Extension Cord Voltage Drop Calculator

Load info:

Prefer a measured or nameplate running current when available; startup surge is not modeled.

Source voltage:

Running current:

Running power:

One-way cord length:

Changing units converts the displayed value and preserves the same physical run.

Cord wire gauge:

Gauge comparisons address voltage drop only; the cord's printed rating and construction remain controlling.

Printed cord rating:

Rating screen:

Voltage drop goal: {{ formatNumber(drop_goal_percent, 1) }}%

Three percent is a strict planning target; choose the threshold that fits the load and applicable guidance.

Conductor temperature:

The 20 °C default leaves published resistance values unchanged.

Use context:

General is neutral; select a context only to tailor the use checklist.

Display precision:

Two decimal places is the neutral presentation default.

Measure	Value	Interpretation	Copy
{{ row.label }}	{{ row.value }}	{{ row.note }}

Gauge	Drop	Load voltage	Cord loss	Max one-way length ({{ cord_length_unit }})	Drop status	Copy
{{ row.gauge }}	{{ row.drop }}	{{ row.loadVoltage }}	{{ row.loss }}	{{ row.maxLength }}	{{ row.status }}

Use and safety guidance

A long extension cord acts as a resistor in series with the appliance. Current flowing through both the outgoing and return conductors loses voltage along the way, so the load receives less than the receptacle supplies. The same resistance turns electrical energy into heat inside the cord.

Length, current, and wire gauge dominate the result. Doubling the one-way distance doubles the round-trip resistance and voltage drop. Doubling current doubles voltage drop but quadruples heat loss. A heavier copper conductor has less resistance, even though American Wire Gauge (AWG) numbers become smaller as the wire gets thicker.

Voltage drop: The volts lost in the cord, often compared with source voltage as a percentage.
Load voltage: Source voltage minus cord drop. Motors, tools, heaters, and electronics can react differently to a low value.
Cord loss: Power dissipated as heat in both current-carrying conductors at the modeled running current.

Voltage-drop and ampere-rating checks answer different questions. A cord may stay within its printed current rating yet deliver too little voltage on a long run. It may also meet a chosen drop goal while being damaged, poorly connected, unsuitable for wet conditions, or underrated for the load. Both checks must pass, and the physical cord still needs inspection.

Running current is not always the hardest condition. Motors and compressors can draw much more current while starting, tools can surge or stall, and watts divided by volts can understate current for equipment with power factor below one. Nameplate data, manufacturer instructions, plug and outlet ratings, environment, and local electrical rules remain part of the decision.

How to Use This Tool:

Use nameplate running current when available, measure the cord one way, and treat the printed cord rating as a separate safety limit.

Choose Running current in amperes for a known nameplate current. Use watts mode only for a simple power-to-current estimate at the entered source voltage.
Enter Source voltage, One-way cord length, and the actual copper Cord wire gauge. Do not double the length yourself; the return conductor is included automatically.
Enter the cord's printed ampere rating and choose the 80% continuous-use planning comparison or 100% printed-rating comparison. Set the voltage-drop goal and conductor-temperature estimate.
Read Cord audit for drop and rating results, then compare Gauge options and Length limits. Shorten the run, reduce the load, or choose a heavier listed cord when either check fails.

Interpreting Results:

A drop result passes when the unrounded percentage is less than or equal to the selected goal. The rating result passes when modeled running current is no greater than the allowed current under the selected rating comparison.

Recommended gauge is the first supported AWG size, from 18 through 6 AWG, that meets the voltage-drop goal. It is not a complete approval of cord ampacity, construction, plug type, or environment.
Max one-way length is the resistance-model boundary for the chosen current, voltage, temperature, and drop goal. It is not permission to exceed the cord's marked rating.
If a tool starts poorly, a connector warms, insulation is damaged, or the cord is wet or pinched, stop using it. A favorable steady-state calculation cannot clear a physical hazard.

Technical Details:

Steady-state drop follows Ohm's law across the complete current path. Copper resistance values are referenced at 20 °C and raised linearly for the selected 40 °C or 60 °C conductor estimate. Metres are converted with 1 m = 3.280839895013123 ft.

Formula Core:

The temperature correction is applied before round-trip resistance, voltage drop, load voltage, heat loss, and length limits are calculated.

\begin{array}{lcl} r_{T} & = & r_{20} [1 + 0.00393 (T - 20)] \\ R & = & \frac{2 L r_{T}}{1000} \\ ΔV & = & I R \\ D & = & \frac{ΔV}{V_{s}} \times 100 \\ P_{loss} & = & I^{2} R \\ L_{\max} & = & \frac{V_{s} (G / 100) 1000}{2 I r_{T}} \end{array}

r₂₀ and r_T are ohms per 1,000 ft at 20 °C and the selected temperature, L is one-way feet, R is round-trip ohms, I is amperes, V_s is source volts, D is drop percent, and G is the drop goal in percent. Load voltage is V_s − ΔV. In watts mode, current is watts divided by source volts.

Lookup and Rule Core:

Copper AWG resistance values at 20 degrees Celsius
Copper gauge	Ω per 1,000 ft at 20 °C
18 AWG	6.385
16 AWG	4.016
14 AWG	2.525
12 AWG	1.588
10 AWG	0.9989
8 AWG	0.6282
6 AWG	0.3951

The continuous-use comparison sets allowed current to 80% of the entered printed rating; the intermittent comparison uses 100%. These are planning comparisons, not universal approval rules. The voltage-drop comparison uses the selected goal from 0.1% to 15% and includes equality. Results are computed at full precision, then displayed with the selected 0 to 4 decimal places.

A 120 V, 12 A load on 50 ft of 14 AWG copper at 20 °C has 100 ft of current path and 0.2525 Ω resistance. Drop is 3.03 V or 2.525%, load voltage is 116.97 V, and cord loss is 36.36 W. The exact 3% drop-goal length is about 59.41 ft.

Safety and Accuracy Notes:

This calculation covers steady-state resistance in copper conductors. It does not inspect the cord, model connector resistance, approve an installation, or replace the equipment manufacturer, cord listing, electrical code, or a qualified electrician.

Inspect plugs, pins, strain relief, jacket, insulation, and grounding before use. Remove damaged cords from service.
Use cords rated for the load and environment, protect connections from moisture and damage, and follow applicable ground-fault protection requirements.
Do not run a heavily loaded cord tightly coiled, covered, pinched, or through a location where it can be crushed.
Motor starting current, nonlinear loads, poor power factor, voltage sag at the outlet, and warm connectors are outside the model.
Inputs and calculations stay in the browser; exported tables contain only the scenario values you save.

Worked Examples:

Warm conductor on the same run

Keeping the 120 V, 12 A, 50 ft, 14 AWG scenario but choosing 60 °C raises modeled copper resistance by 15.72%. Voltage drop rises from 3.03 V to about 3.51 V, so the 3% goal is still narrowly met at about 2.92%. The printed-rating check remains separate and must also pass.

References:

Circular of the Bureau of Standards No. 31, Copper Wire Tables, 2nd edition, National Bureau of Standards.
1910.334 Use of Equipment, U.S. Occupational Safety and Health Administration.
Electrical Safety: Flexible Cords, U.S. Occupational Safety and Health Administration.