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
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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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Long extension cords behave like small resistors placed in series with the load. The longer and thinner the cord, and the higher the current, the more voltage is lost before power reaches the tool, heater, pump, charger, or appliance at the far end.

Voltage drop is not the same as overload protection. A cord can show a tolerable load-end voltage and still be unsuitable because the plug, jacket marking, reel, outlet, or connected equipment is not rated for the current. The reverse can also happen: the printed amp rating may look adequate, but a long light-gauge cord can sag enough to make a motor start poorly or make a sensitive supply run warmer than expected.

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

American Wire Gauge numbers run backward from everyday intuition. A lower AWG number is a thicker conductor with less resistance, so AWG 12 normally drops less voltage than AWG 16 at the same current and length. Copper resistance also rises as the conductor warms, which is why a loaded cord in sun, on a reel, or under a mat can perform worse than a cool reference table suggests.

Planning targets such as 3% or 5% voltage drop are useful efficiency and performance guides, not safety approvals. Final use still depends on the cord marking, equipment instructions, outlet and breaker rating, ground-fault protection where required, moisture exposure, physical condition, and local electrical rules.

How to Use This Tool:

Use the nameplate or measured running load first. Startup surge, damaged conductors, and cord-reel heating need separate judgment.

Choose Load info as Current in amps when the appliance label gives amperes. Choose Power in watts only when watts are the value you have; the calculator estimates current from watts divided by Source voltage.
Enter Source voltage and the physical Cord length from supply to load. Do not double the length yourself because the calculation already counts the outgoing and return conductors.
Select the printed Cord wire gauge, then enter the Printed cord rating in amps. Use the actual cord jacket, plug, or package marking rather than a generic AWG assumption.
Set Rating basis. Use 80% continuous planning for long-running loads such as heaters and pumps, or 100% intermittent check for short-duty use when the cord marking supports it.
Set the Voltage drop goal. A 3% target is stricter; a 5% check may be acceptable for many temporary non-sensitive loads.
Use Advanced only when you have a reason to adjust Cord copper temperature, Load profile, or displayed decimal places. If an input warning appears, fix the out-of-range value before reading the result tabs.

Start with Cord Snapshot, then compare Gauge Options and Length Limit Map when the selected cord is near the voltage-drop or printed-rating limit.

Interpreting Results:

Voltage at load is the main performance estimate. Voltage drop shows the lost volts and percent of source voltage, while Heat in cord estimates conductor power loss under the modeled running current.

Within goal means the modeled voltage drop is less than or equal to the selected Voltage drop goal.
Printed rating use compares load current against the selected rating basis, not against a hidden approval table.
Voltage-drop guide points to the first listed AWG that meets both the drop goal and the built-in reference-current check.
Length Limit Map shows the maximum one-way length for each AWG at the same load, source voltage, temperature, and drop goal.

A pass does not mean the setup is safe in wet, damaged, coiled, buried, daisy-chained, or overloaded conditions. A fail usually means the first fixes are shorter length, lower load, higher source voltage where the equipment permits it, or a heavier and correctly rated cord.

Technical Details:

Single-phase extension-cord voltage drop is an Ohm's law estimate over the round-trip copper path. The physical cord length is one-way, but current flows out to the load and back through another conductor, so the resistance path is twice the entered cord length.

The AWG resistance values are reference copper ohms per 1000 ft. The selected copper temperature adjusts resistance upward or downward with a coefficient of 0.00393 per C from the 20 C reference. The estimate is steady-state; it does not model motor inrush, connector heating, contact resistance, harmonic current, power factor, or thermal protection.

Formula Core:

The voltage estimate starts with round-trip conductor resistance, then applies current to find lost voltage and conductor heat.

\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	Input or output
I	Running load current in amps, entered directly or estimated from watts.	Load current
L	Physical one-way cord length in feet after unit conversion.	Cord length
r_temp	Copper conductor resistance after the selected temperature adjustment.	Selected cord gauge, Cord copper temperature
V_drop	Volts lost across the round-trip cord conductors.	Voltage drop
P_heat	Power converted to heat in the cord conductors at the modeled current.	Heat in cord

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

Extension cord boundary checks
Check	Boundary	Meaning
Voltage drop goal	Pass when drop percent is <= selected goal.	Compares performance against the user's planning limit.
Printed cord rating	Pass when load current is <= printed rating times 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 how far each AWG can run before crossing the drop limit.
Reference current	Compares load current with the built-in AWG reference current.	Supports gauge comparison only; the cord's printed rating remains the user-facing authority.

When the power input mode is watts, current is estimated as watts divided by source voltage. Use amps mode when an equipment label or clamp meter gives current, because real loads may draw more or less current than a simple wattage conversion suggests.

Limitations:

The result is a planning estimate for copper extension cords, not an electrical-code approval or a thermal safety model.

Do not use the result to justify a damaged, undersized, wet, modified, ungrounded, or tightly coiled cord.
Motor and compressor startup can draw much more than running current, so a running-current pass may still produce poor starts.
Connector condition, reel temperature, ambient heat, cord jacket rating, ground-fault protection, and local rules must be checked separately.

Worked Examples:

Shop tool on a 50 ft cord

A 12 A saw on 120 V with AWG 14, 50 ft one-way length, and a 3% goal should show Voltage at load close to 117 V and Voltage drop near 2.5%. Printed rating use still needs the actual cord amp rating, especially if the saw runs for long cuts.

Heater on a light cord

A 1440 W heater at 120 V is treated as about 12 A in watts mode. If the cord rating is 13 A but Rating basis is set to 80% continuous planning, Printed rating use can fail even when voltage drop is acceptable, because the allowed current is only 10.4 A.

Troubleshooting a high drop warning

If Voltage drop is above the 3% goal on a 100 ft AWG 16 run, compare Gauge Options. A lower AWG number or shorter Cord length should raise Voltage at load and reduce Heat in cord.

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 internally for the outgoing and return conductors.

Why does a lower AWG number improve the result?

Lower AWG numbers are thicker copper conductors with lower resistance. Lower resistance reduces Voltage drop and Heat in cord at the same current and length.

Does the calculator approve an extension cord for continuous use?

No. Rating basis helps compare load current against the printed rating, but cord condition, environment, reel use, equipment instructions, and local rules still control actual use.

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

Watts mode divides Load power by Source voltage. Use amps mode when the nameplate lists current, because real equipment current can differ from a simple wattage estimate.

Glossary:

AWG: American Wire Gauge, where lower numbers mean thicker conductors.
Voltage drop: The voltage lost across conductor resistance while current flows.
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, 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.