Extension cords seem simple, but using the wrong one is one of the most common—and dangerous—electrical mistakes we see. Undersized, damaged, or improperly rated extension cords can overheat, damage equipment, trip breakers, or even start fires.
In this guide, we’ll break down how to correctly size an extension cord based on wire gauge, amperage, length, duty rating, environment, and safety features like GFCI protection. We’ll also cover the risks of misuse and what to look for when a cord should be retired.
Why Extension Cord Size Matters
Electricity generates heat as it flows through a conductor. The smaller the wire (higher gauge number), the more resistance it has—and the more heat it produces under load. When a cord is too small for the device or too long for the load, heat builds up inside the insulation where you can’t see it.
This is why extension cords are involved in thousands of residential fires every year.
Correct sizing reduces:
- Overheating and insulation breakdown
- Voltage drop that damages motors and electronics
- Fire and shock hazards
Understanding Wire Gauge (AWG)
Extension cords are sized using American Wire Gauge (AWG). A lower number means thicker wire and higher current capacity.
Common Extension Cord Gauges
| Wire Gauge (AWG) | Typical Use | Notes |
| 18 AWG | Light-duty | Lamps, phone chargers, very low draw only |
| 16 AWG | Light to medium-duty | Small tools, fans, TVs |
| 14 AWG | Medium-duty | Power tools, shop vacs, outdoor equipment |
| 12 AWG | Heavy-duty | High-draw tools, compressors, heaters |
Rule of thumb: When in doubt, go thicker.
Amperage: Matching the Cord to the Load
Every electrical device draws a certain amount of current, measured in amps (A). This information is printed on the device nameplate or in the manual.
Typical Amperage Ratings
| Device | Approx. Amps | ||
| Phone charger | < 1A | ||
| TV | 1–3A | ||
| Shop vacuum | 8–12A | ||
| Circular saw | 12–15A | ||
| Space heater | 12–15A | ||
|
10–15A (startup surge higher) |
Important: Motors draw extra current when starting. Always size cords for startup surge, not just running amps.
Length Matters: Voltage Drop Explained
The longer the extension cord, the more resistance the electricity encounters. This causes voltage drop, which can:
- Overheat cords
- Reduce tool performance
- Shorten motor lifespan
Recommended Cord Gauge by Length and Load
| Amps | 25 ft | 50 ft | 100 ft |
| Up to 7A | 18 AWG | 16 AWG | 14 AWG |
| 8–10A | 16 AWG | 14 AWG | 12 AWG |
| 11–15A | 14 AWG | 12 AWG | 10–12 AWG |
If your cord feels warm to the touch, it’s undersized.
Duty Rating: Light, Medium, and Heavy-Duty
Not all extension cords are designed for continuous use.
Duty Ratings
- Light-Duty: Indoor, temporary use (lamps, electronics)
- Medium-Duty: Power tools, garage use
- Heavy-Duty: Job sites, compressors, heaters, outdoor equipment
Look for markings such as:
- SJTW (junior service, outdoor rated)
- SJEOW (oil and weather resistant)
- SOOW (extra heavy-duty industrial use)
Plug Type and Configuration
Always match the plug to the device:
- 2-prong cords: Double-insulated devices only
- 3-prong cords: Grounded equipment (most tools)
Never remove the grounding pin to make a cord fit.
For high-draw equipment, use:
- Straight-blade heavy-duty plugs
- Molded, strain-relief ends
Indoor vs Outdoor Extension Cords
Indoor Cords
- Lighter insulation
- Not UV or moisture resistant
- Not rated for temperature extremes
Outdoor Cords
- Thicker insulation
- UV-resistant jackets
- Moisture and temperature rated
Using an indoor cord outside—even temporarily—can cause cracking, brittleness, and shock risk.
GFCI Extension Cords: When You Need Them
A GFCI (Ground Fault Circuit Interrupter) protects people from shock by shutting off power if it senses leakage.
Use GFCI extension cords when:
- Working outdoors
- Near water
- On construction sites
- Using tools in damp locations
Some cords have built-in GFCI protection, which is ideal when outlets are not GFCI-protected.
The Risks of Using an Undersized Cord
Running too much current through a small-gauge cord can cause:
- Internal wire overheating
- Melted insulation
- Voltage drop that damages motors
- Fire inside walls, grass, or debris
This is especially dangerous with heaters, compressors, and saws.
Damaged Extension Cords: A Serious Fire Hazard
Never use a cord that shows:
- Cracks or brittleness in the jacket
- Exposed copper
- Loose or broken plugs
- Burn marks or discoloration
UV Damage and Aging
Sunlight degrades many cord jackets over time. UV exposure causes:
- Brittleness
- Cracking
- Loss of insulation flexibility
Outdoor cords should be stored out of direct sunlight when not in use.
Best Practices for Safe Extension Cord Use
- Use the shortest cord possible
- Size for maximum amperage, not average use
- Fully uncoil cords before use
- Never daisy-chain extension cords
- Replace damaged cords immediately
- Use GFCI protection where required
When to Call an Electrician
If you find yourself relying on extension cords regularly, it may indicate:
- Not enough outlets
- Circuits overloaded
- Improper outdoor receptacle placement
A licensed electrician can add properly protected outlets and eliminate the risk entirely.