Extension Cord Calculator: How to Choose the Right Cord Size

An extension cord calculator helps you evaluate the right cord for a portable load by considering the equipment's current, supply voltage, cord length, conductor gauge and voltage drop. A longer cord has more conductor resistance so the same load can experience more voltage drop as the cord length increases.

This guide explains how to calculate extension cord voltage drop, estimate amp draw from watts, understand AWG sizes, compare common cord gauges and check whether an actual extension cord is suitable for the intended load and environment.

Key Takeaways

Choosing an extension cord requires more than matching a wire gauge to the load. You also need to consider cord length, voltage drop, the cord's marked rating and the conditions in which it will be used.

Longer Cords Increase Voltage Drop

A longer cord has more conductor resistance. At the same current increasing the length therefore increases the calculated voltage drop.

Lower AWG Means a Larger Conductor

In the American Wire Gauge system a lower number represents a larger conductor. For example 12 AWG is larger and has lower resistance than 14 AWG.

Ampacity and Voltage Drop Are Different Checks

The cord must be suitable for the load current while voltage drop calculations determine how much voltage is lost along the cord. Passing one check does not automatically make a cord suitable.

The Actual Cord Rating Still Matters

A calculator is a planning tool. Always check the actual cord's marked gauge, conductor count, rating, construction, intended environment and manufacturer information before use.

Extension Cord Size: What Determines the Required Gauge?

The appropriate extension cord depends on several factors:

Load current and cord length are particularly important when evaluating voltage drop. A load that works acceptably on a short cord can experience substantially more voltage loss on a much longer cord.

However voltage drop should not be the only consideration. The actual extension cord must also be rated and constructed appropriately for the load and environment.

How Does an Extension Cord Calculator Work?

An extension cord size calculator combines the load and cord characteristics to estimate voltage drop and depending on the calculator help identify a suitable conductor size.

The basic process is:

This means an extension cord gauge calculator should not choose an AWG size from cord length alone. The load current is equally important.

• Determine the equipment's running current
• Enter the supply voltage
• Enter the one way cord length
• Select the candidate conductor gauge
• Calculate voltage drop
• Compare the result with the selected design target
• Check the actual cord's listed rating and intended use

How to Calculate Amp Draw for an Extension Cord?

If the equipment nameplate provides a rated current in amps use that value as the starting point. If only power in watts is provided a basic current estimate is:

I = P ÷ V

Where:

For example a 1,500W resistive load operating at 120V has an estimated current of:

1,500W ÷ 120V = 12.5A

So the estimated operating current is 12.5A.

For motors and other non resistive loads, watts divided by volts may not give the actual running current because power factor and efficiency affect the relationship between electrical input power and current. Starting current can also be higher than running current.

When the equipment nameplate provides a rated current use the manufacturer's current rating rather than estimating it from wattage.

• I = current in amps
• P = power in watts
• V = voltage in volts

How to Calculate Voltage Drop From an Extension Cord?

An extension cord voltage drop calculator estimates how much voltage is lost as current flows through the cord conductors. For a simplified single phase calculation using conductor resistance:

Vdrop = I × R(total)

The total resistance of the outgoing and returning conductors can be estimated as:

R(total) = (2 × L × R1000) ÷ 1,000

Where:

The percentage voltage drop is:

Voltage Drop % = (Vdrop ÷ Vsource) × 100

The factor of 2 represents the typical outgoing and returning conductor paths in a single phase two wire circuit. For a real AC cord conductor impedance, temperature, power factor and the specific cord construction can affect the actual result.

The simplified resistance calculation is therefore best treated as a planning estimate. For the underlying calculation method see our voltage drop formula and calculation table.

• Vdrop = voltage drop in volts
• I = load current in amps
• R(total) = total round trip conductor resistance in ohms
• L = one way cord length in feet
• R1000 = conductor resistance in ohms per 1,000 feet

Common Copper Resistance Values

Approximate resistance values for common copper conductor sizes are:

These are approximate resistance values for simplified calculations. Actual cord resistance can vary with conductor construction and temperature.

Extension Cord Voltage Drop Example

Consider a 12A power tool operating from a 120V supply through a 100 foot 14 AWG copper extension cord.

Using approximately 2.525 Ω per 1,000 feet:

R(total) = (2 × 100 × 2.525) ÷ 1,000

R(total) = 0.505 Ω

Now calculate the voltage drop:

Vdrop = 12A × 0.505Ω

Vdrop = 6.06V

The percentage voltage drop is:

6.06V ÷ 120V × 100 = 5.05%

The approximate voltage available at the tool is therefore:

120V − 6.06V = 113.94V

This is above a 3% voltage drop design target.

What Happens With a 12 AWG Cord?

Using approximately 1.588 Ω per 1,000 feet for 12 AWG copper:

Rtotal = (2 × 100 × 1.588) ÷ 1,000

Rtotal = 0.3176 Ω

Then:

Vdrop = 12A × 0.3176Ω

Vdrop ≈ 3.81V

The percentage drop is:

3.81V ÷ 120V × 100 ≈ 3.18%

The 12 AWG conductor therefore reduces voltage drop significantly compared with 14 AWG. If a tighter voltage drop target is required a larger conductor or shorter cord may be appropriate.

Extension Cord Voltage Drop by Gauge and Length

The following table shows the calculated resistive voltage drop for a 12A load on a 120V supply at several cord lengths.

These values use the simplified resistance formula and are intended for comparison rather than as cord ampacity ratings. For example at 100 feet and 12A, the calculated resistive drop is about 4.82V for 16 AWG compared with about 1.20V for 10 AWG. This illustrates why conductor size becomes increasingly important as cord length increases.

Does Extension Cord Length Affect Wire Size?

Yes but cord length does not directly change the conductor's physical gauge or its marked rating. Instead increasing length increases the total resistance of the circuit. For the same current greater resistance produces greater voltage drop.

In a simplified resistance calculation:

This is why a long extension cord may require a larger conductor to achieve the desired voltage drop performance even when the load current stays the same.

• Doubling cord length approximately doubles conductor resistance.
• At the same current doubling resistance approximately doubles voltage drop.
• A larger conductor generally has lower resistance.
• A shorter cord generally produces less voltage drop than a longer cord of the same gauge.

Extension Cord Gauge: What Does AWG Mean?

AWG stands for American Wire Gauge, the conductor sizing system commonly used in the United States. The key rule is:

Lower AWG number = larger conductor

For example 12 AWG is larger than 14 AWG while 10 AWG is larger than 12 AWG.

These descriptions are general only. The actual extension cord's marked rating, construction, length and intended application must determine whether the cord is suitable.

How to Use an Extension Cord Calculator?

Before using an extension cord amp calculator, extension cord length calculator or voltage drop tool gather the information below.

1. Find the Load Current

Check the equipment nameplate for its rated current. If only watts are provided a basic estimate is:

I ≈ P ÷ V

For motors and other non resistive equipment, use the manufacturer's rated current when available.

2. Enter the Supply Voltage

For many US household applications the supply is approximately 120V. Some equipment operates at 240V so use the actual supply voltage rather than assuming every extension cord application is 120V.

3. Enter the Cord Length

Enter the one way physical length from the receptacle to the equipment. The voltage drop formula accounts for both conductors. Therefore, if the calculator already includes the return path, do not enter twice the physical cord length.

4. Select the Cord Gauge

Enter the candidate AWG size such as 16, 14, 12 or 10 AWG. Remember that a lower AWG number represents a larger conductor.

5. Check Voltage Drop

The calculator can estimate:

If the calculated drop exceeds your design target consider a larger conductor or shorter cord.

• Voltage drop in volts
• Voltage drop as a percentage
• Approximate load end voltage

6. Verify the Actual Cord Rating

The final step is to check the actual cord's markings and manufacturer specifications. A theoretical conductor calculation does not override the listed rating of the complete extension cord assembly.

Extension Cord Calculator vs Voltage Drop Calculator

An extension cord calculator is designed around a specific portable cord application. It considers factors such as load current, cord length, conductor gauge and voltage drop.

A general Voltage Drop Calculator can be more appropriate when analyzing fixed wiring, branch circuits, feeders or other electrical conductors. For low voltage applications our 12V Voltage Drop Calculator can be used to evaluate voltage loss in a 12V DC circuit.

For more detail on the underlying calculation see our Voltage Drop Formula & Calculation Table. If you are sizing a general wire or cable run rather than a portable extension cord, our Voltage Drop Calculator for Wire & Cable Size is a better fit.

The distinction is important because an extension cord has additional considerations beyond the electrical resistance calculation including the cord assembly's listing, construction, physical condition and intended environment.

What Happens If an Extension Cord Is Too Small?

An extension cord that is unsuitable for the load or application can create several problems.

Excessive Voltage Drop

The equipment may receive substantially less voltage than expected. Motors and other equipment may experience reduced performance when the voltage at the load becomes too low.

Increased Heating

Electrical power dissipated in conductor resistance can be expressed as:

P = I²R

As current increases resistive heating increases rapidly because current is squared in the equation.

Equipment Performance Problems

Power tools, motors, compressors, pumps and similar equipment may not perform as intended when excessive voltage is lost along the cord.

Cord or Connection Damage

An overloaded, damaged or improperly used cord can become hot. If a cord, plug or receptacle becomes unusually hot, stop using the setup and determine the cause rather than continuing operation.

Extension Cord Safety and US Requirements

Extension cords are intended for portable applications and should not be treated as substitutes for a building's fixed wiring. For construction applications OSHA's 29 CFR 1926.405 includes specific requirements for temporary wiring and extension cord sets.

For example, covered extension cord sets used with portable electric tools and appliances are required to be three wire types designed for hard or extra hard usage. OSHA also restricts flexible cords from being used as substitutes for fixed wiring in covered applications.

The US Consumer Product Safety Commission also identifies safety characteristics for extension cords including appropriate minimum wire size, strain relief, polarization, continuity and suitable jacket characteristics for outdoor cords.

Keep these practical points in mind:

The calculator can estimate electrical performance but it cannot determine whether a particular cord is listed and suitable for every physical environment.

• Check the cord marking: Verify the gauge, number of conductors, cord type and rating.
• Use the correct environment rating: Choose a cord intended for the conditions where it will be used.
• Do not exceed the cord's rating: The connected equipment load must remain within the listed rating of the cord.
• Inspect for damage: Do not use cords with damaged insulation, plugs, connectors or other components.
• Protect the cord from physical damage: Avoid crushing, sharp edges, pinch points and similar hazards.
• Avoid covering a heavily loaded cord: Covering can interfere with heat dissipation.
• Avoid unnecessary cord connections: Using one appropriately sized cord is generally preferable to creating a long chain of cords.
• Do not use an extension cord as permanent wiring: Flexible cords should not replace fixed building wiring.
• Use required GFCI protection: Certain outdoor and other locations have specific GFCI requirements.

Common Extension Cord Sizing Mistakes

Choosing Gauge Without Considering Length

A cord that produces acceptable voltage drop over a short distance may produce significantly more voltage drop when the same load is connected through a longer cord.

Looking Only at Current

Current capacity alone does not tell you how much voltage will be lost along a long cord. Voltage drop should be evaluated separately.

Treating 3% as a Universal Legal Limit

A 3% value is commonly used as a voltage drop design target but it should not be presented as a universal legal limit for every extension cord application.

The appropriate target depends on the application, equipment requirements, applicable electrical requirements and design assumptions.

Assuming Watts Always Give Exact Current

The equation I = P ÷ V is useful for basic estimates particularly with resistive loads. Motors and other non resistive loads can involve power factor, efficiency and starting current considerations.

Ignoring the Actual Cord Rating

A theoretical AWG calculation does not override the rating printed on the actual listed extension cord.

Entering Round Trip Length Instead of One Way Length

If the calculator already accounts for the outgoing and returning conductors enter the physical one way cord length. Entering twice the length would double count the return path.

When Should You Use an Extension Cord Calculator?

An extension cord calculator is useful when evaluating a portable cord for applications such as:

Before starting the calculation have the equipment's rated current, supply voltage, cord length and candidate cord gauge available.

• Power tools
• Pumps
• Outdoor equipment
• Workshop equipment
• Portable generators
• Appliances
• Temporary equipment connections
• Long extension cord runs
• Higher current portable loads

Conclusion

An extension cord calculator helps evaluate the relationship between load current, supply voltage, cord length, conductor gauge and voltage drop. Start with the equipment's actual running current, enter the one way cord length and supply voltage, calculate the expected voltage drop and compare the result with your selected design target.

Most importantly do not treat the calculator result as the only requirement. The actual extension cord's marked rating, construction, condition and intended environment must also be suitable for the application.

For general electrical wiring beyond extension cords use our Voltage Drop Calculator and related wire sizing resources to evaluate the appropriate circuit or conductor.

FAQs

How do you calculate amp draw for an extension cord?

Use the equipment's rated current from the nameplate when available. If only wattage is provided a basic estimate is amps = watts ÷ volts. For motors and other non resistive loads the manufacturer's rated current is preferable.

How do you calculate voltage drop from an extension cord?

For a simplified single phase calculation multiply the load current by the total round trip conductor resistance:

Vdrop = I × R(total)

The resistance can be estimated from the one way cord length and the conductor's resistance per 1,000 feet.

What gauge extension cord do I need for a 100 foot run?

There is no single gauge that is correct for every 100 foot extension cord. The appropriate gauge depends on the load current, supply voltage, desired voltage drop performance and the actual cord's listed rating. Higher current loads generally require a larger conductor for longer runs.

Is a 3% voltage drop required for extension cords?

A 3% value is commonly used as a design target for voltage drop calculations but it should not be treated as a universal legal limit for every extension cord application. Applicable electrical requirements, equipment specifications and the cord's listed characteristics should also be considered.

Can I use an extension cord as permanent wiring?

Extension cords should not be used as substitutes for fixed building wiring. OSHA's construction requirements specifically restrict flexible cords from being used as a substitute for fixed wiring in covered applications.