3 Phase Wire Size Calculator: How to Size 3 Phase Wire

A 3 phase wire size calculator helps determine an appropriate conductor size for a three phase circuit by considering load current, system voltage, conductor material, circuit length, voltage drop and installation conditions.

For US electrical systems, the calculated result should also be checked against applicable NEC requirements rather than choosing an AWG size from voltage alone.

Key Takeaways

Three phase wire size depends on current

The phase configuration does not determine the AWG by itself. The load current and required conductor ampacity are the starting points.

480V does not have one fixed wire size

A 480V three phase circuit could use different conductor sizes depending on current, installation conditions and voltage drop requirements.

Ampacity and voltage drop are separate checks

A conductor can have enough ampacity but still require upsizing when a long circuit run creates excessive voltage drop.

Motor and transformer circuits need special attention

Motor and transformer conductors can be subject to additional requirements so a general three phase sizing method should not replace the applicable NEC rules.

What Is a 3 Phase Wire Size Calculator?

A 3 phase wire size calculator estimates an appropriate conductor size for a three phase electrical circuit. Unlike a basic wire gauge lookup a useful calculator considers the conditions that actually affect conductor selection.

Typical inputs include:

The result is commonly expressed as AWG or kcmil for US electrical systems. The basic sizing workflow is:

Load current → required ampacity → conductor size → voltage drop check → final verification

This approach is more reliable than assuming that every 480V or 208V three phase circuit uses the same wire size.

• System voltage
• Load current
• Three phase configuration
• One way circuit length
• Copper or aluminum conductor
• Allowable voltage drop
• Installation conditions

How to Calculate 3 Phase Wire Size?

The first step is to determine the current that the circuit must carry. For a balanced three phase load where real power is known the approximate current relationship is:

I = P ÷ (√3 × V × PF)

Where:

For example consider a 30 kW balanced three phase load at 480V with a power factor of 0.90:

I = 30,000 ÷ (1.732 × 480 × 0.90)

I ≈ 40.1 A

This gives an estimated operating current. It does not automatically mean that a particular AWG conductor is the final answer. The next step is to check the applicable conductor ampacity under the actual installation conditions.

• I = line current in amperes
• P = real power in watts
• V = line to line voltage
• PF = power factor
• √3 ≈ 1.732

How Do You Choose the Wire Size After Finding Current?

Once the circuit current is known, compare the required ampacity with the applicable conductor ampacity data. For US installations conductor ampacity depends on factors such as:

Southwire's published conductor data for example shows different ampacity values for different conductor configurations and references NEC requirements and adjustment factors. The selected conductor therefore needs to satisfy the actual installation, not just a basic chart value.

• Copper or aluminum construction
• Conductor insulation
• Temperature rating
• Ambient temperature
• Number of current carrying conductors
• Installation method
• Terminal temperature limitations
• Applicable correction and adjustment factors

3 Phase Wire Size Chart

A three phase wire size chart should be treated as a reference rather than a universal answer. The following examples show commonly referenced 75°C copper ampacity values under specified base conditions:

These are reference values not automatic recommendations for every three phase installation. Southwire publishes the same NEC based ampacity data with conditions and adjustment notes, demonstrating why a chart value should not be treated as the final installation size. For more general AWG information see our wire size chart guide.

480V 3 Phase Wire Size Chart: What Changes?

A 480V 3 phase wire size chart is useful for estimating conductor requirements but the 480V value alone does not select the wire. For example a 30 kW load at 480V draws substantially less current than the same real power load at a lower voltage. Using the three phase relationship with a power factor of 0.90:

30 kW at 480V:

I ≈ 40.1 A

The conductor still needs to be selected by its applicable ampacity and installation conditions. This is why searching for a single answer such as what wire size for 480V three phase? can be misleading. The correct answer requires the load current and circuit conditions.

Check Voltage Drop Before Finalizing the Size

After confirming that the conductor can carry the required current check voltage drop when circuit length makes it significant. For a balanced three phase circuit the simplified resistance based relationship is:

VD = √3 × I × R × L

Where:

For a more detailed explanation and worked examples see our 3 Phase Voltage Drop Calculator guide. A long three phase feeder may therefore require a larger conductor even when the initial ampacity check passes.

• VD = voltage drop
• I = current
• R = conductor resistance per unit length
• L = one way conductor length

3 Phase Wire Sizing Example

Suppose a balanced three phase load operates at:

The first check is conductor ampacity. A base 75°C copper reference shows 6 AWG at 65A while 4 AWG is 85A under the applicable table conditions. That does not automatically make 6 AWG the final choice. The installation must still be checked for:

If voltage drop requires a larger conductor the final selection may be larger than the minimum ampacity based size.

• Voltage: 480V
• Current: 60A
• Conductor: copper
• Run length: 250 ft
• System: three phase
• Terminal ratings
• Ambient temperature
• Number of current carrying conductors
• Adjustment and correction factors
• Circuit voltage drop

What Size Wire Is Used for 3 Phase?

There is no single wire size used for all three phase circuits. Common US three phase systems include 208V, 240V, 480V and 600V but the conductor size depends primarily on the current and installation requirements.

For example two circuits can both operate at 480V while requiring completely different conductor sizes because one carries 25A and another carries 150A.

The safest approach is therefore to calculate or establish the design current first and then select a conductor that satisfies the applicable requirements.

3 Phase Motor Wire Size Calculator

A motor circuit should not be sized using only the general three phase power equation. For many standard AC motors the NEC motor tables provide the applicable full load current used for conductor sizing.

The site's Motor Wire Size Calculator already covers this dedicated process including three phase motor FLC, conductor ampacity and voltage drop considerations. For a three phase motor the general workflow is:

Motor information → applicable FLC → required conductor ampacity → AWG selection → voltage drop check

Do not simply take motor horsepower and select an AWG from a generic three phase chart.

3 Phase Transformer Wire Size Calculator

Transformer circuits also require more than a simple three phase wire size lookup. For a three phase transformer rated current can be calculated from apparent power using:

I = kVA × 1000 ÷ (√3 × V)

The primary and secondary currents are different because their winding voltages are different. Conductor selection then requires consideration of the applicable transformer, feeder and conductor requirements. Transformer overcurrent protection and conductor sizing are related but are not the same calculation.

For that reason a general 3 phase wire size calculator should not present one universal transformer wire size without knowing the transformer rating, winding voltage, protection arrangement and installation conditions.

What Size Cable Do I Need for 3 Phase?

The required three phase cable size depends on the circuit's design current and installation conditions. Before selecting the cable confirm:

A calculator can make the initial sizing process faster but the final conductor should be verified against the actual installation.

• System voltage
• Load current
• Conductor material
• Insulation and temperature rating
• Number of current carrying conductors
• Installation method
• Circuit length
• Voltage drop requirement
• Applicable NEC and local requirements

Common 3 Phase Wire Sizing Mistakes

Choosing wire size from voltage alone

480V does not automatically mean one AWG size. Voltage affects current for a given power load but conductor selection still depends on the resulting current and installation conditions.

Using a generic ampacity chart without derating

A chart may show a base ampacity but temperature and conductor grouping can change the usable value.

Ignoring circuit length

A conductor can pass an ampacity check and still have excessive voltage drop over a long run.

Treating motor sizing like general load sizing

Motor circuits have additional requirements so a general calculator should not replace the applicable motor sizing procedure.

Assuming the largest wire is always the best answer

Oversizing can increase material cost and installation difficulty. The goal is to select a conductor that satisfies the applicable electrical requirements rather than simply choosing the largest available size.

Conclusion

A 3 phase wire size calculator is useful for narrowing down the appropriate conductor size but three phase wire sizing should not be based on voltage or a single chart value alone. Start with the design and verify conductor ampacity under the actual installation conditions and then check voltage drop when circuit length makes it relevant.

For US applications AWG, NEC ampacity requirements, conductor temperature ratings, adjustment factors and equipment limitations all need to be considered before the final conductor is selected.

FAQs

How to calculate 3 phase cable size?

Determine the three phase load current, establish the required conductor ampacity, select a suitable conductor based on the installation conditions and then check voltage drop for the actual circuit length.

What size wire is used for 3 phase?

There is no universal three phase wire size. The correct AWG depends on current, conductor material, temperature rating, installation conditions and voltage drop requirements.

What size cable do I need for 3 phase?

You need the system voltage, load current, conductor material, installation conditions and circuit length to determine an appropriate cable size.

What wire size is needed for a 3 phase motor?

Motor wire size depends on the motor's applicable full load current, voltage, phase, conductor requirements and installation conditions. Motor circuits should be sized using the applicable motor requirements rather than a generic wire chart.

Does 480V require a specific wire size?

No. A 480V three phase circuit can use different conductor sizes depending on its load current, installation conditions and voltage drop requirements.

Can I use a 3 phase wire size calculator for a motor?

A general calculator can help with preliminary sizing but motor circuits require the applicable motor FLC and code requirements. A dedicated motor wire sizing process is more appropriate.