Extension cords look simple because they are portable, familiar, and cheap compared with fixed wiring. That is exactly why they are so often misused. A portable saw, sump pump, compressor, or pressure washer may run on whatever cord is closest, even if the cord is two wire sizes too small and 100 feet too long for the job. The result is predictable: sluggish motor starts, low torque, nuisance thermal trips, overheated plugs, and users blaming the tool instead of the voltage drop.

The code framework is straightforward once you separate ampacity from performance. The National Electrical Code controls flexible-cord usage under NEC Article 400, especially NEC 400.5(A)(1) for ampacity and NEC 400.12 for uses not permitted. For international comparison, IEC guidance under IEC 60364-5-52 applies the same engineering logic: conductor size, installation condition, and route length all affect terminal voltage and heating. If the load is motor-driven, that performance margin matters even more.

The right field workflow is to check the cord three ways. First confirm the cord can legally and thermally carry the current. Second calculate voltage drop at the actual one-way length. Third remember that the cord drop is added on top of the drop already present in the building branch circuit. The equipment only sees the total.

“The extension cord is the part people treat as temporary, but the motor does not care about your intent. If the cord steals 8 to 12 volts on a 120V tool, the tool experiences that as a real system problem.”
— Hommer Zhao, Technical Director

Why Extension Cords Fail So Often in Real Jobs

Fixed wiring is usually designed once, inspected once, and left alone. Extension cords live a harder life. They are coiled on reels, dragged across concrete, left in direct sun, pinched in doors, and moved from one load type to another. That means the same cord may serve a heater in the morning, a miter saw at noon, and a sump pump in the evening. Those are not equivalent loads. A resistance heater mainly cares about current and heat. A motor-driven load cares about current, heat, voltage, and starting torque.

Motor loads are less forgiving. A saw, compressor, or pump may pull several times running current for a short starting interval. A marginal cord that seems fine on paper can still produce a hard start in the field.
Portable cords often run hotter. A cord wound on a reel or covered by debris sheds heat poorly. That is very different from a fixed conductor in a known installation condition.
120V systems magnify voltage loss. Losing 6V on a 120V tool is already 5%. Losing the same 6V on a 240V load is only 2.5%.
Temporary setups hide stacked losses. A long branch circuit in the wall plus a long extension cord in the yard can easily create a total drop that no single segment looked bad enough to explain by itself.

Fast screening rule

If a 120V motor load will run more than about 50 feet from the receptacle, stop guessing and run the numbers. That single habit prevents a large share of tool-performance complaints.

The Code and Standards Numbers That Matter

Extension-cord sizing is not just a voltage-drop exercise. The main checkpoints are:

NEC 400.5(A)(1): verify the flexible cord ampacity from the correct cord table, not from a building-wire ampacity table.
NEC 400.12: do not use flexible cord as a substitute for permanent wiring inside walls, ceilings, or other prohibited locations.
NEC 590: temporary power still requires proper cord condition, routing, and protection against physical damage.
NEC 210.19(A)(1) and 215.2(A)(1) informational notes: the familiar 3% branch-circuit and 5% combined recommendation is not a hard pass-fail code limit, but it remains very strong design guidance for tool and motor performance.
IEC 60364-5-52 and IEC flexible-cord standards: the logic is the same outside North America: installation method, environment, and allowable drop all influence the final cord selection.

“A cord can pass the ampacity check and still be the wrong answer. When I see a 120V motor circuit near 4% or 5% cord drop before the branch circuit is even counted, I already expect heat and callbacks.”
— Hommer Zhao, Technical Director

Comparison Table: Common Cord Choices That Work and Fail

These examples use practical copper-cord assumptions and focus on screening decisions. They do not replace checking the actual cord listing, attachment plugs, environment, or equipment nameplate.

Application	One-Way Length	Load	Cord Size	Approx. Drop	Field Result
120V miter saw	25 ft	15A	16 AWG	2.5%	Usually acceptable
120V miter saw	100 ft	15A	16 AWG	10.0%	Poor choice
120V miter saw	100 ft	15A	12 AWG	4.0%	Usable
120V pressure washer	50 ft	12A	14 AWG	2.5%	Good
120V temporary feed	100 ft	20A	12 AWG	6.3%	Too much loss
120V temporary feed	100 ft	20A	10 AWG	4.0%	Preferred

Worked Examples with Real Numbers

Example 1: 15A saw on a 100-foot 120V cord

Load: 120V, 15A single-phase, 100 ft one way

16 AWG copper cord resistance is roughly 4.016 ohms per 1000 ft

Round-trip length = 200 ft

Voltage drop ≈ 15 x 4.016 x 0.200 = 12.0V

Drop ≈ 10.0% at 120V: poor for a motor tool

Replace the same cord with 12 AWG copper at about 1.588 ohms per 1000 ft and the drop becomes roughly 4.8V, or 4.0%. That is still not luxurious, but it is a big improvement in starting torque and motor temperature.

Example 2: 12A pressure washer on a 50-foot cord

Load: 120V, 12A, 50 ft one way

14 AWG copper resistance is roughly 2.525 ohms per 1000 ft

Round-trip length = 100 ft

Voltage drop ≈ 12 x 2.525 x 0.100 = 3.03V

Drop ≈ 2.5%: normally workable

This is why 14 AWG often feels acceptable on short 12A outdoor loads. But if the receptacle circuit already drops another 2% to 3%, the combined number can still become mediocre. That is the hidden reason many users report weak pump or washer performance.

Example 3: 20A temporary feed over 100 feet

Load: 120V, 20A, 100 ft one way

12 AWG copper resistance is roughly 1.588 ohms per 1000 ft

Round-trip length = 200 ft

Voltage drop ≈ 20 x 1.588 x 0.200 = 6.35V

Drop ≈ 5.3% before any branch-circuit loss is added

On paper 12 AWG looks respectable because people associate it with a 20A branch circuit. In temporary power, however, 10 AWG is often the better answer at 100 feet because it drops about 4.0% instead of more than 5%, and that leaves at least some room for the wall circuit and connection losses.

“If the cord calculation is already close to 3%, I usually go one size larger. Portable tools rarely complain about extra copper, but they complain very quickly about low voltage.”
— Hommer Zhao, Technical Director

The Practical Decision Process

Start with actual running current from the nameplate or measured load, not the breaker size.
Check whether the load is resistive or motor-driven. Motors deserve more voltage margin.
Use the full one-way cord length and remember the current travels the full round trip.
Add building branch-circuit drop to cord drop. The equipment only experiences the combined result.
Check the cord condition and whether it will be used fully uncoiled. Heat management matters.

If you need a second check, compare the result with this site's wire size calculator, the knowledge-base article on extension-cord voltage drop, and related field guides on air compressor circuits and motor starting voltage drop.

FAQ

What extension cord size should I use for a 15A tool at 100 feet?

For a 120V tool drawing 15A, 12 AWG copper is usually the practical minimum at 100 feet one way. A 16 AWG cord can drop about 10% at full load, while 12 AWG is closer to 4%, which is far kinder to the motor.

Does NEC require the 3% voltage-drop limit on extension cords?

Not as a hard prescriptive rule. The 3% branch-circuit and 5% feeder-plus-branch targets appear in NEC informational notes to 210.19(A)(1) and 215.2(A)(1), but they remain strong design guidance for performance and efficiency.

Can I size an extension cord from breaker rating alone?

No. The breaker protects the branch circuit, not automatically the cord or the equipment performance. You still need to check the actual load current, cord ampacity under NEC 400.5(A)(1), and voltage drop over the full cord length.

Why do motors hate undersized extension cords more than heaters do?

Motors can pull 3 to 6 times running current at startup. A cord that looks acceptable at 12A running current may sag badly during starting, reducing torque and increasing heating. A heater is usually more tolerant because it does not depend on starting torque.

Should I use 10 AWG for a 20A extension cord at 100 feet?

Often yes on 120V temporary-power work. At 20A and 100 feet one way, 12 AWG is usually too close to the edge once branch-circuit drop and connection losses are added. 10 AWG gives much better voltage margin and lower heating.

Do coiled cords need extra caution even if the wire size is correct?

Yes. A coiled cord sheds heat poorly. Even if the conductor size is adequate on paper, a cord reel left mostly wound up at 12A to 15A can run much hotter than the same cord fully uncoiled in free air.

Need a Cord Size You Can Defend in the Field?

Run the actual load, voltage, wire size, and one-way distance in the calculator before you buy the cord reel, then contact us if you want help reviewing a temporary-power layout, long-run branch circuit, or motor-heavy installation.

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Extension Cord Wire Size: Voltage Drop, Ampacity, and NEC 400 Rules