Long runs cost voltage. The NEC recommends keeping branch-circuit drop under 3%, and the arithmetic is short enough to keep in a spreadsheet next to your wire chart.
A 120 ft run at 20 A on #12 copper (1.93 Ω/1000 ft) at 240 V drops 9.26 V — 3.86%, over the 3% guideline.
The example
Three runs on the same job, each with its own length, load, and conductor.
| A | B | C | D | E | F | |
|---|---|---|---|---|---|---|
| 1 | Run | Length (ft) | Amps | Ohms/1000ft | Volts | Drop % |
| 2 | Shop feeder | 120 | 20 | 1.93 | 240 | 3.86% |
| 3 | Well pump | 220 | 12 | 1.21 | 240 | 2.66% |
| 4 | Yard lights | 90 | 6 | 3.07 | 120 | 2.76% |
The formula
The 2 accounts for the current travelling out and back; the 1000 converts the resistance table units:
How it works
Build it one factor at a time:
2*B2doubles the run length — current flows down the hot and back on the neutral, so the wire is twice as long electrically.*C2*D2/1000applies Ohm's law: amps times total resistance gives volts lost. Dividing by 1000 matches the per-1000-feet units in the wire table./E2turns volts lost into a fraction of supply voltage.ROUND(...,4)keeps four decimals so the percent format shows two.
For three-phase runs replace the 2 with 1.732. Everything else in the formula stays put.
Try it: interactive demo
Enter the run length, load, conductor resistance, and system voltage.
Variations
Flag runs over 3%
Let the sheet tell you when to go up a wire size.
Volts lost instead of percent
Sometimes the raw number is what the inspector wants.
Pitfalls & errors
Enter the one-way run length. The formula already doubles it — entering the round trip doubles the drop and sends you up two wire sizes for nothing.
Aluminium has roughly 1.6 times the resistance of copper of the same size. Keep separate resistance columns rather than fudging one number.
Practice workbook
Frequently asked questions
Is 3% a hard code limit?
Where do the ohms-per-1000-feet numbers come from?
Stop fighting formulas. Learn them in a day.
This recipe is one of hundreds of real-world formulas we teach. Our Excel Formulas & Functions class covers lookups, logic, text, and dynamic arrays hands-on — live in Dallas–Fort Worth, Houston, Austin, Oklahoma City, Denver, or online.
See the Formulas & Functions Class