HP to Amps Conversion: Formula, Chart + Free Calculator

Converting horsepower to amps is one of the most tested skills on the Texas Journeyman Electrician exam. Here's the quick answer: For single-phase motors, Amps = (HP × 746) ÷ (Voltage × Efficiency × Power Factor). For three-phase motors, Amps = (HP × 746) ÷ (Voltage × √3 × Efficiency × Power Factor). However—and this is critical—the NEC requires you to use full-load current values from Tables 430.248 and 430.250 for sizing conductors and overcurrent protection devices, not your calculated result.

This guide walks you through the formula, the NEC tables you'll see on the exam, and the worked examples that test your understanding. You'll also learn the common traps that trip up apprentices and journeymen alike.

Quick tool: Use our free HP to Amps Calculator to convert any motor horsepower to amps instantly.

The HP to Amps Formula Explained

Understanding the conversion formula is the foundation of motor calculations. The key is knowing that 1 horsepower equals 746 watts.

Single-Phase Motors

Amps = (HP × 746) / (Voltage × Efficiency × Power Factor)

Variables:

• HP = Motor horsepower rating
• 746 = Watts per horsepower (constant)
• Voltage = Supply voltage (120V, 240V, 277V, etc.)
• Efficiency = Motor efficiency, typically 85–92% (use as decimal: 0.85–0.92)
• Power Factor (PF) = Usually 0.85 for single-phase motors (use as decimal: 0.85)

Example: A 2 HP motor at 240V with 88% efficiency and 0.85 PF:

Amps = (2 × 746) / (240 × 0.88 × 0.85) = 1,492 / 179.5 = 8.3 amps

Three-Phase Motors

Amps = (HP × 746) / (Voltage × √3 × Efficiency × Power Factor)

Variables: Same as above, plus:

• √3 (1.732) = Three-phase constant (accounts for the three-wire system)
• Power Factor (PF) = Usually 0.85–0.90 for three-phase motors

Example: A 5 HP motor at 208V with 90% efficiency and 0.85 PF:

Amps = (5 × 746) / (208 × 1.732 × 0.90 × 0.85) = 3,730 / 259.2 = 14.4 amps

Why 746? One horsepower is defined as 746 watts. Electrical power (P) is calculated as P = V × I × PF (single-phase) or P = V × I × √3 × PF (three-phase). Rearrange to solve for current (amps), and you get the formulas above.

NEC Full-Load Current Tables: The Real Answer on the Exam

Here's what catches most electrician exam candidates: Your calculated amps is rarely the answer on the test. The NEC Article 430 requires you to use full-load current (FLC) tables instead.

Why Use Tables Instead of Calculations?

The NEC tables account for:

1. Standard motor efficiency ratings published by manufacturers
2. Real-world power factors based on motor design
3. Safety margins built into the electrical code
4. Consistency across all motor installations

NEC 430.6(A)(1) states: "The full-load current in Tables 430.247, 430.248, 430.249, and 430.250 shall be used to determine the ampacity of conductors, the ampere rating of switches, and the setting of overload relays protecting the motor."

The only exception is NEC 430.6(A)(2): use the nameplate current for sizing overload protection if it's different from the table value.

Key NEC Motor FLC Tables

• Table 430.247 – AC three-phase squirrel-cage induction motors (full voltage)
• Table 430.248 – AC single-phase motors (120V, 240V, 277V)
• Table 430.249 – AC two-phase motors (four-wire)
• Table 430.250 – AC three-phase motors (except squirrel-cage, low slip)

On the Texas Journeyman exam, Tables 430.248 and 430.250 appear most frequently. You'll be given the table or expected to memorize common values.

HP to Amps Reference Chart

Use this chart for quick lookups of full-load current per NEC Tables 430.248 and 430.250. These are the values you'll use on the exam for conductor and OCPD sizing.

| HP | 120V Single-Phase (A) | 240V Single-Phase (A) | 208V Three-Phase (A) | 460V Three-Phase (A) | |----|-----------------------|-----------------------|----------------------|----------------------| | 1/4 | 3.5 | 1.8 | 0.8 | 0.4 | | 1/3 | 4.1 | 2.1 | 1.0 | 0.5 | | 1/2 | 5.8 | 2.9 | 1.4 | 0.7 | | 3/4 | 8.4 | 4.2 | 2.1 | 1.0 | | 1 | 14 | 8 | 4.6 | 2.1 | | 1.5 | 20 | 10 | 6.5 | 3.2 | | 2 | 28 | 14 | 8.6 | 4.2 | | 3 | 42 | 20 | 13 | 6.3 | | 5 | 70 | 34 | 22 | 11 | | 7.5 | — | 47 | 32 | 16 | | 10 | — | 61 | 44 | 21 | | 15 | — | 83 | 62 | 30 | | 20 | — | 110 | 83 | 41 | | 25 | — | 133 | 102 | 50 |

Note: The "—" indicates that motors of that size are not available at the specified voltage per the NEC.

Worked Examples: From Horsepower to Conductor Sizing

Let's work through three realistic exam questions that test your understanding of motors, NEC tables, and conductor sizing.

Example 1: Single-Phase Motor—Find FLC and Size the Conductor

Question: A 240V single-phase 3 HP motor requires a branch circuit. Find the full-load current and determine the minimum wire size for the motor circuit conductors. (Assume copper wire, 75°C insulation in a raceway, and standard ambient conditions.)

Solution:

1. Find FLC from NEC Table 430.248 (240V single-phase): 3 HP = 20 amps

2. Size conductors at 125% of FLC per NEC 430.22(A):

   • 20A × 1.25 = 25 amps minimum rating required

3. Select wire from NEC Table 310.16 (75°C column):

   • 10 AWG copper carries 30A at 75°C ✓
   • Answer: 10 AWG copper

Example 2: Three-Phase Motor—Calculate Using the Formula, Then Verify with Table

Question: A 10 HP motor operates at 208V three-phase with 90% efficiency and 0.85 power factor. Calculate the full-load current, then verify it against the NEC table.

Solution:

1. Use the three-phase formula:

   Amps = (10 × 746) / (208 × 1.732 × 0.90 × 0.85)
   Amps = 7,460 / 259.2
   Amps = 28.8 amps (calculated)
   

2. Verify against NEC Table 430.250 (208V three-phase, 10 HP): 44 amps

3. Use the NEC table value (44 amps) for all sizing decisions, not your calculated 28.8 amps. The table accounts for a more conservative rating.

Why the difference? The NEC table includes a safety margin and assumes standard efficiency/PF values. Your calculated result is lower because you're using ideal conditions.

Example 3: Full Exam Question—Conductor, OCPD, and Overload Protection

Question: You're installing a 5 HP motor at 240V single-phase. You must size:

1. The branch circuit conductors (copper, 75°C, in conduit)
2. The branch circuit overcurrent protection device (OCPD)
3. The motor overload protection device

Assume the motor nameplate shows 28 amps at full load. (This is given on some exams.)

Solution:

1. Find FLC from NEC Table 430.248 (240V single-phase, 5 HP): 34 amps

   • Use 34 amps (not the nameplate 28 amps) for conductor and OCPD sizing per NEC 430.6(A)(1)

2. Size branch circuit conductors at 125% of FLC per NEC 430.22(A):

   • 34A × 1.25 = 42.5 amps minimum
   • From NEC Table 310.16 (75°C): 6 AWG copper (55A) ✓

3. Size OCPD at 250% of FLC per NEC 430.52(C)(1)(a):

   • 34A × 2.5 = 85 amps
   • Standard OCPD: 90-amp breaker (next standard size above 85A) ✓

4. Size overload protection using motor nameplate (28 amps) per NEC 430.6(A)(2):

   • Class A overload relay rated for 28–32 amps ✓

Key exam point: Conductors and OCPD use Table 430.248 (34A), but overload uses nameplate current (28A). This distinction appears on every exam.

Common Exam Traps to Avoid

Trap 1: Using Calculated Amps Instead of NEC Table Values

• Wrong: Calculate HP to amps, then size the conductor using that value.
• Right: Always use the NEC Table 430.248 or 430.250 value for sizing conductors and OCPDs.

Trap 2: Forgetting the √3 Factor

• Wrong: Using the single-phase formula for a three-phase motor (omitting √3).
• Right: Always multiply by √3 (1.732) for three-phase motors.

Trap 3: Confusing Table Values with Nameplate Amps

• Wrong: Using nameplate amps (from motor sticker) to size the branch circuit conductor.
• Right: Use nameplate amps only for overload protection sizing; use NEC table amps for conductors and OCPD.

Trap 4: Forgetting the 125% Multiplier for Conductors

• Wrong: Selecting a conductor rated exactly at the FLC (e.g., selecting a 34A wire for a 34A motor).
• Right: Multiply FLC by 1.25 per NEC 430.22(A): 34A × 1.25 = 42.5A minimum.

Trap 5: Not Rounding to Standard OCPD Ratings

• Wrong: Installing a 42.5-amp breaker (doesn't exist).
• Right: Round up to the next standard breaker size (50A) per NEC 430.52(C)(1)(a) and 240.6(A).

Key Takeaways for the Texas Journeyman Exam

1. Know the formula for both single-phase and three-phase, but don't rely on it.
2. Use NEC Tables 430.248 and 430.250 for conductor and OCPD sizing.
3. Multiply FLC by 1.25 for conductor ampacity per NEC 430.22(A).
4. Multiply FLC by 2.5 for OCPD sizing per NEC 430.52(C)(1)(a) (or use Table 430.52(C)).
5. Use nameplate amps for overload protection per NEC 430.6(A)(2).
6. Always round up to the next standard size for breakers and fuses.

Related Study Guides

Deepen your motor calculation skills with these complementary guides:

• Motor Calculations & NEC Exam Questions: FLC, Conductor, and OCPD Sizing
• Conductor Sizing: NEC Table 310.16 Explained + Practice Problems
• Texas Electrician Exam Calculations: Complete Breakdown of Every Formula

The HP to amps conversion is a foundation skill. Master the formula, memorize the key NEC table values, and practice the three-step process (FLC → conductors → OCPD). You'll see these questions on your exam. Use this guide as your reference, take the practice quizzes on the Electrician Prep platform, and you'll be ready.