Extension Cord Size Calculator
Pick a practical minimum cord gauge from load amps, length, and voltage drop
Use this extension cord size calculator when you need a temporary cord for a tool, pump, appliance, or backup-power setup and want a safer starting gauge before you buy. Enter the load amps, cord length, voltage, and target voltage-drop limit to estimate the minimum cord gauge. This tool is for temporary extension-cord planning only, not permanent wiring. For a deeper conductor check, use the voltage drop calculator. If nuisance trips are part of the problem, see Why Does My Breaker Keep Tripping?. For heater-sized loads, compare the draw first with How Many Amps Does a 1500W Space Heater Draw?.
Recommended minimum cord: 10 AWG extension cord
Estimated voltage drop: 2.00% (2.40 V)
Ampacity starting point is 14 AWG, but length pushes this example to 10 AWG.
How to choose an extension cord gauge
Extension cord sizing is mostly about two things: amp capacity and voltage drop. A light-duty cord can overheat on a heavy load, and a cord that is technically rated for the amps can still perform poorly if it is too long and too thin for the distance.
That is why a cord that feels fine for a short drill run can become a problem at 100 feet with a shop vac, circular saw, sump pump, or outdoor tool. The farther the power has to travel, the more voltage the cord can lose before it reaches the load.
This calculator gives a practical minimum cord gauge for temporary use. It does not mean an extension cord is always the right solution. High-draw heaters, fixed appliances, and permanent setups usually need a proper branch circuit instead.
- Load amps: heavier loads need thicker cord conductors
- Cord length: longer runs need more copper to control voltage drop
- Supply voltage: 120V loads are more sensitive to cord voltage drop than the same wattage at 240V
- Target drop limit: a tighter target pushes you toward a heavier gauge sooner
When load amps and cord length both climb, go heavier sooner. It is often the long cord, or the harder-starting tool, that changes the answer.
How to convert
Step 1: Find the ampacity starting point
The cord gauge has to handle the load current before anything else.
use the first gauge whose amp limit is at or above the load amps
Step 2: Check voltage drop
Longer cords can force the gauge larger even when the amp load stays the same.
drop volts = (2 x length x amps x resistance) / 1000
drop percent = (drop volts / system voltage) x 100
Step 3: Choose the smallest gauge that passes both checks
The practical answer is the first cord gauge that satisfies current and drop together.
minimum cord gauge = first gauge that passes ampacity and drop target
Worked examples
Question: What cord size for a 12A tool at 100 ft on 120V with a 3% target?
Solution: Ampacity starting point = 14 AWG. 14 AWG drop = (2 x 100 x 12 x 2.525) / 1000 = 6.06 V = 5.05%. That misses 3%, so go heavier. 12 AWG drop = 3.81 V = 3.17%. Still high. 10 AWG drop = 2.40 V = 2.00%. Recommended minimum = 10 AWG.
Question: What cord size for a 5A load at 50 ft on 120V with a 5% target?
Solution: Ampacity starting point = 16 AWG. 16 AWG drop = (2 x 50 x 5 x 4.016) / 1000 = 2.01 V = 1.67%. That passes both checks, so 16 AWG is the practical minimum.
Question: What cord size for a 15A shop-vac load at 25 ft on 120V with a 3% target?
Solution: Ampacity starting point = 14 AWG. 14 AWG drop = (2 x 25 x 15 x 2.525) / 1000 = 1.89 V = 1.58%. That passes the 3% target, so 14 AWG is the practical minimum for this short run, though many homeowners still step up to 12 AWG for a little more margin on hard-starting tools.
Common mistakes and notes
- Do not treat an extension cord as permanent wiring for a room, shed, or appliance.
- Space heaters and other high-draw heaters are usually best plugged directly into a wall outlet unless the manufacturer says otherwise.
- Outdoor use also requires the right cord jacket and weather rating, not just the right gauge.
- If a tool has a harder startup or the cord will be used repeatedly near its limit, one size heavier than the bare minimum can be a reasonable planning choice.
- If a cord, plug, or connection gets hot in normal use, stop and correct the setup before using it again.
Assumptions
- Uses common copper extension-cord gauge starting points for homeowner planning.
- Treats the entered length as one-way cord length and checks round-trip voltage drop internally.
- Results are planning estimates only. Follow equipment instructions and cord labeling before use.
Worked example
Example: A 12A load at 100 ft on 120V starts at 14 AWG for ampacity, but voltage drop pushes the practical minimum to 10 AWG.
FAQ
What gauge extension cord do I need for 15 amps?
It depends on length and how hard the load starts. A short 15A run may still work on 14 AWG, but 12 AWG is often the more comfortable choice for full-load tools, and longer runs often push the answer to 12 AWG or 10 AWG once voltage drop is included.
Is 16 AWG okay for a space heater or other 15A load?
No as a general planning answer. 16 AWG is a light-duty cord and is not the normal choice for a full 15A load. High-draw heaters usually should not be on extension cords at all unless the manufacturer specifically allows it.
Does a longer extension cord need a heavier gauge?
Yes. Length increases resistance, and that increases voltage drop. A cord that is fine at 25 feet can be too light at 100 feet for the same load.
Can I use this for 240V equipment?
Yes for a planning estimate, as long as the cord and equipment are actually rated for 240V use. The math still checks current and voltage drop, but the plugs, connectors, and equipment listing still control the real setup.
What if the cord or plug feels warm?
Stop using it and correct the setup. Warmth can be a sign that the cord is undersized, damaged, loosely connected, or being used with a load it should not carry.
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⚠️ Sanity Check Only
This tool provides informational estimates only. It is not professional engineering advice. Electrical work is dangerous and governed by strict local codes.
Before you start:
- Verify these results with a licensed electrician.
- Cross-reference with the latest Electrical Code (NEC/CEC).
- Never work on live circuits.