Extension Cord Gauge Calculator

Find the minimum wire gauge (AWG) for your extension cord based on appliance wattage, cord length, and voltage. Avoid voltage drop, overheating, and fire hazards with the right AWG size.

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Quick Facts

Lower AWG = Thicker
10 AWG > 16 AWG
Smaller gauge numbers mean thicker wire. 10 AWG carries more current than 16 AWG
Length Doubles Resistance
100 ft = 2\u00D7 50 ft Drop
A longer cord needs thicker wire. Double the length = double the voltage drop
NEC 3% Rule
3.6V Max at 120V
National Electrical Code recommends max 3% voltage drop on branch circuits
Common Mistake
16 AWG for Everything
Cheap 16 AWG cords overheat above 10 amps at 50+ feet

Your Results

Calculated
Minimum Recommended Gauge
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AWG (American Wire Gauge)
Estimated Voltage Drop
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Voltage at device: -
Voltage Drop Percentage
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Max acceptable: -
Power Loss in Cord
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Watts dissipated as heat in the wire

Safety Assessment

Enter your amperage and cord length to find the recommended wire gauge.

Key Takeaways

  • Wire gauge (AWG) is the most important number on an extension cord. A 16 AWG cord at 100 feet carrying 15 amps will lose over 12 volts, leaving your device with 108V instead of 120V \u2014 enough to damage motors and electronics.
  • Voltage drop is proportional to current and distance. If you double the amps or double the cord length, you double the voltage drop. This is Ohm's law in action.
  • The NEC recommends keeping voltage drop under 3% for branch circuits. For a 120V outlet, that means no more than 3.6 volts lost in the cord. A 14 AWG cord handles 15 amps up to about 50 feet before exceeding this threshold.
  • Extension cords coiled on a reel while in use can overheat because the magnetic field from adjacent loops restricts heat dissipation. Always uncoil a cord fully before drawing significant current.

AWG Explained: Why Smaller Numbers Mean Thicker Wire

American Wire Gauge (AWG) is a logarithmic scale where each step of 3 gauge numbers roughly doubles or halves the cross-sectional area of the wire. 10 AWG has about twice the copper of 13 AWG and four times the copper of 16 AWG. More copper means lower resistance, which means less voltage drop and less heat generated in the cord itself.

A 16 AWG cord has a resistance of about 4 ohms per 1000 feet. At 50 feet carrying 15 amps, that is 0.2 ohms of resistance, and the voltage drop is V = I \u00D7 R = 15 \u00D7 0.4 (round trip) = 6 volts. That is 5% of 120V \u2014 above the NEC-recommended limit. A 12 AWG cord at the same length and current drops only about 1.9 volts, or 1.6%.

How This Calculator Works

Voltage drop = 2 \u00D7 current \u00D7 length \u00D7 resistance per foot
Resistance per 1000 ft: 18 AWG = 6.4 ohms, 16 AWG = 4.0 ohms, 14 AWG = 2.5 ohms, 12 AWG = 1.6 ohms, 10 AWG = 1.0 ohms.
Round-trip factor: Current travels to the device and back. The wire length is effectively doubled for voltage drop calculation.
Minimum gauge: The smallest AWG (largest number) that keeps voltage drop within the specified percentage at the given current and length.

Worked Example

15 amps, 100 ft cord, 120V, 3% max drop:

  • 10 AWG resistance at 100 ft round trip: 2 \u00D7 100 \u00D7 (1.0/1000) = 0.2 ohms
  • Voltage drop = 15 \u00D7 0.2 = 3.0V, which is 2.5% \u2014 within the 3% limit
  • 12 AWG would drop 4.8V (4.0%) \u2014 exceeds the limit
  • Minimum gauge: 10 AWG for this scenario

Common Device Amp Draws

LED work light: 0.5\u20131.5 amps. Circular saw: 12\u201315 amps. Space heater: 12.5 amps. Electric leaf blower: 10\u201312 amps. Air compressor: 13\u201315 amps. Miter saw: 13\u201315 amps. Electric chainsaw: 10\u201314 amps. For any 15-amp tool at 50 feet or more, use 12 AWG minimum. For 100 feet, step up to 10 AWG.

Safety Warning

An undersized extension cord is a fire hazard. The cord itself becomes a resistive heating element. A 16 AWG cord carrying 15 amps at 100 feet dissipates roughly 90 watts as heat along its length \u2014 equivalent to a small incandescent light bulb buried inside the cord. If the cord is coiled, under a rug, or against a wall, that heat cannot escape and can melt the insulation or ignite nearby materials.

Helpful products for cord safety

Quality cords and accessories for safe power delivery.

Heavy Duty
12 AWG 50 ft cord

Handles 15 amps at 50 feet with safe voltage drop.

Store
Cord storage reel

Keeps cords organized and prevents kinking damage.

Test
Outlet tester

Verify your outlet is wired correctly before plugging in.

Frequently Asked Questions

What gauge extension cord do I need for a 1,500-watt space heater?
A 1,500-watt heater on a 120V outlet draws 12.5 amps. At 25 feet or less, a 14 AWG cord is adequate. At 50 feet, you need 12 AWG. At 100 feet, use 10 AWG. Space heaters should ideally be plugged directly into a wall outlet — they are one of the leading causes of home fires when used with undersized extension cords because they run at maximum wattage continuously, unlike power tools that cycle on and off.
Why do lower AWG numbers mean thicker wire?
AWG (American Wire Gauge) uses a counterintuitive scale inherited from the wire drawing process: the gauge number corresponds to how many times the wire was drawn through a die to reach its final diameter. More draws (higher number) means thinner wire. A 10 AWG wire has about four times the cross-sectional area of a 16 AWG wire and can carry proportionally more current with less resistance and heat generation.
Is it safe to use an extension cord outdoors?
Only if the cord is rated for outdoor use — look for a "W" rating on the cord label (e.g., "12/3 SJTW" — the W indicates weather resistance). Outdoor-rated cords have thicker, UV-resistant insulation that handles moisture and temperature swings. Indoor cords have thinner insulation that can crack and fail when exposed to sunlight and humidity, creating a shock and fire hazard.
Why should I uncoil an extension cord before use?
A coiled cord carrying current creates an electromagnetic field that traps heat generated by resistance. Thermal energy cannot dissipate normally when the wires are packed together, and the temperature inside the coil can rise significantly above what the insulation is rated for. At high enough loads, this melts insulation and can cause a fire. Always fully uncoil before using, especially with high-wattage tools and appliances.