Extension Cord Voltage Drop Calculator

Load info:

Choose the nameplate value you have for the tool or appliance.

Source voltage:

Common examples are 120 V, 230 V, and 240 V.

Load current:

Enter the normal running load in amperes.

Load power:

Enter the running power in watts for a current estimate.

Cord length:

Use one-way cord length, not the round-trip wire path.

Cord wire gauge:

Select the AWG printed on the cord jacket or packaging.

Printed cord rating:

Use the cord's actual amp rating for the overload check.

Rating basis:

Choose how conservative the printed-rating check should be.

Voltage drop goal:

Set the maximum percent of source voltage to lose in the cord.

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Cord copper temperature:

Use 20 C for published copper resistance, or a warmer estimate for loaded cords.

Load profile:

Use the closest load type so the checklist reflects the application risk.

Decimal places:

Use 0-4 places for displayed numbers and exports.

places

Metric	Value	Note	Copy
{{ row.metric }}	{{ row.value }}	{{ row.note }}

Gauge	Drop	Load voltage	Heat	Max length	Status	Copy
{{ row.gauge }}	{{ row.drop }}	{{ row.loadVoltage }}	{{ row.heat }}	{{ row.maxLength }}	{{ row.status }}

Check	Status	Action	Copy
{{ row.check }}	{{ row.status }}	{{ row.action }}

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An extension cord is part of the electrical circuit, not just a convenient way to move an outlet closer to the work. Current has to travel from the source to the load and back again through copper conductors. Every foot of that path has resistance, so some of the source voltage is lost in the cord before it reaches the tool, heater, pump, charger, or appliance.

Voltage drop becomes more noticeable when a load draws higher current, the cord is long, or the wire is thin. A saw that runs acceptably on a short AWG 12 cord may start poorly on a long AWG 16 cord because the motor sees less voltage while the cord converts more power into heat. The same idea applies to heaters, compressors, shop equipment, outdoor tools, battery chargers, and electronics with narrow input tolerances.

Extension cord setup showing source, one-way cord length, outgoing and return conductors, and load

American Wire Gauge, or AWG, is often the part that trips people up. Lower numbers mean thicker copper, so AWG 12 has less resistance than AWG 16. Cord length has the same kind of effect in the other direction: doubling the physical run roughly doubles the conductor resistance because the current still needs an outgoing and return path.

Practical factors that change extension cord voltage drop
Factor	What usually happens	Common mistake
Higher load current	More volts are lost and more heat is produced in the cord.	Using a watt label without checking whether the running amps are known.
Longer cord	The round-trip conductor resistance increases with distance.	Entering twice the physical length when only one-way length is needed.
Smaller conductor	A higher AWG number drops more voltage at the same load.	Assuming all cords with the same plug shape have the same capacity.
Warmer copper	Resistance rises as the conductor warms under load.	Leaving a loaded reel coiled or covered and trusting a cool-table estimate.

Voltage drop is a performance estimate, not a full safety approval. The cord's printed amp rating, jacket type, plug condition, outlet and breaker rating, equipment instructions, moisture exposure, ground-fault protection, and local electrical rules still matter. A cord can meet a voltage-drop target and still be the wrong cord for the setting.

Planning targets such as 3% and 5% help turn a messy setup into a clear comparison. They are useful for choosing a shorter run, a heavier gauge, or a lower load, but they do not replace inspection or code judgment. Treat the result as a way to spot sag and heat risk before plugging in, especially for motors, continuous heaters, and equipment that starts hard.

How to Use This Tool:

Start with the load information printed on the equipment or measured during normal operation. Running current gives the cleanest estimate; watts are useful when current is not listed.

Choose Load info. Use Current in amps when the label or meter gives amperes. Use Power in watts when watts are the available value and the calculator should estimate current from watts divided by source voltage.
Enter the Source voltage before the extension cord. Typical examples include 120 V, 230 V, and 240 V, but use the actual supply when you know it.
Enter the physical Cord length from the source to the load. Do not double this number, because the voltage-drop calculation already counts the outgoing and return conductors.
Select the cord's printed Cord wire gauge, then enter the Printed cord rating from the jacket, plug, tag, or packaging. The rating check uses the cord marking you provide rather than approving a cord from AWG alone.
Set Rating basis. Use 80% continuous planning for loads that may run for a long time, such as heaters, pumps, and chargers. Use 100% intermittent check only for short-duty use where the cord marking and equipment instructions support it.
Choose a Voltage drop goal. A 3% goal is a stricter planning target; a 5% check is often used as a looser upper guide for temporary loads that are not especially sensitive.
Open Advanced when you need to adjust Cord copper temperature, choose a Load profile for checklist language, or change displayed decimal places. If a validation warning appears, correct the input before relying on the result tabs.

Read Cord Snapshot first, then use Gauge Options and Length Limit Map when the selected cord is near the voltage-drop goal or printed-rating limit.

Interpreting Results:

Voltage at load is the estimated steady-state voltage after the cord drop is subtracted. Voltage drop shows the lost volts and the percent of source voltage, while Heat in cord estimates the conductor loss at the modeled running current.

Within goal means the selected AWG and length are at or below the chosen voltage-drop percentage.
Printed rating use compares the running current with the selected 80% or 100% rating basis.
Voltage-drop guide points to the first listed AWG that meets both the drop goal and the built-in reference-current check.
Gauge Options compares AWG choices at the same load, voltage, length, temperature, and goal.
Length Limit Map shows how far each listed AWG can run one way before crossing the selected voltage-drop goal.

A voltage pass and a rating pass answer different questions. The voltage pass says the modeled sag is inside the chosen planning target. The rating pass says the entered load current fits the selected fraction of the printed cord rating. Neither result says the cord is undamaged, suitable for wet locations, safe while coiled, or allowed by the equipment instructions.

When the result is marginal, the usual fixes are physical rather than mathematical: shorten the cord, reduce the load, use a heavier lower-AWG cord with the right printed rating, or place the equipment closer to a suitable outlet. Motors and compressors deserve extra margin because starting current can be much higher than running current.

Technical Details:

Extension-cord voltage drop is a round-trip copper-resistance problem. The entered length is the physical one-way distance from source to load, but current flows through two conductors in a complete circuit. The resistance used for the drop estimate is therefore based on twice the physical length.

The model is steady-state and single-phase. It assumes copper conductors, uses listed AWG resistance values in ohms per 1000 ft, and adjusts resistance for the selected conductor temperature. It does not model motor inrush, connector contact resistance, damaged strands, harmonics, power factor, breaker trip curves, insulation temperature limits, or reel heat buildup.

Formula Core:

The calculation first adjusts conductor resistance for temperature, then applies Ohm's law across the outgoing and return path.

\begin{array}{lll} r_{temp} & = & r_{20C} \times (1 + 0.00393 \times (T - 20)) \\ R_{round} & = & \frac{2 \times L \times r_{temp}}{1000} \\ V_{drop} & = & I \times R_{round} \\ V_{load} & = & V_{source} - V_{drop} \\ P_{heat} & = & I \times V_{drop} \end{array}

Extension cord voltage drop variable map
Symbol	Meaning	Where it appears
I	Running load current in amps, either entered directly or estimated from watts divided by source voltage.	Load current
L	One-way cord length in feet after unit conversion.	Cord length
r_temp	Copper conductor resistance after the selected temperature adjustment.	Selected AWG and cord temperature
R_round	Resistance of the outgoing plus return conductors.	Round-trip resistance
V_drop	Voltage lost in the cord conductors.	Voltage drop
P_heat	Electrical power converted to heat in the cord conductors.	Heat in cord

The listed AWG choices use the following copper resistance values at the 20 C reference point. The reference-current values support comparison inside the gauge table, but the cord's printed amp rating remains the value to use for the user-facing rating check.

AWG resistance and reference-current values used for extension cord comparison
AWG	Ohms per 1000 ft at 20 C	Reference current
18	6.385	10 A
16	4.016	13 A
14	2.525	18 A
12	1.588	25 A
10	0.9989	30 A
8	0.6282	40 A
6	0.3951	55 A

A 12 A load on a 50 ft one-way AWG 14 copper cord at 20 C uses about 2.525 ohms per 1000 ft. The round-trip resistance is about 0.2525 ohm, so the drop is about 3.03 V. From a 120 V source, the load-end voltage is about 116.97 V, the drop is about 2.52%, and conductor heat is about 36.36 W.

Extension cord boundary checks
Check	Boundary	Meaning
Voltage drop goal	Passes when the calculated drop percent is at or below the selected goal.	Compares modeled performance against the chosen planning limit.
Printed cord rating	Passes when load current is at or below printed rating multiplied by 0.8 or 1.0.	Applies the selected continuous or intermittent rating basis.
Maximum one-way length	Solves the voltage-drop equation for length at the selected goal.	Shows where each AWG crosses the drop limit for the same load.
Reference current	Compares load current with the listed AWG reference current.	Supports gauge comparison only; it does not replace the cord marking.

When power is entered in watts, current is estimated as watts divided by source voltage. That is useful for a first pass, but equipment labels and clamp-meter measurements can be better for real loads because power supplies, motors, and heaters may not behave like a simple fixed-resistance load in every condition.

Limitations and Privacy:

Use the result as a planning estimate for copper extension cords. It is not an electrical-code approval, a thermal safety model, or a substitute for the cord and equipment instructions.

Do not use the result to justify a damaged, modified, ungrounded, wet, buried, daisy-chained, undersized, or tightly coiled cord.
Motor and compressor startup current can be much higher than running current, so a running-current pass may still lead to hard starts or nuisance trips.
Connector condition, cord-reel temperature, ambient heat, jacket type, outdoor rating, ground-fault protection, and local rules must be checked separately.
The calculation itself runs in the browser. Exported tables, chart images, JSON, or shared values should be treated like any other job note if they contain site or equipment details.

For any permanent wiring, high-current load, repeated overheating, tripped breaker, damaged insulation, wet location, or uncertain cord marking, use a qualified electrical professional or the equipment manufacturer's instructions rather than relying on a voltage-drop estimate alone.

Advanced Tips:

Use amps mode when a nameplate or clamp meter gives running current. Watts mode divides power by source voltage, which is useful for a first pass but can misread motors and power supplies.
Keep the 80% continuous rating basis for heaters, pumps, chargers, and other loads that may run for long periods. Short-duty use is the only reasonable place to test the 100% intermittent basis.
Compare Gauge Options before changing only the voltage-drop goal. A heavier lower-AWG cord often improves both load-end voltage and conductor heat, but it still needs the right printed amp rating.
Use Length Limit Map for route planning. If the selected run is just under the limit, shorten the path or choose the next heavier cord so small source-voltage changes do not erase the margin.
Raise Cord copper temperature only to make the estimate more conservative. A warmer setting does not make a coiled, covered, damaged, wet, or underrated cord acceptable.

Worked Examples:

Shop tool on a 50 ft cord

A 12 A saw on a 120 V source with a 50 ft AWG 14 cord and a 3% drop goal should land near 117 V at the load with a drop around 2.5%. That is a reasonable performance estimate, but the actual cord rating still has to fit the selected rating basis and the saw's duty cycle.

Heater on a light cord

A 1440 W heater at 120 V is estimated as a 12 A load in watts mode. If the cord is rated 13 A and Rating basis is set to 80% continuous planning, the rating check can fail because the allowed planning current is only 10.4 A.

Long outdoor run with high drop

A 100 ft AWG 16 run can cross a 3% goal even when the connected equipment is below the cord's printed rating. Compare Gauge Options to see how AWG 14, AWG 12, or a shorter physical route changes load-end voltage, heat, and maximum one-way length.

FAQ:

Should cord length be one-way or round-trip?

Enter the physical one-way Cord length from source to load. The calculator doubles that length for the outgoing and return conductors.

Why does a lower AWG number usually improve the result?

Lower AWG numbers represent thicker copper conductors. Thicker copper has less resistance, so it loses less voltage and produces less conductor heat at the same current and length.

Is a 3% voltage drop limit required for every extension cord?

No. It is a planning target, not a universal approval rule for every temporary cord use. The right limit depends on the load, sensitivity, duty cycle, cord rating, and applicable rules.

Does a passing result mean a cord is safe for continuous use?

No. The rating basis check is only one screen. Cord condition, markings, heat, moisture, reel use, plug fit, equipment instructions, and local rules still control actual use.

Why did watts mode estimate a different current than my equipment label?

Watts mode divides Load power by Source voltage. Use amps mode when a nameplate or measurement gives current, because real equipment can draw current differently from a simple wattage conversion.

Glossary:

AWG: American Wire Gauge, a wire-size system where lower numbers mean thicker conductors.
Voltage drop: The voltage lost across conductor resistance while current flows to the load.
Load-end voltage: The estimated voltage available at the equipment after cord drop is subtracted.
Round-trip resistance: The combined resistance of the outgoing and return cord conductors.
Load current: The running current drawn by the connected tool or appliance.
Printed cord rating: The amp rating marked on the cord, plug, tag, or packaging.

References:

Construction Electrical Incidents: Flexible Cords, Occupational Safety and Health Administration.
Extension Cords: 5 Things to Know, Occupational Safety and Health Administration.
Recommended Practices for Designing and Installing Copper Building Wire Systems, Copper Development Association.