.8	68.0	34.0	27.0
30	22	88.0	80.0	40.0	32.0
40	30	114	104	52.0	41.0
50	37	143	130	65.0	52.0
60	45	169	154	77.0	62.0
75	55	211	192	96.0	77.0
100	75	273	248	124	99.0
125	90	343	312	156	125
150	110	396	360	180	144
200	150	528	480	240	192

Standards covering motor sizing and protection

Document	Scope	Key clause
NEC Article 430	Motor circuits	FLA tables, 1.25 × conductor, 250 % OCPD
NEMA MG 1	Motor and generator construction	Frame, NEMA Design A/B/C/D
NEMA MG 10	Energy management guide	Premium efficiency calculations
IEEE 112	Polyphase induction motor test method	Methods A, B, E1, F1
IEC 60034-1	Rotating electrical machines — rating	IE classes IE1–IE5
UL 1004-1	Rotating electrical machines — general	Listing baseline

Read the nameplate Locate Voltage (V), HP / kW rating, phases, RPM, FLA / amperes, service factor (SF), and efficiency. NEMA MG 1 requires every motor manufactured for sale in the U.S. to display these.
Compare nameplate FLA against NEC 430.250 For motor branch-circuit and overcurrent protection, NEC 430.6(A)(1) requires using the table FLA, not the nameplate FLA. Pull the value from NEC Table 430.250 (3-phase) or 430.248 (single-phase).
Compute branch-circuit conductor Wire the motor at 1.25 × FLA per NEC 430.22(A). For a 10 HP / 230 V 3-phase motor (FLA = 28 A), conductor ampacity must be ≥ 35 A → 8 AWG copper.
Size the OCPD Inverse-time breaker: 250 % × FLA per NEC 430.52. 28 A × 2.50 = 70 A breaker. Round up to next standard size if needed.
Verify torque and RPM at the load Compute T = 5252 × HP / RPM (in lb·ft) and confirm the load torque is below motor rated torque at the design RPM. Use a tachometer or current-clamped efficiency reading to validate during commissioning.

Worked example — 25 HP, 460 V, 3-phase motor

FLA from NEC 430.250: 34 A.
Conductor sizing: 1.25 × 34 = 42.5 A → 8 AWG Cu THHN (50 A at 75 °C).
Branch-circuit OCPD: 2.50 × 34 = 85 A → next std up = 90 A inverse-time breaker.
Overload relay: 1.15 × nameplate FLA. With nameplate 32 A: 36.8 A trip setpoint.
Disconnect: 1.15 × FLA → 40 A NEMA-rated, fused or unfused per design.
Synchronous speed (4-pole, 60 Hz): 1800 RPM; loaded 1760 RPM (slip 0.022).
Rated torque: T = 5252 × 25 / 1760 = 74.6 lb·ft.

Comparison — single-phase vs. three-phase electric motor

Aspect	1-phase (3 HP shown)	3-phase (3 HP shown)
FLA at 230 V	17 A	9.6 A
Starting method	Capacitor-start / capacitor-run	Direct, soft-start, or VFD
Starting torque	~150 % of rated	~200 % of rated (NEMA B)
Efficiency at rated load	72–82 %	87–94 % (Premium)
Power factor	0.65–0.75	0.85–0.90
Typical use	Pumps, fans < 5 HP, residential	Industrial > 1 HP, commercial HVAC

Related queries — electric motor HP, sizes, ratings

Electric motor HP — picking the right rating

HP is the rated mechanical output, not the input electrical draw. Always select an electric motor HP equal to or 10–15 % above the load's actual shaft demand at design point. Oversizing past 25 % drops the motor below its peak efficiency window and increases idle losses; undersizing causes chronic overload trips and shortened insulation life.

Electric motor size chart — NEMA frame to HP

NEMA frame size encodes the shaft height, mounting bolt pattern, and shaft diameter. A 56 frame is the smallest commercial size (1/3–1.5 HP), 143/145T covers 1–2 HP, 184T covers 5 HP, 254T covers 15 HP, 326T covers 40 HP. The "T" suffix means the modern (post-1964) imperial standard. IEC machines use D-flange or B3 foot mountings sized in millimetres.

How to calculate hp of electric motor?: For a 3-phase motor: HP = (√3 × V × I × η × PF) / 746 — where V is line voltage, I is full-load current, η is efficiency (0.85–0.95 for NEMA Premium), PF is power factor (0.80–0.90 typical). For a single-phase motor drop the √3: HP = (V × I × η × PF) / 746. Example: 460 V, 12 A, η = 0.91, PF = 0.85 → HP = (1.732 × 460 × 12 × 0.91 × 0.85) / 746 = 9.93 → round to 10 HP.
How to calculate horsepower electric motor (alternative formulation)?: If you have shaft torque and speed instead of electrical inputs, use HP = (T × N) / 5252, where T is torque in lb·ft and N is RPM. For metric: kW = (T × N) / 9549, with T in N·m. A 10 HP motor at 1750 RPM produces 30 lb·ft of torque (10 × 5252 / 1750).
How to measure electric motor torque?: Three practical methods. (1) Indirect: T = 5252 × HP / RPM, using nameplate HP and a tachometer reading of actual RPM. (2) In-line torque transducer: a strain-gauge-based shaft coupling that streams instantaneous torque digitally — most accurate, used in test stands. (3) Reaction-arm load cell: bolt the motor on a pivoting bracket, measure the reaction force at a known lever-arm length. Method 1 is fine for everyday troubleshooting; methods 2 and 3 are required for warranty disputes and certified efficiency tests.
How to calculate rpm of electric motor?: Synchronous RPM of an AC induction motor: Nₛ = 120 × f / P, where f is the supply frequency (Hz) and P is the number of poles. A 4-pole 60 Hz motor: Nₛ = 1800 RPM. The actual shaft speed under load is slightly lower because of slip: N = Nₛ × (1 − s), with s ≈ 0.02–0.05 for NEMA B motors. So a "1750 RPM" 4-pole motor runs at 1800 × (1 − 0.0278) = 1750.
What is the electric motor FLA chart?: The FLA chart in NEC Table 430.250 (3-phase induction) lists full-load amperes for standard motor HP ratings at 200, 208, 230, 460, and 575 V. It is the legally required value for sizing motor branch-circuit conductors and overcurrent devices, regardless of the slightly different number printed on the nameplate. Common entries: 1 HP @ 230 V = 3.6 A, 10 HP @ 230 V = 28 A, 50 HP @ 460 V = 65 A, 100 HP @ 460 V = 124 A.

Other than for motors built for low speeds (less than 1200 RPM) or high torques, the values given in Tables 430.247, 430.248, 430.249, and 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate.

NEC 430.6(A)(1) → NFPA 70 — National Electrical Code, 2023

Sources

NFPA 70 — NEC Article 430 (motor circuits, 2023).
NEMA MG 1-2021 — Motors and Generators.
IEEE Std 112 — Test Procedure for Polyphase Induction Motors.
IEC 60034-1 / 60034-30-1 — Rotating machines and IE efficiency classes.
U.S. DOE 10 CFR 431 — Electric motor energy conservation rules.
NEMA Premium Efficiency Motors program documentation.