Extension Cord Helper
Find the right gauge for your load
Load: 12.5 amps at 120V
Recommended Gauge
12 AWG
Heavy-duty: saws, compressors
Reference Chart
Quick Facts
- •Lower AWG = thicker wire = more capacity
- •12 AWG handles up to 20A at 50ft
- •Never exceed 80% of cord rating
- •Don't use indoor cords outdoors
Extension Cord Safety
Using an undersized extension cord can cause overheating and fire. The required wire gauge depends on the load (amps) and cord length. Longer runs need thicker wire (lower AWG number). Never daisy-chain extension cords or exceed the rated capacity.
Understanding the extension cord helper
An extension cord that is too thin for the load drops voltage along its length, which makes motors run hot and can melt insulation. Gauge is counted backwards: a 12 gauge cord is thicker than a 16 gauge one. Length matters just as much as gauge, because voltage drop rises with distance.
Worked examples
Gauge for a 13 amp saw on a 100 ft cord
- 1Load is 13 amps, which sits in the 11 to 13 amp band.
- 2At 25 ft, 16 gauge would do. At 100 ft, that band requires 12 gauge.
- 3Confirm the cord is rated for outdoor use if the saw is outside.
A 12 gauge, 100 ft outdoor rated cord.
Voltage drop on a 50 ft, 14 gauge cord at 12 amps
- 114 gauge copper has about 2.53 ohms per 1,000 ft.
- 2The current travels out and back, so use twice the length: 100 ft.
- 3Resistance: 100 ÷ 1,000 × 2.53 = 0.253 ohms.
- 4Drop = amps × ohms = 12 × 0.253 = 3.04 volts, or 2.5 percent of 120 V.
About 3 volts of drop, within the 3 percent guideline but close to the limit.
Minimum wire gauge by load and cord length at 120 V
| Amps | 25 ft | 50 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 0 to 5 | 16 | 16 | 16 | 14 |
| 6 to 8 | 16 | 16 | 14 | 12 |
| 9 to 10 | 16 | 16 | 14 | 12 |
| 11 to 13 | 16 | 14 | 12 | 10 |
| 14 to 15 | 14 | 12 | 12 | 10 |
| 16 to 20 | 12 | 12 | 10 | 10 |
Lower gauge numbers mean thicker wire. When in doubt, go thicker.
Cord jacket ratings
| Code | Meaning |
|---|---|
| S | Flexible service cord |
| J | 300 volt insulation, standard household |
| T | Vinyl thermoplastic jacket |
| O | Oil resistant |
| W | Rated for outdoor and wet locations |
| E | Elastomer, stays flexible in cold |
SJTW is the common outdoor household cord. SJOOW handles oil and cold on a job site.
Practical tips
- •Uncoil the cord fully before use. A coiled cord under load traps heat and can reach damaging temperatures.
- •Do not daisy chain cords. Use one longer cord instead, since each connection adds resistance and a failure point.
- •Check the cord for warmth after ten minutes of use. Anything warmer than mildly is a sign the gauge is too small.
Frequently asked questions
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This calculator is for educational and reference purposes only. Always verify critical calculations independently.