Extension Cord Size Calculator — Free Online Calculator
Find the right extension cord gauge for your power tools. Enter wattage and cord length for a safe recommendation.
How to Use This Calculator
Enter the tool wattage from its nameplate, select cord length and voltage.
The Formula Explained
Extension cord sizing is based on both amperage and distance. Longer cords need thicker gauges to prevent excessive voltage drop, which can damage motors and overheat the cord.
Extension Cord Sizing: Safety and Performance
Extension cords are designed for temporary use, not permanent wiring, but they remain one of the most common causes of residential electrical fires. The problem is almost always a combination of undersized wire, heavy loads, and extended run length. A 16 AWG cord running a space heater 50 feet away from the outlet is effectively a long resistor — most of the power heats the cord rather than the heater, and the insulation can ignite if the cord is coiled or covered. Understanding cord sizing and matching the right cord to the job prevents both fires and nuisance problems like underpowered equipment.
The sizing is based on two factors: the load current (amps) and the cord length (feet). Longer cords need thicker wire to maintain voltage and avoid overheating. Higher loads need thicker wire for both reasons. The general rule: 16 AWG for light duty under 25 feet, 14 AWG for medium duty or longer runs, 12 AWG for heavy loads or 50+ feet, 10 AWG for extremely heavy loads or runs over 100 feet. Outdoor cords should always be rated SJTW or equivalent for wet/damp conditions.
Worked Example: Space Heater Extension
A 1,500W space heater needs to reach an outlet 50 feet away. Current: 1,500 / 120 = 12.5 A. Continuous duty (heater runs for hours), so safety margin is important.
Cord options: 16 AWG (rated 13A max): borderline for 12.5A continuous, definitely undersized for 50 feet due to voltage drop. 14 AWG (rated 15A): adequate current capacity, but 50 ft of 14 AWG at 12.5A drops about 3% voltage. 12 AWG (rated 20A): comfortable current capacity, 50 ft drops about 2% voltage. Best choice: 12 AWG heavy-duty cord.
Better practice: do not use an extension cord for space heaters at all. NEC recommends permanent wiring for any continuously-operated appliance over 1,200W. Extension cords are explicitly prohibited for space heaters by most manufacturers and by good practice. If you need a heater in a location without a nearby outlet, add an outlet — it is cheaper in the long run than the fire risk.
Worked Example: Outdoor Power Tools
A construction site needs a 100-foot extension cord to power a circular saw (15A continuous) and occasional drill (7A).
Worst case load: the saw at 15A. Cord sizing for 100 ft at 15A: 12 AWG gives about 4% voltage drop — acceptable but marginal. 10 AWG gives 2.5% drop — better for continuous use. Choose 10 AWG outdoor-rated (SJTW or SOOW jacket) 100-ft cord.
Construction-grade cords are rated for rugged use, moisture, and impact. Typical cost: 100-ft 10 AWG SJTW cord is about 80-120 USD, 12 AWG about 50-70 USD. The upgrade to 10 AWG is worth it for daily job-site use because a saw running at 10% low voltage (115V instead of 120V) cuts slower and strains the motor, shortening tool life.
For generators on remote sites, the same sizing applies. A 100-ft run from a 5,000W generator to a 15A saw through 12 AWG cord can cause noticeable voltage sag at tool startup.
Extension Cord Mistakes
1. Daisy-chaining cords. Connecting multiple extension cords in series multiplies voltage drop and increases connection failure points. UL and NEC prohibit this practice. Use one cord of appropriate length.
2. Running cords through walls or ceilings. Extension cords are not approved for building wiring and violate NEC when run through structures. For permanent needs, install permanent wiring.
3. Coiling cord while in use. A coiled cord traps heat and can exceed its insulation temperature rating even at rated current. Uncoil cords fully during use, especially for high loads.
4. Using indoor cords outside. Indoor-rated cords (SVT, SPT) deteriorate quickly in sunlight, moisture, and temperature extremes. Outdoor use requires SJTW, SOOW, or similar weather-rated cords.
5. Ignoring the load label. Every cord has a printed maximum load (typically 13A or 15A). Exceeding the rating overheats the cord. Hair dryers, space heaters, and microwaves together can easily exceed the rating.
Cord Sizing Quick Reference
25 ft cord:
Under 7A load: 16 AWG okay.
7-10A: 16 AWG minimum, 14 AWG preferred.
10-13A: 14 AWG.
13-15A: 12 AWG.
50 ft cord:
Under 7A: 16 AWG.
7-10A: 14 AWG.
10-13A: 12 AWG.
13-15A: 12 AWG or 10 AWG.
100 ft cord:
Under 7A: 14 AWG.
7-10A: 12 AWG.
10-13A: 10 AWG.
13-15A: 10 AWG or 8 AWG.
150 ft cord: Rarely advisable. Consider permanent wiring instead. If required: 10 AWG minimum, 8 AWG for loads over 10A.
Cord Type Codes
SJT: Light-duty indoor, thermoplastic jacket. Ok for occasional light loads.
SJTW: Weather-rated outdoor. W = weather-rated.
SJOW: Oil-resistant outdoor. For workshop or garage use.
SJOOW: Oil-resistant both jacket and insulation, heavy duty.
SOOW: Heavy-duty oil-resistant, industrial use.
STW or STOW: Thermoset insulation, high-temperature outdoor.
Cord amperage ratings: SJT typically 10-13A. SJTW typically 13-15A. SJOW/SJOOW typically 15-20A. SOOW typically 20-30A. Always check the printed rating on the cord itself.
Safety standards: UL 62 (flexible cords), UL 817 (cord sets), NEC 400 (flexible cords usage). OSHA 1926.405 governs temporary wiring at construction sites including extension cord requirements.
Extension cord gauge: matching cord to load and length without starting a fire
Extension cords cause more home fires than people think. Most failures trace to one of three things: overloading (15 A heater on a 16-gauge cord rated 13 A), undersizing for length (10-foot cord works fine, the 100-foot version of the same gauge does not), or daisy-chaining cords. The math here is the same as fixed wiring, just with a much shorter list of acceptable answers.
The calculator gives the minimum cord gauge for your load and length using the same NEC 310 ampacity logic and the 3 percent voltage-drop guideline that fixed wiring uses, plus the flexible-cord allowance from NEC Article 400.
The formula and what it does
NEC Table 400.5(A) lists allowable ampacity for flexible cords (SJOOW, SOOW, etc.). These are derated below comparable in-conduit conductors because cords spend time coiled and have less heat dissipation. 16 AWG SJOOW: 13 A. 14 AWG: 18 A. 12 AWG: 25 A. 10 AWG: 30 A.
Worked example
Scenario: 15 A circular saw on a 75 ft cord, 120 V.
Ampacity check: 15 A continuous load (saw under cut is intermittent peak but lock-rotor is high) needs at least 14 AWG per Table 400.5(A). Voltage drop check: 75 ft x 12.9 x 15 x 2 / CM at 3 percent of 120 V (3.6 V allowed). CM required = 2 x 12.9 x 15 x 75 / 3.6 = 8,063. 10 AWG (10,380 CM) clears, 12 AWG (6,530 CM) does not. The voltage-drop limit pushes you to 10 AWG even though ampacity alone says 12 AWG.
Common mistakes to avoid
Coiling the cord while in use. A coiled cord under load builds heat faster than a stretched-out one because the surface area for cooling is reduced. Stretch out cords during heavy use; pull excess into loose figure-eights, not tight coils.
Trusting the cord nameplate without checking length. A 16-gauge cord rated 13 A is rated at short length. At 100 ft, voltage drop and heat buildup make that same cord unsafe at 13 A.
Daisy-chaining cords. Each cord connection is a potential failure point and a place for voltage drop to accumulate. OSHA 1926.405 prohibits daisy-chained cords on jobsites for good reason.
Frequently asked questions
What gauge cord do I need for a window AC unit?
Window ACs typically draw 8-15 A. Use 14 AWG for runs under 50 ft, 12 AWG for 50-100 ft. Never run a window AC through an extension cord longer than 100 ft, and never share that cord with anything else.
Why are construction cords so much heavier than household cords?
They are usually SOOW or SJOOW jacket (oil-resistant, abrasion-resistant) rated for harsh environments. The conductor inside is the same copper, but the jacket is thicker for durability under foot traffic and weather.
Can I use a cord with an EV charger?
Manufacturers explicitly say no. The 8 A draw on a Level 1 EVSE for 8-12 hours overnight is exactly the worst case for cord heating. Plug an EVSE direct into a wall receptacle on its own circuit.
What is the longest cord I can use for a hair dryer?
A 1500 W hair dryer pulls 12.5 A. On 14 AWG you can run 40 ft. On 12 AWG, 75 ft. Beyond that, voltage drop slows the heating element enough that the dryer runs hot and slow. Manufacturers usually warn against extension cord use entirely.
How do I tell if a cord is rated for outdoor use?
The jacket marking. SJTW or SJOW (where W means weather-resistant) is rated for outdoor and damp locations. SJT or SJ is indoor-only. The W is the only letter that matters for water rating.
Should I unplug cords when not in use?
For high-draw appliances yes, since a connected cord is a parasitic load on the receptacle (slight) and a fault risk. For low-draw electronics on a power strip, the savings are negligible.