Home Projects

Extension Cord Voltage Drop Calculator

Voltage drop depends on current and the full out-and-back conductor length. A long or undersized extension cord can reduce equipment voltage and increase heating, especially during motor startup. This calculator uses an entered conductor resistance and does not select a safe cord: ampacity, temperature, duty, connectors, grounding, environment, and equipment instructions still apply.

Planning estimate only. Check measurements and real-world constraints before buying materials or making a commitment.

Last Updated: July 9, 2026

Calculate your scenario

Change any input. Results update immediately.

Running currentMeasured or nameplate running load.AOne-way cord lengthSource to equipment distance.ftConductor resistanceUse a suitable value for wire material and size.ohms/1,000 ftSupply voltageMeasured or nominal source voltage.VStartup current multiplierMotor or compressor starting scenario.×Planning drop limitComparison value, not a universal code rule.%

Your results

Running voltage drop

3.81 V (3.18%)

Out-and-back conductor path included.

Estimated load voltage

116.19 V

Supply voltage minus modeled running drop.

Startup voltage drop

11.43 V

Running drop exceeds the entered comparison.

How the calculation works

The calculator applies this relationship to the inputs above. Keep every measurement in the unit shown.

voltage drop = current × resistance per length × two-way conductor length

Running current12 A

One-way cord length100 ft

Conductor resistance1.588 ohms/1,000 ft

Supply voltage120 V

Startup current multiplier3 ×

Planning drop limit3 %

Worked example

Use this example to check the calculator by hand before relying on a result.

Find loop resistance

Current travels out and back.

1.588 ÷ 1,000 × 100 × 2 = 0.3176 Ω

Find running drop

This is 3.18% of 120 volts.

12 × 0.3176 = 3.81 V

Stress motor startup

Actual starting behaviour depends on source and motor characteristics.

3.81 × 3 = 11.43 V

Assumptions behind the result

• Single-phase two-conductor path is modeled.
• Resistance value is suitable.
• Current is stable.
• Connections add no extra resistance.
• Cord selection and code compliance are checked separately.

Mistakes that change the answer

• Using one-way length in resistance.
• Confusing voltage drop with ampacity.
• Coiling a loaded cord without considering heat.

Questions about extension cord voltage drop calculator

Does a thicker cord reduce voltage drop?

A larger conductor generally has lower resistance, but safe selection also depends on ampacity, insulation, environment, connectors, and equipment.

Why is startup drop higher?

Motors and compressors can draw several times running current briefly, multiplying the drop in the supply path.

Can adapters solve voltage drop?

No. An adapter does not reduce conductor resistance or create additional circuit capacity.

What to calculate next

Campsite Power Load Budget Calculator Shelf Load Safety Margin Calculator Garage Storage Bin Shelf Capacity Calculator

Calculator methods and editorial structure reviewed July 11, 2026. Results are estimates; verify regulated rates, eligibility rules, and professional decisions with the cited primary source.