Electrical & Lighting · Calculator
Extension Cord Gauge Calculator
Use this extension cord gauge chart and calculator to pick the right wire size for your tool, its amps and the length of your run. It checks the cord's amp rating and the voltage drop, then tells you the thinnest cord that will keep your motor safe.
Your tool and cord
Preset amps are typical nameplate ratings. Your tool's label or manual is the final word.
Standard US outlets are 120 V.
Assumes copper conductors at 68 F (16 AWG 4.02, 14 AWG 2.53, 12 AWG 1.59, 10 AWG 1.00 ohms per 1,000 ft) and the amp ratings printed on most cords (18 AWG 10 A, 16 AWG 13 A, 14 AWG 15 A, 12 AWG 20 A).
- 18 AWG16.0% · 100.8 VOver rating
- 16 AWG10.0% · 107.9 VOver rating
- 14 AWG6.3% · 112.4 VHigh drop
- 12 AWG4.0% · 115.2 VOK
- 10 AWG2.5% · 117.0 VOK
Extension cord gauge chart (amps vs length)
Find your tool's amps on the left and your cord length across the top. Each cell is the thinnest cord that stays within its printed amp rating and keeps voltage drop at or under 5 percent on a 120 V circuit. The numbers come from the same math as the calculator above, using the top of each amp range as the worst case.
| Tool amps | 25 ft | 50 ft | 75 ft | 100 ft | 150 ft |
|---|---|---|---|---|---|
| 0 to 5 A | 18 AWG | 18 AWG | 18 AWG | 16 AWG | 14 AWG |
| 5.1 to 7 A | 18 AWG | 18 AWG | 16 AWG | 16 AWG | 14 AWG |
| 7.1 to 10 A | 18 AWG | 16 AWG | 14 AWG | 14 AWG | 12 AWG |
| 10.1 to 13 A | 16 AWG | 16 AWG | 14 AWG | 12 AWG | 10 AWG |
| 13.1 to 15 A | 14 AWG | 14 AWG | 14 AWG | 12 AWG | 10 AWG |
Stricter 3 percent chart. Use this one for motors that start under load (air compressors, table saws, pressure washers) and for any cord that stays plugged in for hours. "Too long" means even a 10 AWG cord drops more than 3 percent.
| Tool amps | 25 ft | 50 ft | 75 ft | 100 ft | 150 ft |
|---|---|---|---|---|---|
| 0 to 5 A | 18 AWG | 18 AWG | 16 AWG | 14 AWG | 12 AWG |
| 5.1 to 7 A | 18 AWG | 16 AWG | 14 AWG | 14 AWG | 12 AWG |
| 7.1 to 10 A | 18 AWG | 14 AWG | 12 AWG | 12 AWG | 10 AWG |
| 10.1 to 13 A | 16 AWG | 14 AWG | 12 AWG | 10 AWG | Too long |
| 13.1 to 15 A | 14 AWG | 12 AWG | 12 AWG | 10 AWG | Too long |
How to calculate extension cord voltage drop
Current flows out on the hot wire and back on the neutral, so the electricity travels the cord length twice. That is why the formula doubles the length. Multiply by the wire's resistance per foot and by the current, and you have the volts lost before power reaches the tool.
Then check the amp rating. Voltage drop is only half the test. A short 16 AWG cord shows almost no drop on a 15 A saw, but it is rated for 13 A and will run hot. The calculator picks the thinnest gauge that passes both checks.
Pick 3 percent or 5 percent. Five percent is the common limit for tools. Three percent is better for long runs, for motors that start under load, and for cords that get warm, since hot copper has more resistance than the 68 F values used here.
What AWG means on an extension cord
AWG (American Wire Gauge) runs backwards: the lower the number, the thicker the wire. A 10 AWG conductor is about twice the cross section of 13 AWG and roughly four times that of 16 AWG. Every three gauge steps halves or doubles the copper.
The jacket code tells you the rest. On a cord marked 12/3 SJTW: 12 is the gauge, 3 is the number of conductors (hot, neutral, ground), S means service cord, J is junior service (300 V rated), T is a thermoplastic (vinyl) jacket and W means it is rated for outdoor and wet locations.
- SJTW: the everyday outdoor cord. Vinyl gets stiff in the cold.
- SJOW / SJEOW: oil resistant rubber or elastomer jackets that stay flexible in winter.
- SPT-2: lamp cord. Indoor only, never for tools.
| Gauge | Ohms / 1,000 ft | Amp rating | Max length at 13 A (5%) |
|---|---|---|---|
| 18 AWG | 6.39 | 10 A | Not rated |
| 16 AWG | 4.02 | 13 A | 57 ft |
| 14 AWG | 2.53 | 15 A | 91 ft |
| 12 AWG | 1.59 | 20 A | 145 ft |
| 10 AWG | 1.00 | 30 A | 231 ft |
Why an undersized cord burns up motors
A universal or induction motor has to make the same power no matter what voltage it gets. When the cord drops the voltage, the motor pulls more current to compensate. More current means more heat in the windings, and heat is what breaks down the insulation and kills motors.
The damage is worst at startup. A motor draws several times its running current for the first moments, so a long, thin cord can sag the voltage enough that the motor struggles to get up to speed. Air compressors that start against tank pressure are the classic victim, which is why compressor manuals tell you to add air hose instead of extension cord. Our air compressor guide covers models that are easier on a circuit.
The cord suffers too. Every watt lost in the cord becomes heat along its length. A coiled cord cannot shed that heat, so always unroll a reel fully before running a big tool on it.
Tools that belong on a heavy cord
Extension cord safety tips
- Do not daisy-chain cords. Each plug connection adds resistance and heat, and the thinnest cord sets the limit for the whole chain. The same goes for plugging a power strip into a cord. See surge protector vs power strip for what those strips can and cannot handle.
- Use a W-rated cord outside and plug it into a GFCI outlet. Keep connections off wet ground.
- Match the plug to the job. A cord is only as good as its 15 A plug. Anything over 15 A needs a 20 A circuit and a cord built for it.
- Retire damaged cords. Cut jackets, missing ground pins and warm plugs are signs the cord is done.
- Running tools off a generator? Size the generator first with our generator wattage calculator, then use this chart for the cord.
Protecting electronics instead of tools? A good surge protector is the right answer there. For saws, compressors and outdoor gear, bookmark this extension cord gauge chart and buy one heavy 12 AWG cord that covers most of your jobs.
Frequently asked questions
What gauge extension cord do I need for a 15 amp tool?
At the usual 5 percent voltage drop limit, a 15 amp tool needs a 14 AWG cord up to 75 feet, 12 AWG up to 100 feet and 10 AWG beyond that. For compressors and other motors that start under load, use the 3 percent limit: 14 AWG only to 25 feet, then 12 AWG to 75 feet. A 16 AWG cord is rated for 13 amps, so it is never right for a 15 amp tool.
Is a 12 gauge or 14 gauge extension cord better?
A 12 gauge cord is thicker, carries more current (20 amps versus 15) and loses less voltage over distance. A 14 gauge cord is lighter and cheaper and handles 15 amp tools up to about 50 to 75 feet. If you only own one heavy cord, make it a 12 gauge.
What does 16/3 or 12/3 mean on an extension cord?
The first number is the wire gauge (AWG) and the second is the number of conductors. A 12/3 cord has three 12 AWG wires: hot, neutral and ground. Lower gauge numbers mean thicker wire.
How much voltage drop is acceptable on an extension cord?
Keep voltage drop at 5 percent or less, and 3 percent or less for long runs and motor tools that start under load. On a 120 volt circuit, 5 percent is 6 volts, leaving about 114 volts at the tool.
Can I plug two extension cords together?
You should not. Every connection adds resistance and heat, the total length raises voltage drop, and the thinnest cord in the chain sets the amp limit. Buy one cord long enough and heavy enough for the job.
Does a longer extension cord use more electricity?
Slightly. The watts lost in the cord turn into heat, so a thin 100 foot cord on a 15 amp saw can waste 100 watts or more. The bigger problem is the low voltage at the tool, which makes motors run hot.
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