When voltage drop forces a larger AWG
If you only look up an ampacity chart, long runs will quietly size you one gauge too small.
How to run this check
- Get the smallest size that clears ampacity.
- Check that size for voltage drop at the real one-way length.
- If it is over target, go up one standard size and check again.
The two-check method
8 AWG copper has a 75 °C effective planning ampacity of 50 A, so 40 A often clears ampacity.
The same 8 AWG on 180 ft, 240 V, 40 A drops about 11.3 V (4.7%). That is why this site prints “What limited this result.”
What to do after a size-up
A larger conductor can create a new conduit-fill or termination-range problem. After Wire Size sizes up, open Conduit Fill with the new AWG.
Do not treat 3% as a law. Treat it as the target you entered, then verify the current code, the equipment nameplate, and the AHJ.
240 V, 40 A, 180 ft, copper, 3% target
| Check | 8 AWG | Recommended |
|---|---|---|
| Ampacity | 50 A planning value | 6 AWG |
| Voltage drop | 11.3 V / 4.7% | 7.1 V / 2.9% |
| Governing limit | Over target | Voltage drop |
Related
FAQ
Why did the calculator pick a larger AWG than the chart?
Voltage drop. Ampacity can pass while the long-run percent fails.
Should I always upsize when the run looks long?
No. Enter the feet. A 240 V feeder and a 120 V branch of the same length are not the same problem.
Planning reference only. Verify against the current NEC, manufacturer data, and the authority having jurisdiction. This is not engineering advice or a license substitute.