I = (HP × 746) / (V × η × PF) (1φ) · I = (HP × 746) / (√3 × V × η × PF) (3φ)NEC Quick Reference (3φ, 460 V)
| HP | FLA (A) | HP | FLA (A) |
|---|---|---|---|
| 1 | 2.1 | 30 | 40 |
| 2 | 3.4 | 40 | 52 |
| 3 | 4.8 | 50 | 65 |
| 5 | 7.6 | 60 | 77 |
| 7.5 | 11 | 75 | 96 |
| 10 | 14 | 100 | 124 |
| 15 | 21 | 125 | 156 |
| 20 | 27 | 200 | 240 |
Source: NEC Table 430.250 (2023). Values are for standard-speed squirrel-cage motors.
FAQ
Why do I need PF and efficiency?
HP is shaft output. Amps is electrical input. PF and η bridge the gap between mechanical and electrical power.
Can I just use NEC tables?
Yes for motor protection sizing. But for exact current, use the formula with your motor’s actual nameplate PF and η.
Every time you size a breaker, select a cable, or set up overload protection for a motor, you need to convert HP to Amps. The formula itself is straightforward — multiply horsepower by 746 and divide by voltage, efficiency, and power factor (plus √3 for three-phase). But the real challenge is knowing which values to plug in: NEC table values for protection sizing, or actual nameplate data for exact calculations. This HP to Amps calculator handles both scenarios and covers single-phase, three-phase, and DC motors.
Below you will find the complete NEC full-load current tables (430.248 and 430.250), the formulas explained variable by variable, six solved examples with real equipment, and direct answers for every common HP-to-Amps lookup from 1 HP through 60 HP.
HP to Amps — NEC Full-Load Current Tables
These values come from NEC Tables 430.248 (single-phase) and 430.250 (three-phase). Use them for sizing branch-circuit conductors, overcurrent protection, and motor controllers — not for setting overload relays (use nameplate FLA for that). The tables assume standard-speed, normal-torque squirrel-cage induction motors at 60 Hz.
Table 1 — AC Induction Motors: HP to Amps (60 Hz)
| HP | 1φ 115 V | 1φ 230 V | 3φ 200 V | 3φ 230 V | 3φ 460 V | 3φ 575 V |
|---|---|---|---|---|---|---|
| ½ | 9.8 | 4.9 | 2.5 | 2.2 | 1.1 | 0.9 |
| ¾ | 13.8 | 6.9 | 3.7 | 3.2 | 1.6 | 1.3 |
| 1 | 16 | 8 | 4.8 | 4.2 | 2.1 | 1.7 |
| 1½ | 20 | 10 | 6.9 | 6.0 | 3.0 | 2.4 |
| 2 | 24 | 12 | 7.8 | 6.8 | 3.4 | 2.7 |
| 3 | 34 | 17 | 11.0 | 9.6 | 4.8 | 3.9 |
| 5 | 56 | 28 | 17.5 | 15.2 | 7.6 | 6.1 |
| 7½ | 80 | 40 | 25.3 | 22 | 11 | 9 |
| 10 | 100 | 50 | 32.2 | 28 | 14 | 11 |
| 15 | — | — | 48.3 | 42 | 21 | 17 |
| 20 | — | — | 62.1 | 54 | 27 | 22 |
| 25 | — | — | 78.2 | 68 | 34 | 27 |
| 30 | — | — | 92 | 80 | 40 | 32 |
| 40 | — | — | 120 | 104 | 52 | 41 |
| 50 | — | — | 150 | 130 | 65 | 52 |
| 60 | — | — | 177 | 154 | 77 | 62 |
| 75 | — | — | 221 | 192 | 96 | 77 |
| 100 | — | — | 285 | 248 | 124 | 99 |
| 125 | — | — | 359 | 312 | 156 | 125 |
| 150 | — | — | 414 | 360 | 180 | 144 |
| 200 | — | — | 552 | 480 | 240 | 192 |
Key point: NEC table values are intentionally conservative. They are used to size conductors and overcurrent protection. For setting the overload relay (which protects the motor from sustained overload), always use the full-load amperes (FLA) stamped on the motor nameplate — per NEC 430.6(A).
Table 2 — DC Motors: HP to Amps
| HP | 90 V | 120 V | 180 V | 240 V | 500 V |
|---|---|---|---|---|---|
| ¼ | 4.0 | 3.1 | 2.0 | 1.6 | — |
| ½ | 6.8 | 5.4 | 3.4 | 2.7 | — |
| 1 | 12.2 | 9.5 | 6.1 | 4.7 | — |
| 2 | — | 17 | 10.8 | 8.5 | 3.6 |
| 3 | — | 25 | 16 | 12.2 | 5.2 |
| 5 | — | 40 | 27 | 20 | 8.3 |
| 7½ | — | 58 | — | 29 | 13.6 |
| 10 | — | 76 | — | 38 | 18 |
| 15 | — | 110 | — | 55 | 27 |
| 20 | — | 148 | — | 72 | 34 |
DC motor values come from NEC Table 430.247. DC motors draw more current per HP than AC motors because they lack the √3 voltage advantage of three-phase systems.
Step-by-Step Formulas: How to Convert HP to Amps
The core idea is simple: horsepower is mechanical output, amps is electrical input. To bridge the gap, you need three quantities — voltage, motor efficiency (η), and power factor (PF). Each formula below starts from the fundamental relationship P(W) = V × I × PF × η (for single-phase), with HP × 746 converting horsepower to watts.
Formula 1 — Single-Phase AC
Where V is line voltage (typically 115 V or 230 V), η is motor efficiency as a decimal (e.g., 0.90), and PF is power factor as a decimal (e.g., 0.85). The constant 746 converts HP to watts (1 HP = 746 W per NEMA convention).
Example: A 5 HP single-phase motor at 230 V, η = 0.88, PF = 0.85.
I = (5 × 746) ÷ (230 × 0.88 × 0.85) = 3730 ÷ 171.96 = 21.69 A.
Compare with NEC table value of 28 A — the table is intentionally higher to provide a safety margin.
Formula 2 — Three-Phase AC
The √3 (1.7321) factor accounts for the 120° phase offset in a balanced three-phase system. V is the line-to-line voltage (e.g., 460 V, 480 V). This formula is defined in IEEE standards and referenced throughout IEEE C37 for motor circuit protection.
Example: A 10 HP three-phase motor at 460 V, η = 0.91, PF = 0.86.
I = (10 × 746) ÷ (1.7321 × 460 × 0.91 × 0.86) = 7460 ÷ 623.67 = 11.96 A.
NEC Table 430.250 lists 14 A for a 10 HP motor at 460 V.
Formula 3 — DC Motors
DC motors have no power factor (there is no reactive component in DC circuits). Only voltage and efficiency matter. η for DC motors is typically 0.80–0.92 depending on size and type.
Example: A 5 HP DC motor at 240 V, η = 0.85.
I = (5 × 746) ÷ (240 × 0.85) = 3730 ÷ 204 = 18.28 A.
NEC table lists 20 A for 5 HP at 240 V DC.
Variables Explained — Power Factor, Efficiency, and Voltage
The formula is only as good as the values you plug into it. Here is a practical reference for each variable, so you do not have to guess.
| Variable | Symbol | Typical Range | Where to Find It |
|---|---|---|---|
| Voltage | V | 115–600 V (LV) | Motor nameplate, supply transformer secondary |
| Efficiency | η | 0.80–0.96 | Nameplate, or manufacturer datasheet (IE class) |
| Power Factor | PF | 0.70–0.95 | Nameplate (often listed at full load), or measured with power analyzer |
Typical PF by motor size: Small motors (1–5 HP): PF ≈ 0.75–0.85. Medium motors (7.5–30 HP): PF ≈ 0.82–0.90. Large motors (40–200 HP): PF ≈ 0.85–0.92. Motors at partial load have lower PF — a 50% loaded motor might drop to PF 0.65.
Typical efficiency by frame: NEMA standard efficiency motors: η ≈ 0.85–0.90. NEMA premium (or IEC IE3): η ≈ 0.91–0.96. Check our motor efficiency calculator for exact values by HP and frame.
When to use NEC tables vs formulas: Use NEC table FLA values for conductor sizing and overcurrent protection per Article 430. Use the formula with actual nameplate PF and η when you need the true operating current for energy analysis, VFD programming, or load monitoring. Per IEC 60034-30, motor efficiency classes (IE1–IE5) must be stamped on the nameplate in IEC markets.
Inverse Conversion: Amps to HP
To go from amps back to horsepower, multiply instead of dividing.
HP = (I × √3 × V × η × PF) ÷ 746 — three-phase
| Amps | HP at 230 V 1φ (η=0.90, PF=0.85) | HP at 460 V 3φ (η=0.90, PF=0.85) |
|---|---|---|
| 5 | 2.35 | 4.07 |
| 10 | 4.71 | 8.14 |
| 14 | 6.59 | 11.40 |
| 20 | 9.42 | 16.28 |
| 28 | 13.18 | 22.80 |
| 40 | 18.83 | 32.57 |
| 52 | 24.48 | 42.34 |
| 77 | 36.25 | 62.71 |
| 124 | 58.35 | 100.95 |
You can also use the calculator above in reverse mode, or visit our dedicated Amps to HP calculator for a full treatment.
6 Solved Examples — Real-World HP to Amps Conversions
Example 1 — 1 HP Single-Phase Pool Pump at 230 V
Data: 1 HP, 230 V, single-phase, η = 0.82, PF = 0.80.
Formula: I = (1 × 746) ÷ (230 × 0.82 × 0.80)
Result: 4.94 A
Residential pool pumps are typically 1 HP single-phase. The NEC table lists 8 A at 230 V — substantially higher than the calculated 4.94 A because the table accounts for worst-case motor designs. For branch circuit sizing, use 8 A × 1.25 = 10 A minimum conductor ampacity.
Example 2 — 3 HP Three-Phase Compressor at 230 V
Data: 3 HP, 230 V, 3-phase, η = 0.88, PF = 0.84.
Formula: I = (3 × 746) ÷ (1.7321 × 230 × 0.88 × 0.84)
Result: 7.59 A
A 3 HP compressor motor is common in small shops. NEC lists 9.6 A at 230 V 3φ. Size a 15 A breaker (9.6 × 1.25 = 12 A → next standard size is 15 A) and 14 AWG copper THHN per NEC 430.22.
Example 3 — 10 HP Three-Phase Motor at 460 V
Data: 10 HP, 460 V, 3-phase, η = 0.91, PF = 0.86.
Formula: I = (10 × 746) ÷ (1.7321 × 460 × 0.91 × 0.86)
Result: 11.96 A
The 10 HP, 460 V motor is a workhorse for fans, pumps, and conveyor drives. NEC table FLA is 14 A. Branch circuit breaker: 14 × 2.5 = 35 A (inverse-time breaker per NEC 430.52), so a 35 A breaker is the first choice. Conductor: 12 AWG copper minimum (14 × 1.25 = 17.5 A ampacity).
Example 4 — 30 HP Three-Phase Motor at 480 V
Data: 30 HP, 480 V, 3-phase, η = 0.92, PF = 0.87.
Formula: I = (30 × 746) ÷ (1.7321 × 480 × 0.92 × 0.87)
Result: 33.62 A
For branch circuit sizing, NEC uses 40 A (at 460 V). Breaker: 40 × 2.5 = 100 A (inverse-time). Conductor: 40 × 1.25 = 50 A → 6 AWG copper THHN. Always verify your actual system voltage — 480 V vs 460 V changes the NEC table column you reference.
Example 5 — 40 HP Three-Phase at 380 V (IEC System)
Data: 40 HP (29.8 kW), 380 V, 3-phase, η = 0.92, PF = 0.86.
Formula: I = (40 × 746) ÷ (1.7321 × 380 × 0.92 × 0.86)
Result: 55.18 A
In IEC 380/400 V systems (EU, Asia), a 40 HP motor draws about 55 A. Size cables per IEC 60364 — typically 16 mm² copper. The MCCB trip setting should be approximately 55 × 1.3 = 72 A for motor starting allowance.
Example 6 — 60 HP Three-Phase at 575 V (Canada)
Data: 60 HP, 575 V, 3-phase, η = 0.93, PF = 0.88.
Formula: I = (60 × 746) ÷ (1.7321 × 575 × 0.93 × 0.88)
Result: 54.78 A
Canada’s 600 V class runs at 575 V nominal. NEC lists 62 A at 575 V for 60 HP. The higher voltage means significantly less current than the same motor at 460 V (77 A), allowing smaller cables and bus bars. Size a 4 AWG copper conductor.
HP to Amps on Motor Nameplates — What You Actually See
When you read a motor nameplate, you will see HP (or kW), voltage, FLA, service factor, NEMA design letter, and sometimes efficiency and PF. Here is how these relate to the HP-to-Amps conversion.
Nameplate FLA vs NEC Table FLA
The nameplate FLA is the actual current that specific motor draws at full rated load. The NEC table FLA is a generic, conservative value that represents the highest current any standard motor of that HP rating might draw. You use the nameplate FLA for overload protection (OL relay) and the NEC table FLA for branch circuit conductors and overcurrent protection. Mixing them up is one of the most common code violations on motor installations.
Service Factor (SF)
A service factor of 1.15 means the motor can continuously deliver 115% of nameplate HP without overheating. At SF load, the motor draws more current than nameplate FLA. When setting overload relays on a motor with SF ≥ 1.15, NEC 430.32 allows the overload trip to be set at 125% of nameplate FLA (vs 115% for SF 1.0 motors).
Dual-Voltage Motors
Many motors are stamped “230/460 V.” At 230 V, the motor draws twice the amps it draws at 460 V — same HP, half the voltage, double the current. The NEC table reflects this. Always verify which voltage tap your motor is wired for before looking up FLA.
For wire sizing based on the current you just calculated, check our AWG to mm² equivalence table and our Amps to Watts calculator.
Quick Equivalences — Most-Searched HP to Amps Values
These are the specific HP values people search for most. Each result uses NEC table values (the go-to numbers for electricians) plus a formula-calculated value for reference.
1 HP to Amps
NEC: 16 A at 115 V (1φ) · 8 A at 230 V (1φ) · 2.1 A at 460 V (3φ)
Standard fractional motor. 1 HP at 230 V single-phase is the most common residential motor size.
1 HP to Amps at 220 V
Formula: ~4.78 A (3φ, η=0.90, PF=0.85) · NEC: ~4.2 A (3φ 230 V) · ~8 A (1φ 230 V)
The 220 V query usually refers to 230 V nominal. For single-phase, NEC says 8 A.
2 HP to Amps
NEC: 24 A at 115 V (1φ) · 12 A at 230 V (1φ) · 3.4 A at 460 V (3φ)
Common for garage door openers, sump pumps, and small shop tools.
3 HP to Amps
NEC: 34 A at 115 V (1φ) · 17 A at 230 V (1φ) · 4.8 A at 460 V (3φ)
Drill presses, small air compressors. At 115 V single-phase, 34 A requires a dedicated 40 A circuit.
5 HP to Amps
NEC: 56 A at 115 V (1φ) · 28 A at 230 V (1φ) · 7.6 A at 460 V (3φ)
Air compressors, centrifugal pumps. 5 HP single-phase at 115 V is rare — 56 A demands heavy wiring.
10 HP to Amps
NEC: 50 A at 230 V (1φ) · 14 A at 460 V (3φ) · 28 A at 230 V (3φ)
Elevator hoists, large fans, chillers. 10 HP is the practical upper limit for single-phase motors.
30 HP to Amps
NEC: 80 A at 230 V (3φ) · 40 A at 460 V (3φ) · 32 A at 575 V (3φ)
Conveyor systems, HVAC chillers, large compressors. Size a 100 A breaker at 460 V (40 × 2.5).
40 HP to Amps
NEC: 104 A at 230 V (3φ) · 52 A at 460 V ( 3φ) · 41 A at 575 V (3φ)
Crushers, large mixers. Conductor sizing at 460 V: 52 × 1.25 = 65 A → 6 AWG copper.
60 HP to Amps
NEC: 154 A at 230 V (3φ) · 77 A at 460 V (3φ) · 62 A at 575 V (3φ)
Mine ventilation, cooling towers. At 460 V, breaker: 77 × 2.5 = 192.5 → use 200 A inverse-time.
HP to Amps 3-Phase Formula
I = (HP × 746) ÷ (√3 × V × η × PF)
This is the universal three-phase formula. Use NEC table values for protection; use this formula with nameplate data for exact operating current.
Frequently Asked Questions
How do I convert HP to Amps?
Use I = (HP × 746) ÷ (V × η × PF) for single-phase, or I = (HP × 746) ÷ (√3 × V × η × PF) for three-phase. Example: 10 HP at 460 V 3φ, η = 0.91, PF = 0.86 → I = 7460 ÷ 623.67 = 11.96 A.
How many amps does a 1 HP motor draw?
Per NEC Table 430.248: 16 A at 115 V single-phase, 8 A at 230 V single-phase. Per NEC Table 430.250: 4.2 A at 230 V three-phase, 2.1 A at 460 V three-phase. These are full-load values for standard-speed motors.
What is the HP to Amps formula for 3-phase?
I = (HP × 746) ÷ (√3 × V × η × PF). The √3 factor (1.7321) accounts for the three-phase power relationship. V is line-to-line voltage. Example: 30 HP at 480 V, η = 0.92, PF = 0.87 → 33.62 A.
Why do I need efficiency and power factor?
HP is mechanical output power. Amps is electrical input current. A motor is not 100% efficient — it loses energy to heat, friction, and magnetic losses. Efficiency (η) accounts for those losses. Power factor (PF) accounts for the phase difference between voltage and current in AC circuits. Without both, your calculation will underestimate the actual current draw.
Can I use NEC table values instead of the formula?
Yes, for conductor and overcurrent protection sizing. NEC Table 430.248 (single-phase) and 430.250 (three-phase) give conservative FLA values that already account for worst-case PF and η. But for exact operating current (energy audits, VFD setpoints, load monitoring), use the formula with actual nameplate data.
How many amps is 10 HP at 460 V three-phase?
14 A per NEC Table 430.250. Formula-calculated with typical values (η=0.91, PF=0.86): 11.96 A. The NEC value is higher because it represents the maximum FLA across all standard motor designs at that HP and voltage.
Why is the NEC table current higher than the formula result?
NEC tables use a worst-case motor (lowest efficiency, lowest PF) to ensure conductors and breakers are adequate for any standard motor of that HP rating. The formula with your specific motor’s η and PF gives the actual operating current, which is usually lower.
How do I size a breaker for a motor based on HP?
Step 1: Find the NEC table FLA (e.g., 14 A for 10 HP at 460 V 3φ). Step 2: Multiply by 2.5 for inverse-time breakers or 8.0–13.0 for instantaneous-trip per NEC 430.52. Step 3: Round to next standard breaker size. Example: 14 × 2.5 = 35 A → use a 35 A breaker.
What wire size do I need for a motor based on HP?
Take the NEC table FLA, multiply by 1.25 (per NEC 430.22), then select a conductor from NEC Table 310.16 with ampacity at or above that value. Example: 10 HP at 460 V 3φ → 14 A × 1.25 = 17.5 A → 12 AWG copper THHN (rated 30 A). See our AWG to mm² conversion calculator for cross-reference.
Is the HP to Amps conversion the same for single-phase and three-phase?
No. Three-phase includes a √3 (1.7321) divisor that single-phase does not. This means a three-phase motor draws significantly less current per phase than a single-phase motor of the same HP and voltage. For example, a 5 HP motor at 230 V: single-phase = 28 A, three-phase = 15.2 A (NEC values).
How does voltage affect the HP to Amps result?
Higher voltage = lower current for the same HP. Doubling the voltage approximately halves the current. That is why industrial motors run at 460 V or 575 V — it reduces conductor size, voltage drop, and I²R losses. A 100 HP motor draws 248 A at 230 V but only 124 A at 460 V.
What does 746 mean in the HP to Amps formula?
746 is the number of watts in one mechanical horsepower. James Watt defined 1 HP as 550 ft·lbf/s, which equals 745.6999 W — rounded to 746 W for engineering use. This conversion factor is standardized by NEMA and IEEE.
Related Conversions
- Amps to HP Calculator — the inverse of this page
- Amps to kW Calculator — from current to kilowatts
- Amps to Watts Calculator — single-phase and three-phase
- Amperes to VA Calculator — apparent power from current
- Motor Efficiency Calculator — look up IE class and η by HP
- AWG to mm² Conversion Calculator — size your conductors
- Balanced and Unbalanced Load Calculation — handle phase imbalance