Extension Cord Gauge Chart: Watts by AWG and Length
Extension cord gauge chart by length: UL 817 rates 16 AWG at 13 A to 50 ft and 10 A beyond. Watts each AWG carries, SJTW markings, and why 1,500 W heaters fail.
Short answer: gauge and length are one limit, not two. UL 817 rates a 16 AWG cord at 13 A (1,560 W) up to 50 ft and only 10 A (1,200 W) beyond it. For 15 A / 1,800 W use 14 AWG up to 50 ft and 12 AWG past it; for 20 A / 2,400 W use 12 AWG, then 10 AWG. Watts are simply amps × 120 V.
Gauge and length are one limit, not two
Almost every extension cord article treats wire gauge and cord length as separate topics. They are not. They are two inputs to a single number: how many amps that specific cord, at that specific length, can carry without overheating or starving your tool of voltage.
The clearest published proof comes from UL 817, the safety standard for cord sets and power-supply cords. Its cord ampacity table rates a 16 AWG cord at 13 A up to 50 ft, and 10 A over 50 ft. Same copper, same jacket, same cord, 20 percent less capacity purely because it is longer. Its companion cord-set table says the same thing from the other direction, and both are tabulated below.
So the 16-gauge cord in your garage is not simply “a 13-amp cord.” It is a 13-amp cord at 25 ft and a 10-amp cord at 100 ft.
The table: gauge, length, amps, watts
Two sources matter here, and they are not interchangeable.
NEC Table 400.5(A)(1) gives the ampacity of the flexible cord itself, in free air, independent of length. This is the correct table for SJTW, SJOOW and similar flexible cords. Table 310.16, which many blogs quote instead, is for building wire in raceways and does not apply to cord sets.
| AWG | Column A (3 conductors) | Column B (2 conductors) |
|---|---|---|
| 18 | 7 A | 10 A |
| 16 | 10 A | 13 A |
| 14 | 15 A | 18 A |
| 12 | 20 A | 25 A |
| 10 | 25 A | 30 A |
Note the column definitions honestly: they are defined by conductor count, and sources differ on whether the equipment grounding conductor in a 3-wire cord counts. For a standard three-prong extension cord, use the Column A figure. It is the conservative read and it lines up with real-world cord markings.
UL 817 then layers length on top. Watts below are simply amps multiplied by 120 V.
| Rating you need | Up to 50 ft | Over 50 ft |
|---|---|---|
| 10 A / 1,200 W | 16 AWG | 16 AWG |
| 13 A / 1,560 W | 16 AWG | not permitted in 16 AWG — step up |
| 15 A / 1,800 W | 14 AWG | 12 AWG |
| 20 A / 2,400 W | 12 AWG | 10 AWG |
That is the whole guide in one grid. A missing row is missing because the standard does not publish it.
Why length changes the number
Resistance in a conductor is proportional to its length. Double the run, double the resistance. That produces two problems at once.
The first is voltage drop. NEC 210.19(A) Informational Note recommends that voltage drop on a branch circuit to any load not exceed 3 percent, with the combined branch-circuit and feeder drop kept under 5 percent. On a 120 V circuit, 3 percent is 3.6 V. The standard single-phase calculation is VD = (2 × K × I × L) ÷ CMA, where K is 12.9 ohm-cmil per foot for copper, I is the load current, L is the one-way length in feet, and CMA is the conductor’s circular mil area from NEC Chapter 9 Table 8. Notice that L appears in the numerator and CMA in the denominator: going longer and going thinner do the exact same thing to your result.
The second problem is heat, and it is the one that starts fires. Power dissipated in the cord is I²R, and because R scales with length, a 100 ft cord dissipates roughly four times the heat of a 25 ft cord of the same gauge at the same current. A motor that browns out on a long thin cord then draws more current trying to make up the shortfall, which makes the heating worse, not better.
Check 12 AWG outdoor extension cords on Amazon
The space heater case
A 1,500 W resistive heater on a 120 V circuit draws 12.5 A, and unlike a drill it draws it continuously for hours. That is the single worst load you can put on a cord.
The manufacturers are blunt. Vornado instructs users to plug the heater directly into a wall outlet, never an extension cord or relocatable power tap; De’Longhi warns that a cord may overheat and create a fire risk, and specifies no smaller than No. 14 AWG rated not less than 1,875 W if one must be used. CPSC says the same to consumers, and NFPA 1 requires it.
The stakes justify the tone. NFPA’s home heating fire data for 2020 through 2024 puts the annual average at roughly 37,365 home fires involving heating equipment, with about 417 civilian deaths, 1,260 injuries and $1.2 billion in property damage. Space heaters and heating stoves account for roughly 30 percent of those fires but about 73 percent of the deaths.
Now compare 12.5 A against the table above. A 100 ft 16 AWG cord is a 10 A cord — 25 percent over rating, continuously, coiled, behind furniture.
How to read your own cord
Walk out to the garage and look at the jacket. Everything you need is printed on it.
- 16/3, 14/3, 12/3 — conductor gauge, then conductor count. 12/3 is three 12 AWG conductors: hot, neutral, ground.
- S — service-grade cord. J adds “junior,” meaning a 300 V rating instead of 600 V. Most household and light-construction cords are J.
- T — thermoplastic (PVC) jacket. No T means a rubber jacket, which stays flexible in the cold.
- O — oil-resistant jacket. OO means both the jacket and the conductor insulation are oil-resistant.
- W — weather-resistant, suitable for outdoor use. This is the marking that decides indoor versus outdoor, not the color of the cord.
- UL or ETL mark — the cord was tested to a recognized safety standard. An unmarked cord has no verified rating at all.
Check 14 AWG indoor extension cords on Amazon
Coiled cords, honestly
The advice to fully unwind a reel before loading it is correct, but the usual explanation is wrong. Inductive heating gets the blame; at 60 Hz it is essentially negligible, because the return conductor sits in the same jacket and cancels most of the field.
The real mechanism is thermal. The cord generates I²R heat along its whole length whether coiled or not, but a tight coil buries most of that length inside the bundle with little surface area to shed it, so the inner turns run hottest. US Patent 9,238,416 on cord-reel thermal management says the same: heat does not escape as readily from coiled cord, and the more cord left on the reel, the more is trapped.
Hence the two ratings — wound and unwound — on many reels, and why a thermal cutout is worth paying for on compressor and heater duty.
Check cord reels with thermal cutout on Amazon
Outdoors: the W mark and GFCI
Outdoor use adds two requirements on top of gauge and length. The cord must carry the W marking, and the circuit needs ground-fault protection.
For homes, NEC 210.8(A)(3) requires GFCI protection for receptacles installed outdoors at dwelling units. On job sites, OSHA 29 CFR 1926.404(b)(1)(ii) requires it for 120 V, single-phase, 15 and 20 A receptacle outlets that are not part of the structure’s permanent wiring, with an assured equipment grounding conductor program as the only alternative. If your exterior outlet is older and unprotected, an inline GFCI adapter at the plug end covers you.
Those two are different kinds of rule, and only one is uniform. UL 817 and the cord ampacity tables above are product standards, not federal law: UL is a private organisation and one of the 21 laboratories OSHA currently recognises as Nationally Recognized Testing Laboratories, alongside Intertek/ETL, CSA, TÜV and others. What is uniform is the requirement to be listed at all — OSHA 29 CFR 1910.303(a) demands approved equipment, and a listing from any NRTL satisfies it, so no federal rule singles out the UL mark. On that basis a 16 AWG cord is rated the same way in all fifty states. Installation and workplace rules are not. The NEC is a model code with no legal force until a state, county or city adopts it; per NFPA’s 3 August 2026 enforcement maps the 2026 edition is in force in six states, the 2023 in 20, the 2020 in 15, the 2017 in three and the 2008 edition in two, and Chicago — usually cited as running its own code instead — actually adopts NFPA 70 (2017) by reference under Municipal Code §14E-1-010 and then amends it. So 210.8(A)(3) applies in whatever edition your AHJ enforces, and only your building department can tell you which. OSHA 1926 is narrower still: a construction workplace standard under the OSH Act, which reaches only an employer who has employees (OSH Act §3(5) and §3(6)). OSHA’s letter of interpretation of 23 May 2001 is explicit that a genuinely self-employed person with no employees is outside its authority, so 1926.404 binds a contractor working on your house and does not reach you working on your own. Twenty-two OSHA-approved State Plans — 21 states plus Puerto Rico — cover private-sector employers and may be stricter, with seven more covering public employees only. The physics does not care which names you — put a GFCI between yourself and a wet cord either way.
If you want to know what your tools actually draw rather than guessing from a nameplate, a clamp meter around one conductor of a line splitter settles it in seconds.
Summary
Buy for the length you will actually use, not the length you wish you needed. If a run is over 50 ft and the load is over 10 A, 16 AWG is out, and 14 AWG is out for anything asking 15 A. A 100 ft 12 AWG SJTW cord covers nearly every homeowner case and costs less than one drywall repair after a melted jacket.
For space heaters, skip the arithmetic entirely and follow the manufacturers: plug it into the wall.
Next, see how these cord limits interact with the branch circuit they plug into in the 15 A circuit limit and what you can actually run on one, or browse more buying guides at our English section.
Sources: NEC Table 400.5(A)(1), NEC 210.19(A) Informational Note, NEC 210.8(A)(3), NEC Chapter 9 Table 8, UL 817 cord ampacity and cord-set rating tables, OSHA 29 CFR 1926.404(b)(1)(ii), NFPA 1 and NFPA home heating fire statistics (2020–2024), CPSC space heater guidance, and published Vornado and De’Longhi owner’s manuals. This article is general information, not a substitute for a licensed electrician.
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Frequently Asked Questions
- Q: Can I run a 1,500 W space heater on a 16-gauge extension cord?
- A: No. A 1,500 W heater draws 12.5 A continuously at 120 V, and UL 817 rates a 16 AWG cord at 13 A only up to 50 ft, dropping to 10 A past that. Beyond the numbers, manufacturers say not to do it at all: Vornado instructs users to plug the heater directly into a wall outlet and never into an extension cord or relocatable power tap, and De'Longhi's manuals warn that an extension cord may overheat and cause a fire risk. If one is truly unavoidable, De'Longhi specifies no smaller than No. 14 AWG and rated not less than 1,875 W.
- Q: Why does the same cord have a lower amp rating when it is longer?
- A: Two effects stack. Resistance is proportional to length, so a 100 ft run has roughly four times the resistance of a 25 ft run of the same gauge, which means four times the voltage drop at the same current and four times the heat dissipated in the cord. NEC 210.19(A) Informational Note recommends keeping branch-circuit voltage drop under 3 percent, and UL 817's cord-set tables build the same idea in by requiring a heavier gauge past 50 ft for the same ampere rating.
- Q: What does SJTW actually mean on my cord?
- A: S is service-grade cord, J is junior service at a 300 V rating rather than 600 V, T means a thermoplastic (PVC) jacket, and W means weather-resistant and suitable for outdoor use. SJOOW swaps the thermoplastic jacket for rubber and adds oil resistance on both the jacket and the conductor insulation. A cord without the W is an indoor cord, no matter how heavy it looks.
- Q: Do I really have to unroll a cord reel before using it?
- A: Yes, if you are drawing meaningful current. The mechanism is not exotic: the cord generates I-squared-R heat along its whole length, and a tightly wound coil has far less exposed surface area to shed that heat, so the inner turns run hottest. US Patent 9,238,416, covering cord-reel thermal management, describes exactly this, that heat does not escape as readily from coiled cord and more heat is trapped as more cord remains on the reel. The often-repeated inductive-heating explanation is largely a myth at 60 Hz because the return conductor in the same jacket cancels most of the field.
- Q: Does an outdoor extension cord need GFCI protection?
- A: The receptacle it plugs into generally does. NEC 210.8(A)(3) requires ground-fault circuit-interrupter protection for receptacles installed outdoors at dwelling units, and on job sites OSHA 1926.404(b)(1)(ii) requires GFCI protection for 120 V, single-phase, 15 and 20 A receptacle outlets that are not part of the building's permanent wiring, or an assured equipment grounding conductor program instead. If the outlet you are using is not protected, an inline GFCI adapter at the plug end is the practical fix.
- Q: What gauge extension cord do I need, and how many watts can it carry?
- A: Pick the gauge from the amps you need and the length you will actually run, because UL 817 rates the same cord lower past 50 ft. For 10 A / 1,200 W, 16 AWG works at any length. For 13 A / 1,560 W, 16 AWG is fine up to 50 ft and not permitted beyond it. For 15 A / 1,800 W, use 14 AWG up to 50 ft and 12 AWG over 50 ft. For 20 A / 2,400 W, use 12 AWG up to 50 ft and 10 AWG beyond. Multiply amps by 120 V to get watts.
- Q: What is the capacity of an extension cord, and why does length change it?
- A: Capacity is set by the flexible cord's ampacity and then reduced by length. NEC Table 400.5(A)(1) gives the cord itself: 7 A for 18 AWG, 10 A for 16, 15 A for 14, 20 A for 12 and 25 A for 10 in the three-conductor column used by a standard three-prong cord. Length then bites because resistance is proportional to it — a 100 ft run has roughly four times the resistance of a 25 ft run of the same gauge, so four times the voltage drop and four times the heat dissipated in the cord at the same current.