AmpSizerWhat gauge extension cord do you need?
Extension cord gauge
10 AWG cord, copper
for a 100 ft cord carrying 12 A · 2.5% drop, leaving 117 V at the tool
2.5% drop at 100 ft
- Ampacity wants14 AWG
- Voltage drop wants10 AWG
- Current the conductor has to carry
- 12 A
- Usable ampacity after derating
- 15 A (table 15)
- Longest run before the drop passes 3%
- 121 ft
- In metric sizes
- 5.3 mm²
— had the ampacity and 240.4(D) still rejected it, at 0 A.
A 50 ft cord and two 25 ft cords joined are not the same thing: every joint is another connection to lose voltage across.
A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.
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MonthlyOne thing a month worth measuring, testing or recalculating on a home electrical system — the part of sizing that happens after the install. No product pitches, and nothing that needs a live panel.
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What gauge extension cord do you need?
The AmpSizer extension cord result for 12 amps over 100 feet on 120 volts is 10 AWG copper. A 14-gauge cord would carry that current without overheating, but it loses 6.3% of the voltage over that distance; 10 gauge holds the loss to 2.5% and leaves 117 volts at the tool.
Length is what makes cords different from fixed wiring. Nobody runs a 100 ft branch circuit to a receptacle in a garage, and everybody runs a 100 ft cord across a yard.
A motor is the load that punishes this. Voltage down means current up to deliver the same work, and a universal motor on a soft cord runs hot, starts badly under load, and eventually gives up — long before the cord itself shows any sign of trouble.
The table below is the practical version: how much voltage each common cord gauge loses at a real load, at the three lengths cords are actually sold in.
| Load | 25 ft | 50 ft | 100 ft | Buy |
|---|---|---|---|---|
| 5 A — lamp, laptop, fan | 16 ga: 1.0% | 16 ga: 2.1% | 16 ga: 4.2% | 16 ga short, 14 ga long |
| 10 A — circular saw, vacuum | 14 ga: 1.3% | 14 ga: 2.6% | 14 ga: 5.2% | 14 ga short, 12 ga long |
| 12 A — mitre saw, pressure washer | 14 ga: 1.6% | 12 ga: 2.0% | 10 ga: 2.5% | 12 ga to 50 ft, 10 ga beyond |
| 15 A — table saw, compressor | 12 ga: 1.2% | 12 ga: 2.5% | 10 ga: 3.1% | 12 ga to 50 ft, 10 ga beyond |
| 20 A — welder, large compressor | 12 ga: 1.7% | 12 ga: 3.3% | 10 ga: 4.1% | 10 ga, and keep it short |
Percentages are at 120 V with copper conductors. The “buy” column is the size that keeps the loss near or under 3% at that length, which is the same target used for fixed wiring.
Why the tool notices before the cord does
A cord that is running hot is obvious. A cord that is quietly dropping 7% of the voltage is invisible, and it is the more common problem by far.
At 7% down, a 120 V tool sees 112 V. An induction motor makes roughly the square of that in torque, so it loses about 13% of its pulling power and makes up the difference by drawing more current — which heats the motor, not the cord. Compressors and pressure washers show it first, because they start against load. That is the failure mode people blame on the tool.
Frequently asked questions
What gauge extension cord for a 20 amp tool?
12 gauge up to about 50 feet and 10 gauge beyond that, on 120 V. At a genuine 20 A draw, a 12-gauge cord loses 3.3% over 50 ft and 6.6% over 100 ft. Most cords sold as ‘heavy duty’ are 12 gauge, which is the right choice for short runs and one size short for long ones.
Can I plug two extension cords together?
Electrically the drop simply adds up, so two 50 ft cords behave like one 100 ft cord of the same gauge — plus the loss across the joint. The practical problems are the joint itself: it is the part that pulls apart, gets wet and arcs. If you need 100 ft, buy 100 ft.
Does a coiled cord matter?
Yes, in two ways. A coil traps its own heat, so a cord that is fine unrolled can get hot while wound on a reel — reels usually carry a lower rating for exactly that reason. It also becomes an inductor, which does very little at 60 Hz but is another reason not to leave a working cord on the drum.
Why is my cord rated 13 amps but my breaker is 20?
Because the cord is protected by the breaker, not matched to it. Table 400.5(A)(1) gives a 16-gauge cord 13 A, and a 20 A breaker will not trip at 15 A — the cord is meant to be used within its own rating. That mismatch is why light cords fail on heavy tools without ever tripping anything.
Is 16 gauge ever enough?
For a lamp, a laptop or a fan on a short run, yes. For anything with a motor, a heating element or a compressor in it, no — and not because of safety first, but because there is no useful voltage left. Treat 16 gauge as a household cord and 12 or 10 as a tool cord.
How we calculate this
2 × amps × R × cord length ÷ 1,000 using the direct-current resistance of stranded
uncoated copper from NEC Chapter 9, Table 8, against a 3% target. For allowable current this page uses
the 60 °C column of Table 310.16 rather than the higher figures for flexible cord in Table
400.5(A)(1), which is a deliberately conservative choice: cords are coiled, walked on, run through
sunlight and used at their limit for hours, and the 60 °C column lands roughly one safety step below
the cord table. Cord assemblies also carry their own listed rating on the jacket, and where that is
lower than either figure, it is the one that governs.Worked example
With Tool current 12, One-way run 100, System voltage 120 V, Metal Copper, this page works out 10 AWG cord, copper. for a 100 ft cord carrying 12 A · 2.5% drop, leaving 117 V at the tool
- Current the conductor has to carry12 A
- Usable ampacity after derating15 A (table 15 )
- Longest run before the drop passes 3%121 ft
- In metric sizes5.3 mm²
Sources:
Reading a cord before you buy it
The number on the jacket is gauge and conductor count. “12/3” is three 12-gauge conductors — two current-carrying plus a ground. A cord without the third conductor has no ground and no place on a tool with a metal case.
SJTW, SJOOW and the rest are jacket codes, not sizes. The W matters outdoors; it means the cord is rated for wet locations and sunlight. The rest describe the rubber.
Longer is not free. A cord one size heavier than you need is a small extra cost. A cord one size lighter is a permanent tax on every tool you plug into it.