Feeder Size Calculator: How to Calculate Feeder Conductor Size

A feeder size calculator helps determine the conductor size needed to supply power from service equipment or one distribution point to a downstream panel, transformer, motor group or other loads. The process starts with the calculated feeder load and required ampacity then checks voltage drop when the feeder run is long enough for voltage loss to matter.

For US electrical installations, feeder sizing can involve NEC requirements, continuous and noncontinuous loads, conductor temperature ratings, correction and adjustment factors and special rules for dwelling and motor feeders. This guide explains how to calculate feeder size step by step and how to check the conductor before treating the calculator result as final.

Key Takeaways

Feeder sizing involves more than choosing a wire based on the panel rating. The calculated load, conductor ampacity, installation conditions and voltage drop all need to be considered.

Feeder Ampacity Sets the Minimum

The conductor must have sufficient allowable ampacity for the calculated feeder load including the required treatment of continuous loads.

Voltage Drop Is a Separate Check

A conductor can meet its ampacity requirement and still have too much voltage drop on a long feeder run. The actual load current and feeder length should be checked separately.

Special Applications Have Different Rules

Qualifying dwelling feeders, motor feeders, transformers and parallel conductors can require additional NEC requirements beyond a basic feeder calculation.

What Is a Feeder?

A feeder is a set of conductors that carries power from service equipment or another power source to downstream distribution equipment. A common example is a feeder running from a main panel to a subpanel.

Feeders can supply:

A feeder is different from a branch circuit because a branch circuit normally supplies the final loads while a feeder supplies distribution equipment or multiple downstream circuits.

For a subpanel installation the feeder conductor is not automatically sized to match the panel's maximum rating. The required conductor size depends on the calculated load and the applicable electrical requirements.

• Subpanels
• Distribution panels
• Transformers
• Motor groups
• Detached buildings
• Groups of branch circuits

How to Calculate Feeder Size?

To calculate feeder size first determine the load the feeder must supply and calculate the minimum required ampacity. Then select a conductor that meets that ampacity under the actual installation conditions.

For a typical feeder:

Required Ampacity = Noncontinuous Load + (Continuous Load × 1.25)

A continuous load is generally one expected to operate at its maximum current for three hours or more.

Step 1: Calculate the Required Feeder Ampacity

Separate the feeder load into continuous and noncontinuous portions then apply the 125% factor to the qualifying continuous load.

For example if a feeder supplies:

The required ampacity is:

80A + (40A × 1.25) = 130A

The feeder conductors therefore need an allowable ampacity of at least 130A subject to applicable NEC requirements and installation conditions.

• 80A of noncontinuous load
• 40A of continuous load

Step 2: Select the Feeder Conductor

After calculating the required ampacity select a conductor whose allowable ampacity meets or exceeds the calculated requirement.

The allowable ampacity can vary with:

US installations commonly use AWG for smaller conductors and kcmil for larger conductors. A feeder sizing calculator can help with the calculation but the selected conductor should still be verified against the applicable NEC requirements and actual installation conditions.

• Copper or aluminum conductor
• Conductor temperature rating
• Termination limitations
• Ambient temperature
• Number of current carrying conductors
• Correction and adjustment factors
• Installation method

Feeder Size Example

Consider a 120/240V single phase feeder supplying a downstream panel.

Assume:

First calculate the required conductor ampacity:

80A + (40A × 1.25) = 130A

The conductor must then have an allowable ampacity that meets or exceeds 130A under the applicable termination and installation conditions.

For the voltage drop calculation however you should use the actual load current not the 130A required ampacity value. In this example the actual load current is:

80A + 40A = 120A

If a simplified resistance based calculation is used with 1 AWG copper and a circular mil area of approximately 83,690 CM:

VD = (2 × 12.9 × 120 × 40) ÷ 83,690

VD ≈ 1.48V

At 240V:

Voltage Drop % = (1.48 ÷ 240) × 100 ≈ 0.62%

This example shows why feeder ampacity and voltage drop should not be treated as the same calculation. The 130A value establishes an ampacity requirement while the actual 120A load is used to evaluate the voltage loss.

The exact conductor selection still depends on the applicable NEC ampacity table, terminal ratings, conductor type, installation conditions and any correction or adjustment factors.

• 80A noncontinuous load
• 40A continuous load
• 40 ft one way feeder length
• Copper conductors
• No correction or adjustment factors for this simplified example

Checking Voltage Drop When Sizing a Feeder

Ampacity is only one part of feeder conductor selection. A long feeder can have significant voltage drop even when the conductor has enough ampacity for the load. For a simplified single phase or DC calculation using circular mil conductor area:

VD = (2 × K × I × L) ÷ CM

Where:

For a balanced three phase circuit a commonly used simplified form is:

VD = (√3 × K × I × L) ÷ CM

These formulas are simplified resistance based calculations. More complete AC calculations can also consider conductor impedance, power factor, operating temperature and installation conditions.

• VD = voltage drop in volts
• K = conductor resistivity constant
• I = actual load current in amps
• L = one way length in feet
• CM = conductor circular mil area

Why Feeder Voltage Drop Matters

A feeder can pass its ampacity check while still delivering lower voltage to the downstream equipment. If voltage drop is higher than the design target possible solutions include:

For a detailed wire and cable voltage drop calculation use our Voltage Drop Calculator for Wire & Cable Size rather than duplicating the full voltage drop sizing process on this page. A feeder voltage drop calculator is therefore best viewed as a separate check from the feeder ampacity calculation.

• Increasing conductor size
• Reducing the feeder length where practical
• Reducing the load where appropriate
• Reviewing the system voltage or distribution arrangement

Feeder Loss and Voltage Drop

Voltage drop is also related to electrical power loss in the feeder conductors. For a simplified resistive circuit:

Power Loss = I² × R

This means conductor resistance and load current both affect feeder losses. A feeder loss calculator can estimate these losses but power loss by itself does not determine the required conductor size. Ampacity, voltage drop, installation conditions and applicable code requirements still need to be considered.

The Dwelling Feeder 83% Rule

Certain qualifying dwelling services and feeders can use a special conductor sizing provision under NEC 310.12. For qualifying feeder installations rated from 100A through 400A the conductors supplying the entire load of a qualifying dwelling unit may be permitted to have an ampacity of at least 83% of the feeder rating when the requirements of the section are satisfied.

For example:

200A × 0.83 = 166A

This is a conductor ampacity calculation not a blanket rule that every 200A feeder can use a conductor rated for only 166A. The provision has specific conditions relating to the dwelling type, feeder arrangement, rating, system and conductor requirements.

The 2026 NEC also clarified the use of Table 310.12 for qualifying conductors or cables rated 75°C or greater. Commercial, industrial and other nonqualifying feeders should not automatically use the 83% provision.

How to Size a Motor Feeder?

Feeders supplying multiple motors require a different calculation from a basic feeder supplying ordinary loads. Under NEC 430.24 the feeder conductor calculation generally includes:

125% of the highest rated motor FLC + 100% of the other motor FLCs

Other nonmotor loads on the same feeder can also affect the calculation. Continuous and noncontinuous nonmotor loads have their own treatment. For a simplified example with two applicable motor full load currents of 35A and 20A:

(35A × 1.25) + 20A = 63.75A

The conductor must then be selected using the applicable ampacity requirements. The important point is that the 125% factor should not simply be applied to every motor.

Motor feeder calculations have their own rules and the applicable motor full load current should be determined according to the NEC requirements for the installation. A motor feeder calculator can be useful for organizing these inputs but it should not replace the applicable motor and feeder requirements.

Feeder Size for Transformers

A transformer feeder requires additional consideration because the primary and secondary sides have different voltage and current relationships. The conductor selection can depend on:

For the same power level a lower voltage produces a higher current. This means the primary and secondary conductors of a transformer cannot simply be assumed to use the same conductor size.

A transformer feeder calculator can help organize transformer voltage, current and conductor inputs but the final design must also be checked against the applicable transformer and overcurrent protection requirements.

• Transformer rating
• Primary voltage
• Secondary voltage
• Calculated current
• Conductor material
• Feeder length
• Overcurrent protection
• Transformer specific requirements
• Applicable NEC provisions

Parallel Feeder Conductors

Large electrical loads may require multiple conductors connected in parallel instead of one very large conductor. A parallel feeder calculator can help estimate the current carried by each parallel conductor and compare the combined capacity with the feeder requirement.

However parallel conductors must satisfy the applicable NEC requirements for conductor size, installation, arrangement, termination and other conditions.

Do not assume that simply adding the ampacity of two conductors makes them acceptable as a parallel feeder. The conductors and installation must meet the applicable requirements as a complete system.

Common Feeder Sizing Mistakes

Using the Panel Rating as the Feeder Load

A 200A panel does not automatically mean the feeder carries a 200A calculated load. The feeder should be sized from the calculated load and applicable requirements.

Applying 125% to Every Load

For a basic feeder calculation the 125% factor applies to qualifying continuous loads. It should not simply be multiplied by every load. Motor feeders also have separate rules for applying the 125% factor.

Using Required Ampacity as Voltage Drop Current

The required ampacity and actual operating load are not always the same number. For voltage drop calculations use the actual load current represented by the circuit conditions rather than automatically using the conductor's required ampacity.

Ignoring Installation Conditions

A conductor's published ampacity does not automatically apply to every installation. Temperature, conductor grouping, terminations, conductor type, correction factors and adjustment factors can affect the allowable ampacity.

Checking Ampacity but Ignoring Voltage Drop

A feeder can meet its ampacity requirement and still have excessive voltage drop on a long run. This is particularly important for feeders supplying distant buildings, remote panels or large loads.

Applying the 83% Rule Everywhere

The dwelling feeder provision has specific conditions. It should not be treated as a general reduction for commercial or industrial feeders.

Confusing Feeder Sizing With MCA

Minimum Circuit Ampacity is commonly associated with individual equipment such as HVAC equipment while feeder sizing concerns conductors supplying distribution equipment or groups of loads. These are related conductor sizing concepts but they are not interchangeable calculations.

How to Use a Feeder Size Calculator Correctly

A feeder size calculator is most useful when you have the basic electrical information needed to describe the feeder. Work through the calculation in this order.

1. Determine the Calculated Load

Identify the loads supplied by the feeder and determine the applicable demand load. Separate continuous and noncontinuous loads where required.

2. Calculate Required Ampacity

Apply the appropriate feeder sizing rules to determine the minimum required conductor ampacity.

For a basic feeder calculation:

Required Ampacity = Noncontinuous Load + 125% of Continuous Load

Other applications such as motor feeders can require additional calculations.

3. Select the Conductor

Choose a conductor whose allowable ampacity meets the calculated requirement under the actual installation conditions. Check conductor material, temperature ratings, terminations, correction factors, adjustment factors and applicable ampacity tables.

4. Check Voltage Drop

For longer feeder runs calculate voltage drop using the actual load current, conductor characteristics, circuit configuration and one way length. If the voltage drop is higher than the design target a larger conductor may be appropriate.

5. Verify the Complete Installation

Finally check the complete design against the current NEC edition, equipment requirements and local code adoption. A calculator provides a calculation result. It does not automatically approve the installation or replace the applicable electrical requirements.

Conclusion

A feeder size calculator helps organize the process of selecting conductors for feeders supplying subpanels, distribution panels, transformers, motor groups and other loads.

The first step is to determine the calculated feeder load and required conductor ampacity. The conductor is then selected based on its allowable ampacity under the actual installation conditions.

For longer runs voltage drop should also be checked using the actual load current and feeder length. Special applications such as qualifying dwelling feeders, motor feeders, transformers and parallel conductors may require additional NEC rules.

The best approach is to use a feeder calculator as a calculation aid to verify the inputs and installation conditions and confirm the final conductor selection against the current NEC and applicable local requirements.

FAQs

What size wire do I need for a 200 amp feeder?

There is no single conductor size that applies to every 200A feeder. The answer depends on the calculated load, conductor material, temperature and termination requirements, installation conditions and whether a special provision such as NEC 310.12 applies.

Does feeder length affect conductor size?

Yes. A longer feeder generally produces greater voltage drop. A conductor that meets the required ampacity may need to be increased in size when voltage drop becomes too high for the intended design.

Is a feeder calculator the same as a voltage drop calculator?

No. A feeder calculator can evaluate load and conductor ampacity and may also include voltage drop. A voltage drop calculator focuses specifically on the voltage lost along the conductor run. The two calculations can be used together.

Can a motor feeder use the normal feeder formula?

Not always. Feeders supplying multiple motors have specific motor conductor requirements including the 125% factor applied to the highest rated motor and the applicable currents of the other motors. Other loads may also need to be included.

Can parallel conductors be used to increase feeder capacity?

Yes when the installation meets the applicable NEC requirements for parallel conductors. Conductor size, arrangement, terminations, installation method and ampacity all need to be considered.

What information should I enter into a feeder size calculator?

Typical inputs include the calculated load, supply voltage, circuit type, feeder length, conductor material, continuous load information and conductor installation conditions. The required inputs can vary depending on whether the feeder supplies ordinary loads, motors, transformers or another special application.