100 ft Extension Cord Voltage Drop
Plain-English answer
A 100 ft extension cord is long enough that voltage drop often changes the answer in a real way. At that distance, a cord that looks acceptable for current alone can still be too light for good performance.
For many 120V loads, 14 AWG is already too weak at 100 ft once the current climbs. 12 AWG can still be marginal on heavier loads. 10 AWG often becomes the more comfortable starting point for 12A to 15A temporary runs.
The key issue is not only heat. Long cords also steal voltage before it reaches the tool, pump, or heater, and that can show up as weaker startup, poorer performance, and extra stress on the equipment.
Quick starting point
Homeowners often think of 100 feet as just "a longer cord." Electrically, it is a different class of setup. The same 12A or 15A load that feels fine on a short cord can behave much worse at 100 feet, especially on 120V tools and motor loads.
That is why 100-foot planning is less about the label on the breaker and more about the combination of current, cord gauge, and how sensitive the load is to reduced voltage at the far end.
Worked examples
Example 1: 12A load on a 100 ft cord at 120V
Use a common 14 AWG resistance of 2.525 ohms per 1000 ft:
14 AWG drop = (2 x 100 x 12 x 2.525) / 1000 = 6.06V
That is about 5.05% drop. Move to 12 AWG and the drop becomes about 3.18%. Move to 10 AWG and it lands around 2.00%. That is why a 100-foot 12A tool run often pushes the better answer toward 10 AWG if you want a tighter result.
Example 2: 15A load on a 100 ft cord at 120V
Now raise the load to a full 15A:
12 AWG drop = (2 x 100 x 15 x 1.588) / 1000 = 4.76V
That is about 3.97% drop. On 10 AWG, the same load drops about 2.50%. That is why a full 15A run at 100 feet often shifts from 12 AWG on short runs to 10 AWG on long ones.
Common 100 ft cord cases at a glance
| 100 ft temporary setup | Typical load | Voltage-drop pressure | Practical starting point |
|---|---|---|---|
| Light-duty electronics or lights | Low amps | Usually low | 16 AWG or 14 AWG may still work |
| Moderate 120V tool load | Around 10A | Noticeable | 12 AWG often better than 14 AWG |
| Heavier 12A to 15A 120V run | High temporary load | Strong | 10 AWG often the better starting point |
| Hard-starting motor or heater-like load | High and demanding | High plus performance risk | Heavier cord or change approach |
These are practical planning ranges. Actual cord resistance, supply voltage, equipment sensitivity, and real startup behavior can still move the answer.
Use both calculators for the real decision
Start with the Extension Cord Size Calculator when you are choosing a temporary cord. Then use the Voltage Drop Calculator if you want to test the voltage-loss side directly with a specific load and conductor resistance.
What changes the answer?
- Load current: the difference between 5A and 15A at 100 feet is huge.
- 120V vs 240V: the same voltage drop hurts a 120V setup more in percentage terms.
- Load type: motors and compressors can feel drop more sharply than simple electronics.
- Cord quality and terminations: cheaper cords and tired plugs can make a marginal setup worse.
- Supply conditions: a generator or already-low supply voltage leaves less room for a long cord to waste any more voltage.
When not to use a 100 ft extension cord
If the cord will be used often, if the load is heavy every time, or if the equipment struggles to start on the long run, it is time to change approach. A closer receptacle, shorter run, or proper branch circuit is usually smarter than forcing a long cord to do permanent-duty work.
Stop and rethink the setup if the cord gets warm, the tool sounds strained, lights dim, or breakers trip. Long runs are exactly where small electrical compromises turn into frustrating real-world problems.
Related cord and voltage-drop tools
FAQ
Is 100 feet too long for an extension cord?
Not automatically, but it is long enough that voltage drop becomes a real planning issue. The heavier the load, the more seriously you need to treat cord gauge at that distance.
What gauge is better for 100 feet at 15 amps?
For many 120V full-load temporary runs, 10 AWG is often the better starting point at 100 feet. That is where 12 AWG can start feeling marginal.
Why do long cords make motors struggle?
Because the cord can steal voltage before it reaches the motor. Lower delivered voltage can make startup weaker and increase the current stress on the equipment.
Can I just accept the voltage drop if the cord is temporary?
Sometimes on lighter loads, yes. But if the equipment is heavy, sensitive, or already near the edge, "temporary" does not make a weak electrical setup harmless.
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.