Voltage Drop Calculator
You already have a size. See if the run stays inside your voltage-drop target.
What this calculator answers
It tells you the volts and percent of drop for a known AWG, then whether that size stays inside your target.
How to read the percent
Compare drop in volts to your target percent of system voltage.
Inside target means the known size is acceptable under these assumptions.
Over target means consider the suggested larger size, then verify terminations and ampacity.
This is not a promise that 3% is required by the NEC.
Example: 6 AWG copper, 240 V, 30 A, 80 ft
Single-phase planning math, K = 12.9, 6 AWG = 26240 CM:
Voltage drop ≈ 2 × 12.9 × 30 × 80 / 26240 ≈ 2.4 V, about 1.0% of 240 V.
Example labels: Voltage drop about 2.4 V. Percent about 1.0%. Vs 3% target: Inside target. Suggested larger size: none required by this voltage-drop check.
Still verify ampacity, continuous load, and the AHJ. A short 30 A run can pass voltage drop and still fail another rule.
Related tools
Guides for this tool
Voltage drop FAQ
Do I enter one-way feet or the round trip?
Enter one-way feet. The single-phase formula already uses a 2-way path.
Is 3% required by the NEC?
No. It is a common planning target, not a blanket requirement.
Why is my drop higher on 120 V than 240 V?
The same volts of drop are a larger percent of a 120 V system.
Does this replace the wire size calculator?
No. Use this when the size is known. Use the wire size calculator when you need the next AWG.
Is this free?
Yes. The voltage drop calculation stays free. You do not need an account.