Motor Cable Size for 5 HP & 10 HP: Complete Selection Guide with Charts

Selecting the correct cable size for electric motors is critical for safe operation, optimal performance, and compliance with electrical codes. Whether you're installing a 5 HP motor for a workshop compressor or a 10 HP motor for industrial equipment, proper wire sizing ensures your motor receives adequate power while protecting against overheating and electrical hazards.

This practical guide provides the essential information you need to select the right cable size for 5 HP and 10 HP motors, including quick reference charts, voltage considerations, and step-by-step sizing calculations.

Quick Reference: Motor Cable Sizing Charts

5 HP Motor Cable Size Chart

Motor Type	Voltage	Full Load Amps (FLA)	Minimum Wire Size (Copper)	Recommended Breaker Size
Single-Phase	115V	56A	6 AWG	70A
Single-Phase	230V	28A	10 AWG	40A
Three-Phase	230V	15.2A	14 AWG	20A
Three-Phase	460V	7.6A	14 AWG	15A

10 HP Motor Cable Size Chart

Motor Type	Voltage	Full Load Amps (FLA)	Minimum Wire Size (Copper)	Recommended Breaker Size
Single-Phase	115V	100A	2 AWG	125A
Single-Phase	230V	50A	6 AWG	70A
Three-Phase	230V	28A	10 AWG	40A
Three-Phase	460V	14A	14 AWG	20A

Understanding Motor Cable Sizing Basics

Why Proper Motor Cable Sizing Matters

Undersized motor cables create serious problems:

• Voltage drop: Reduces motor torque and efficiency
• Overheating: Damages wire insulation over time
• Motor damage: Insufficient voltage causes overheating and premature failure
• Fire hazard: Overloaded conductors can ignite
• Code violations: Results in failed inspections

Key Factors in Motor Cable Selection

1. Motor Full Load Amperage (FLA) The continuous current the motor draws during normal operation, found on the motor nameplate.

2. NEC 125% Rule According to NEC Article 430.22, motor circuit conductors must have ampacity at least 125% of motor FLA:

$$\text{Required Ampacity} = \text{Motor FLA} \times 1.25$$

3. Voltage Level Higher voltage significantly reduces current requirements:

• 460V three-phase: Lowest current, smallest wire
• 230V three-phase: Moderate current
• 230V single-phase: High current, larger wire needed
• 115V single-phase: Highest current, largest wire required

4. Wire Run Length Longer distances require larger wire to minimize voltage drop:

• Under 100 feet: Use minimum code-required size
• 100-200 feet: Upsize one wire gauge
• Over 200 feet: Calculate voltage drop and upsize accordingly

Step-by-Step Cable Sizing Process

Step 1: Determine Motor Specifications

Locate the motor nameplate and record:

• Horsepower rating (HP)
• Voltage (115V, 230V, 460V)
• Phase (single or three-phase)
• Full Load Amperage (FLA)
• Service Factor (typically 1.0 or 1.15)

Step 2: Calculate Minimum Wire Ampacity

Apply the NEC 125% rule:

Example - 5 HP Three-Phase Motor at 230V:

• Motor FLA: 15.2A (from nameplate)
• Required ampacity: 15.2A × 1.25 = 19A
• Minimum wire size: 14 AWG copper (20A rating)

Example - 10 HP Single-Phase Motor at 230V:

• Motor FLA: 50A (from nameplate)
• Required ampacity: 50A × 1.25 = 62.5A
• Minimum wire size: 6 AWG copper (65A rating)

Step 3: Check Voltage Drop

For runs over 100 feet, calculate voltage drop:

$$\text{VD\%} = \frac{2 \times K \times I \times L}{CM \times V} \times 100$$

Where:

• K = 12.9 (copper) or 21.2 (aluminum)
• I = Current in amperes
• L = One-way length in feet
• CM = Circular mils of conductor
• V = Voltage

Acceptable voltage drop limits:

• Feeder circuits: 2% maximum
• Branch circuits: 3% maximum
• Combined: 5% maximum total

Step 4: Select Final Wire Size

Choose wire size that satisfies both:

1. Minimum ampacity requirement (125% of FLA)
2. Voltage drop requirement (≤3% for branch circuits)

If voltage drop is excessive, upsize the wire even if ampacity is adequate.

Detailed Sizing Examples

Example 1: 5 HP Three-Phase Motor at 460V

Motor Specifications:

• Power: 5 HP
• Voltage: 460V three-phase
• FLA: 7.6A (from nameplate)
• Distance: 150 feet

Step 1 - Calculate Minimum Ampacity:

$$\text{Required Ampacity} = 7.6A \times 1.25 = 9.5A$$

Step 2 - Initial Wire Selection: 14 AWG copper (15A rating) meets ampacity requirement

Step 3 - Check Voltage Drop: For 14 AWG (4,110 CM):

$$\text{VD\%} = \frac{2 \times 12.9 \times 7.6 \times 150}{4110 \times 460} \times 100 = 1.55\%$$

Result: 14 AWG copper is adequate (voltage drop under 3%)

Final Selection:

• Cable size: 14 AWG copper, 3-conductor with ground
• Conduit: ¾" EMT or PVC
• Breaker size: 15A

Example 2: 10 HP Single-Phase Motor at 230V

Motor Specifications:

• Power: 10 HP
• Voltage: 230V single-phase
• FLA: 50A (from nameplate)
• Distance: 75 feet

Step 1 - Calculate Minimum Ampacity:

$$\text{Required Ampacity} = 50A \times 1.25 = 62.5A$$

Step 2 - Initial Wire Selection: 6 AWG copper (65A rating) meets ampacity requirement

Step 3 - Check Voltage Drop: For 6 AWG (26,240 CM):

$$\text{VD\%} = \frac{2 \times 12.9 \times 50 \times 75}{26240 \times 230} \times 100 = 1.61\%$$

Result: 6 AWG copper is adequate (voltage drop under 3%)

Final Selection:

• Cable size: 6 AWG copper, 2-conductor with ground
• Conduit: 1¼" EMT or PVC
• Breaker size: 70A

Voltage Considerations for Motor Applications

Single-Phase vs. Three-Phase Impact

The difference in current draw between single-phase and three-phase motors is significant:

10 HP Motor Comparison:

Configuration	Current Draw	Wire Size Needed	Cost Impact
Single-Phase 230V	50A	6 AWG	High
Three-Phase 230V	28A	10 AWG	Medium
Three-Phase 460V	14A	14 AWG	Low

Key Takeaway: Three-phase motors at higher voltage require significantly smaller (and less expensive) wire.

Benefits of Higher Voltage Operation

Operating motors at higher voltage provides:

• Smaller wire sizes: Reduces material costs
• Lower current: Reduces losses and heat
• Better efficiency: Less voltage drop
• Lower installation cost: Smaller conduit and components

Special Considerations for Motor Circuits

Motor Starting Currents

Motors draw 5-7 times FLA during startup, affecting:

• Breaker selection: Must handle inrush without nuisance tripping
• Wire sizing: Based on FLA, not starting current
• Voltage drop: May be significant during starts on long runs

Soft Starters and VFDs

When using soft starters or variable frequency drives:

• Size wire to drive output rating, not motor FLA
• Follow manufacturer specifications
• Consider harmonic effects on wire heating
• May require oversized neutral for single-phase VFDs

Temperature and Ambient Conditions

High Temperature Locations: Apply temperature correction factors:

Ambient Temperature	Correction Factor
86-95°F (30-35°C)	0.91
96-104°F (36-40°C)	0.82
105-113°F (41-45°C)	0.71

Example: 10 AWG rated 30A at 90°C ambient requires derating:

$$\text{Adjusted Ampacity} = 30A \times 0.91 = 27.3A$$

Installation Best Practices

Cable Type Selection

THHN/THWN-2 (Most Common):

• Temperature rating: 90°C
• Suitable for dry and wet locations
• Cost-effective
• Available in all sizes

XHHW-2:

• Temperature rating: 90°C
• Excellent moisture resistance
• Direct burial rated
• Premium option

MC Cable (Metal-Clad):

• Pre-assembled with ground
• Faster installation
• Higher cost
• Good for exposed runs

Conduit Sizing

Proper conduit size prevents damage and aids heat dissipation:

For Three 14 AWG Conductors + Ground:

• Minimum: ½" EMT or PVC
• Recommended: ¾" for easier pulling

For Three 6 AWG Conductors + Ground:

• Minimum: 1" EMT or PVC
• Recommended: 1¼" for easier pulling

Protection Requirements

Overload Protection:

• Required by NEC Article 430.32
• Typically built into motor starter
• Set at 115-125% of motor FLA

Branch Circuit Protection:

• Inverse time circuit breaker: 250% of FLA
• Instantaneous trip breaker: 800% of FLA
• Must protect conductors and motor

Common Cable Sizing Mistakes to Avoid

Mistake 1: Using Motor Nameplate HP Instead of FLA

Wrong Approach: "It's a 10 HP motor, so I'll use wire for 10 amps."

Correct Approach: Always use the actual FLA from nameplate (50A for 10 HP single-phase at 230V).

Mistake 2: Ignoring Voltage Drop on Long Runs

Problem: 150-foot run with minimum code-size wire causes 8% voltage drop.

Result:

• Motor runs hot
• Reduced torque
• Premature failure

Solution: Calculate voltage drop and upsize wire as needed.

Mistake 3: Not Applying the 125% Rule

Wrong: 50A motor with 50A-rated wire

Correct: 50A motor requires wire rated for 50A × 1.25 = 62.5A minimum

Mistake 4: Confusing Starting Current with FLA

Remember:

• Wire is sized for continuous operation (FLA × 125%)
• Starting current affects breaker selection only
• Motor protection handles overload conditions

Quick Decision Guide

When to Choose Each Wire Size

14 AWG Copper:

• 5 HP three-phase at any voltage
• 10 HP three-phase at 460V
• Short runs only
• Maximum 15A continuous load

10 AWG Copper:

• 5 HP single-phase at 230V
• 10 HP three-phase at 230V
• Runs up to 150 feet
• Maximum 30A continuous load

6 AWG Copper:

• 10 HP single-phase at 230V
• Large three-phase motors
• Long distance runs
• Maximum 65A continuous load

Cost Considerations

Wire Cost Comparison (per 100 feet)

Wire Size	Copper Cost	Aluminum Cost	Relative Savings
14 AWG	$30-40	$20-25	Aluminum 35% less
10 AWG	$60-80	$35-45	Aluminum 40% less
6 AWG	$150-200	$80-100	Aluminum 45% less

Approximate retail cost ranges. Actual prices vary by supplier, metal market, and location.

Aluminum Considerations:

• Must upsize two wire gauges vs. copper
• Requires special terminations (anti-oxidant compound)
• Not recommended below 6 AWG for safety
• Cost-effective for large motors and long runs

Conclusion

Proper cable sizing for 5 HP and 10 HP motors ensures safe, efficient operation and compliance with electrical codes. The key points to remember:

1. Always verify motor FLA from the nameplate, don't assume standard values
2. Apply the 125% rule for conductor ampacity per NEC requirements
3. Calculate voltage drop for runs over 100 feet and upsize as needed
4. Consider voltage level - higher voltage allows smaller, more economical wire
5. Choose three-phase when possible to reduce current and wire size requirements

For 5 HP motors, 14 AWG copper typically suffices for three-phase applications, while 10 AWG is needed for single-phase. For 10 HP motors, sizes range from 14 AWG (three-phase 460V) to 6 AWG (single-phase 230V).

When in doubt, consult with a licensed electrician to verify your calculations and ensure compliance with local electrical codes. Proper motor cable sizing is not just about meeting minimum requirements—it's about ensuring reliable, safe operation for years to come.

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Credits

• Photo by Mika Baumeister on Unsplash

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