How to check voltage drop on a long run
A long run is not a special formula. It is the same voltage-drop check with honest footage.
Open the voltage drop calculator
How to run this check
- Measure one-way feet from source to load, not the coil on the truck.
- Enter voltage, load current, material, and the AWG already chosen.
- Compare the percent with the target you are using on that job.
- If it fails, go one size up and check fill before you buy the spool.
What “long” actually means
There is no universal foot count that makes a run long. 80 ft on 120 V can be worse than 150 ft on 240 V.
The 40 A / 180 ft example on this site is a long run because 8 AWG lands at 4.7%, not because 180 is a magic number.
Keep ampacity and voltage drop separate
Ampacity asks whether the conductor can carry the current. Voltage drop asks what the load still sees at the far end.
Mixing them into one mental chart is how people undersize feeders to sheds, shops, and subpanels.
Same 40 A copper load, different jobs
| Job | Tool to open | What you learn |
|---|---|---|
| Need the next size | Wire Size | This example recommends 6 AWG |
| Already have 8 AWG | Voltage Drop | Over target |
Related
FAQ
Do I double the length before I enter it?
No. Enter one-way feet. The calculator already uses the 2-wire or √3 path.
Is 3% a code requirement?
No. It is a common planning target. Your equipment and AHJ decide the real limit.
Planning reference only. Verify against the current NEC, manufacturer data, and the authority having jurisdiction. This is not engineering advice or a license substitute.