Motor Wire Size Calculator: How to Size Motor Wiring

A Motor Wire Size Calculator helps determine the appropriate conductor size for a motor circuit by considering horsepower, voltage, phase, full load current, conductor ampacity, installation conditions and voltage drop.

For US applications motor wire sizing should not be based on horsepower alone because the same motor rating can require different conductor sizes at different voltages and phases.

For NEC based design the process starts with the applicable motor full load current value then checks the required conductor ampacity and the actual conductor size under the installation conditions.

Key Takeaways

The most important points to remember when sizing motor wiring are:

Motor horsepower is only the starting point

The required wire size also depends on voltage, phase, motor type, conductor material, installation conditions and circuit length.

Use the appropriate motor FLC basis

For many AC motors NEC Tables 430.248 and 430.250 provide the standardized full load current values used for conductor sizing.

A motor circuit is not sized like an ordinary load

For a single continuous duty motor NEC 430.22 generally requires branch circuit conductor ampacity of at least 125% of the applicable motor FLC subject to the specific exceptions in Article 430.

Voltage drop still matters

A conductor can satisfy an ampacity calculation but still produce excessive voltage drop on a long motor run. Motor circuits should therefore receive a separate voltage drop check when circuit length makes it significant.

What Is a Motor Wire Size Calculator?

A Motor Wire Size Calculator determines a suitable conductor size for a motor circuit based on the motor’s electrical characteristics and applicable installation requirements.

Unlike a basic wire gauge calculator motor wire sizing considers factors such as motor voltage, phase, full load current (FLC), conductor ampacity and installation conditions.

The calculator helps establish the minimum conductor requirements before selecting the appropriate AWG or kcmil size. Because motor current changes with voltage, phase and motor type the correct motor wire size cannot be determined from horsepower alone.

How to Calculate Motor Wire Size?

A practical motor wire sizing process can be broken into several steps.

1. Identify the motor horsepower, voltage and phase

Start with the motor nameplate and identify its horsepower, rated voltage and whether it is single phase or three phase.

A 10 HP motor at 230 V does not have the same current as a 10 HP motor at 460 V so these values must be established before selecting a conductor.

2. Determine the applicable full load current

For many standard AC motors covered by the NEC tables, the applicable FLC comes from the appropriate motor table rather than simply using the nameplate current for branch circuit conductor sizing.

NEC Table 430.248 covers single phase AC motors while Table 430.250 covers three phase AC motors. Special motor types and applications can have different rules so the motor construction and application should be verified before applying a table value.

3. Calculate the minimum conductor ampacity

For a single continuous duty motor the general starting point is:

Minimum conductor ampacity = 125% × motor FLC

For example if the applicable FLC is 52 A:

52 A × 1.25 = 65 A

The conductor must then satisfy the applicable ampacity requirements rather than simply being selected because its nominal AWG rating appears close to 65 A. NEC 430.22 establishes the 125% requirement for a single continuous duty motor with exceptions and special cases in the same section.

4. Select the conductor size

Compare the calculated minimum ampacity with the applicable conductor ampacity table. NEC Table 310.16 provides base ampacity for copper and aluminum conductors under specified conditions and includes separate 60°C, 75°C and 90°C values.

The final conductor size may therefore be affected by:

• Conductor material
• Terminal temperature limitations
• Ambient temperature
• Number of current carrying conductors
• Raceway or cable installation
• Conductor insulation
• Other applicable adjustment or correction factors

5. Check voltage drop

For longer runs, perform a separate voltage drop check after selecting the conductor based on ampacity. For this step use the site's Voltage Drop Calculator for Wire & Cable Size as the next check.

Motor Wire Size Formula

The simplest starting formula for a single continuous duty motor is:

Required conductor ampacity = Motor FLC × 1.25

This does not directly produce an AWG number. Instead it produces the minimum ampacity that the selected conductor must satisfy before other applicable requirements are considered.

Single Phase Motor Wire Size Calculator

A single phase motor wire size calculator uses the same general sizing concept but obtains the motor FLC from the appropriate single phase motor data. NEC Table 430.248 provides full load current values for single phase AC motors at standard rated voltages.

For example the table lists a 1.5 HP single phase motor at:

These are very different current values for the same horsepower rating. That means the answer to what wire size does a 1.5 HP motor need? cannot be determined correctly without knowing the motor voltage and other circuit conditions.

• 115 V: 20 A
• 230 V: 10 A

3-Phase Motor Wire Size Calculator

A 3 phase motor wire size calculator uses the motor's horsepower and voltage to establish the applicable three phase full load current (FLC) before conductor sizing. NEC Table 430.250 provides standardized full load current values for three phase AC motors.

The applicable FLC is then used to determine the minimum conductor ampacity before selecting the appropriate AWG or kcmil conductor. The final conductor size also depends on terminal ratings, installation conditions, correction and adjustment factors and voltage drop.

Motor Wire Size Example: 40 HP, 3-Phase Motor

Consider a 40 HP three phase motor supplied at 460 V. According to the three phase motor FLC table the applicable current is 52 A. For a single continuous-duty motor:

52 A × 125% = 65 A

The calculated minimum conductor ampacity is therefore 65 A. A conductor ampacity table can then be checked to identify an appropriate AWG size under the actual installation conditions.

For example the base 75°C copper ampacity values commonly shown in NEC Table 310.16 include:

Thus 6 AWG copper reaches 65 A at the 75°C column under the table's base conditions, while 4 AWG provides additional ampacity.

The actual installation must still be checked for terminal ratings, correction and adjustment factors, conductor count, ambient temperature, installation method and voltage drop.

This example shows why a motor wire size calculator should use the complete set of motor and installation inputs rather than simply assigning one wire size based on horsepower.

• 8 AWG — 50 A
• 6 AWG — 65 A
• 4 AWG — 85 A

DC Motor Wire Size Calculator

DC motors require a different approach from standard single phase and three phase AC motor calculations. NEC Table 430.247 provides full load current values for direct current motors based on horsepower and armature voltage. For example a 5 HP DC motor is listed at 40 A at 120 V, 27 A at 180 V and 20 A at 240 V.

DC motor circuits can also have special conductor sizing provisions depending on how the motor is supplied, including rectifier fed arrangements. For that reason a DC motor wire size calculator should not simply reuse an AC motor formula without identifying the supply arrangement.

What Size Wire for a 200 HP Motor?

A 200 HP motor does not have one universal wire size. For example the three phase motor table lists a 200 HP motor at 240 A at 460 V and 192 A at 575 V. Using the 125% conductor sizing starting point:

460 V:
240 × 1.25 = 300 A

575 V:
192 × 1.25 = 240 A

The final conductor size must then be selected using the applicable ampacity rules and installation conditions.

A large motor can also introduce additional design considerations such as parallel conductors, termination ratings, voltage drop, raceway requirements and equipment limitations. Those decisions should be verified against the applicable NEC provisions and equipment documentation.

How Do You Size a Motor Cable?

Motor cable sizing follows the same basic electrical principles as motor wire sizing but the cable's construction, insulation, installation method and operating environment must also be considered.

The selected cable must have an allowable ampacity that meets the motor circuit requirements under the actual installation conditions. When sizing a motor cable consider:

Cable ampacity:

The cable must have sufficient allowable ampacity for the motor circuit based on the applicable sizing requirements.

Conductor material:

Copper and aluminum cables have different ampacity characteristics which can affect the required cable size.

Insulation and temperature rating:

The cable's insulation must be suitable for the expected operating temperature and applicable terminal limitations.

Installation conditions:

Ambient temperature, number of current carrying conductors, raceway or cable configuration and other correction or adjustment factors can affect usable ampacity.

Circuit length:

Longer cable runs can produce greater voltage drop and may require a larger conductor than the minimum ampacity calculation indicates.

Cable construction and environment:

The cable should be suitable for the installation environment, equipment connection and applicable listing or code requirements.

For this reason a motor cable size calculator should consider more than the motor's horsepower. The final cable selection should satisfy the applicable ampacity requirements and where necessary be checked for voltage drop and installation conditions.

Motor Wire Size vs Motor Breaker Size

The motor conductor and the motor branch circuit overcurrent device are not sized by simply matching their ampere ratings. The conductor sizing calculation and motor short circuit/ground fault protection have different requirements under NEC Article 430.

A conductor may therefore have an ampacity that looks smaller than the rating of the branch circuit protective device without automatically being incorrectly sized. Motor overload protection is another separate part of the motor protection system.

This is one reason a motor wire size calculator should not be used as a substitute for a complete motor circuit design.

Motor Wire Size and Voltage Drop

Voltage drop becomes increasingly important as motor circuit distance increases. A long motor run has more conductor resistance and therefore greater voltage loss. The resulting voltage at the motor can be lower than the supply voltage. A practical sizing workflow is therefore:

Motor FLC → conductor ampacity → AWG selection → voltage drop check

For voltage drop calculations use the site's Voltage Drop Calculator. For three phase circuits the site's 3-Phase Voltage Drop Calculator can be used when checking the voltage loss on a three phase motor run.

NEC voltage drop informational notes provide design recommendations rather than a universal motor specific mandatory voltage drop limit so the actual design target should be selected based on the equipment, installation and applicable requirements.

Motor Wire Size Chart: Why One Chart Is Not Enough?

A simple Motor Wire Size Chart can be useful as a quick reference but it should not be treated as a universal sizing authority. The same horsepower motor can have substantially different FLC values at different voltages. For example the NEC three phase table gives these values for a 40 HP motor:

The final AWG selection also depends on the conductor's allowable ampacity and installation conditions. Therefore a calculator is generally more useful than a single horsepower to wire size chart.

What Information Does a Motor Wire Size Calculator Need?

A reliable calculator should collect enough information to avoid giving a misleading one size fits all answer. Useful inputs include:

Motor horsepower

HP establishes the motor's rated mechanical output and helps identify the applicable FLC.

Voltage

Motor current can change substantially with voltage so the rated motor voltage is essential.

Phase

Single phase and three phase motors use different FLC data and calculations.

Motor type

Standard AC motors, DC motors, multispeed motors and special motor arrangements can have different sizing provisions.

Conductor material

Copper and aluminum conductors have different ampacity characteristics.

Circuit length

Run length becomes important when checking voltage drop.

Installation conditions

Ambient temperature, conductor count, insulation rating, raceway or cable configuration and termination temperature can affect the usable ampacity.

Conclusion

A Motor Wire Size Calculator should do more than convert motor horsepower into an AWG number. A reliable motor wire sizing process considers the motor's voltage, phase, applicable full load current, required conductor ampacity, conductor characteristics, installation conditions and voltage drop.

For US applications the correct approach is to determine the applicable motor FLC first, apply the relevant conductor sizing requirement, select a conductor that satisfies the applicable ampacity rules and then verify voltage drop and other installation requirements.

When the motor circuit is long or the electrical conditions are unusual use a dedicated voltage drop calculation and verify the complete installation against the current NEC, equipment instructions and local requirements.

FAQs

Which mm wire is required for a 1.5 HP motor?

There is no single answer. A 1.5 HP motor can require different conductor sizes depending on whether it is 115 V or 230 V, single phase or three phase and on the installation conditions.

What size wire is needed for a 40 HP 3-phase motor?

It depends on voltage. A 40 HP three phase motor has an NEC table FLC of 104 A at 230 V and 52 A at 460 V so the conductor calculation is different for each voltage.

What size wire is needed for a 200 HP motor?

There is no universal wire size. For example a 200 HP three phase motor has table FLC values of 240 A at 460 V and 192 A at 575 V producing different conductor sizing requirements.

Can I size motor wire from the nameplate amps?

Not always. For many standard motors covered by NEC Article 430 the applicable table FLC is used for branch circuit conductor sizing while the nameplate information has separate roles in motor protection.

Does motor wire size depend on cable length?

Yes. Cable length affects voltage drop so a longer motor circuit may require a larger conductor even when the minimum ampacity calculation is already satisfied.

Is a motor wire size the same as the breaker size?

No. Motor conductors, overload protection and branch circuit, short circuit/ground fault protection are governed by different requirements.