What Gauge Extension Cord for 20 Amps?
Plain-English answer
For a true 20A temporary load, the common short-run planning answer usually starts at 12 AWG. Once the cord gets longer, the load starts harder, or you are trying to run near the full rating, 10 AWG often becomes the better answer.
For long 100-foot runs or hard-starting equipment, even 10 AWG can be the more comfortable choice over 12 AWG, and sometimes the better move is to stop relying on an extension cord at all.
The important distinction is that a 20A breaker or 20A receptacle does not automatically make a lighter cord safe. The cord still has to handle the actual current and the voltage-drop hit from the run length.
For a quick load-and-length check, use the Extension Cord Size Calculator.
Quick starting point
This question usually comes up around heavier temporary equipment such as compressors, pressure washers, table saws, or jobsite tools. At that point, you are no longer in casual lamp-cord territory. Even when the plug and receptacle look ordinary, the current is high enough that a weak or overly long cord can show up quickly as heat, weak startup, or nuisance trips.
For practical planning, short 20A temporary runs often start at 12 AWG. Longer or harder-working setups often move to 10 AWG. If the load is truly near 20A for long periods, the safest answer may be a closer outlet or a proper branch circuit instead of a long cord.
Worked examples
Example 1: 16A tool at 50 ft on 120V
Say a tool draws 16A and the cord length is 50 ft. With 12 AWG resistance around 1.588 ohms per 1000 ft:
V drop = (2 x 50 x 16 x 1.588) / 1000 = 2.54V
That is about 2.12% drop on 120V, which is why 12 AWG still works as a good short-to-medium-run answer for many 20A-style temporary loads.
Example 2: 18A load at 100 ft on 120V
Now take an 18A temporary load and stretch the run to 100 ft.
12 AWG drop = (2 x 100 x 18 x 1.588) / 1000 = 5.72V
That is about 4.76% drop, which is pretty soft for a near-full load. Move to 10 AWG and the drop becomes about 3.00%. That is why 100-foot runs near 20A often point toward 10 AWG, not 12 AWG.
Common 20A cord cases at a glance
| Temporary setup | Typical load | Distance effect | Practical starting point |
|---|---|---|---|
| Short 25 ft heavy tool run | 15 to 16A | Usually modest | 12 AWG |
| 50 ft compressor or pressure washer | 16 to 18A | Noticeable on startup | 12 AWG, sometimes 10 AWG for margin |
| 100 ft near-full 120V run | 18 to 20A | Strong voltage-drop effect | 10 AWG often the better answer |
| Repeated near-20A daily use | High, sustained load | Heat and wear add up | Usually change approach instead of leaning on a cord |
These are homeowner planning baselines only. Cord listing, plug style, equipment instructions, and the actual load profile still control the real setup.
Use the calculators before you commit to a cord
The Extension Cord Size Calculator is the quickest way to test your actual amps and length. If you want to inspect the voltage-loss side more directly, use the Voltage Drop Calculator. If you are already nuisance-tripping a breaker, compare the load with the Breaker Size Calculator too.
What changes the answer?
- Actual current draw: a tool that peaks near 20A is a different problem from one that only runs around 13A.
- Motor startup: compressors, saws, and other hard-starting equipment can be less tolerant of a thin or long cord.
- Cord length: the 25-foot answer is not the 100-foot answer.
- Plug and cord rating: even if the branch circuit is 20A, the cord and connectors must also be suitable for that use.
- Heat, coiling, and outdoor conditions: a cord used in hot sun, left coiled, or dragged across a jobsite is not working under ideal conditions.
When not to use an extension cord
Do not treat a long extension cord as the permanent answer for a garage tool, freezer, heater, or shed setup. That is especially true if the equipment runs near full load, if the cord will stay in place, or if the same setup is used every day.
Change approach if the equipment manual wants a dedicated circuit, if the cord gets warm, if the motor struggles to start, or if the breaker is already tripping. In those cases, a closer outlet, shorter run, or proper circuit upgrade is the better move.
Related extension-cord planning tools
FAQ
Is 12 AWG enough for 20 amps?
Often yes as a short-run starting point, which is why it is the common first answer. But length and startup behavior can still push the better practical answer to 10 AWG.
Can I use 14 AWG on a 20A circuit if the load is temporary?
Not as the normal planning answer for a true 20A extension-cord setup. If you are really near 20A, 14 AWG is too light to treat casually.
What gauge is better for 20 amps at 100 feet?
At 100 feet, 10 AWG is often the better starting point for a near-full 120V load because voltage drop changes the answer quickly.
Does the breaker size decide the cord gauge by itself?
No. The cord choice also depends on the actual current, the cord length, the load type, and whether the equipment is sensitive to voltage drop.
Disclaimer: Results are informational estimates for learning and planning only. Always follow the applicable electrical code and consult a qualified licensed electrician for safety-critical work.