Voltage Drop Calculator
Calculate voltage drop in an AC or DC circuit given wire gauge, voltage, current, and length. Also find the correct conductor size for your run.
Last updated 2026-08-21
What Is a Voltage Drop Calculator?
A voltage drop calculator tells you how many volts a circuit loses along the conductor over a given distance. It converts that loss to a percentage of the supply voltage so you can judge whether a run is acceptable. Long runs and small wire cause the biggest drops, and code recommends keeping them in check.
How It Works
Drop (V) = (2 × 12.9 × Current × Distance) ÷ cmil. Drop % = Drop ÷ Voltage × 100.
Example
20 A over 100 ft of 12 AWG at 120 V: 7.9 V drop ≈ 6.6%.
How to Calculate
- Enter the current, say 20 A, and the distance, for example 100 ft one way.
- Choose the wire size you plan to use, such as 12 AWG copper.
- Enter the voltage, commonly 120 V. The tool calculates the drop with the formula 2 × 12.9 × current × distance divided by the circular mil area of the wire.
- For this example the drop works out to about 7.9 V, which is roughly 6.6% of 120 V. Since that exceeds the 3% branch circuit recommendation, switch to a larger wire or shorten the run.
Recommended voltage drop limits
| Circuit type | Maximum drop | Source |
|---|---|---|
| Branch circuit | 3% | NEC recommendation |
| Feeder circuit | 3% | NEC recommendation |
| Total from service to outlet | 5% | NEC recommendation |
| Sensitive electronics | 1 to 2% | Manufacturer guidance |
Sources
Frequently Asked Questions
What is an acceptable voltage drop?
NEC recommends no more than 3% on a branch circuit and 5% total from the service.
Which wire sizes are in the list?
Standard AWG sizes from 14 AWG up to 4/0 are included in the lookup table.
How do I fix excessive drop?
Use a larger wire, shorten the run, or raise the circuit voltage.
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