Voltage Drop Calculator
Check whether a cable run is too long for the load.
Enter your values
ⓘ Live results
Results
- Voltage drop
- 4.764 V
- Percent drop
- 3.97%
- Voltage at the load
- 115.24 V
- Within 3% recommendation?
- No — use a thicker wire or shorten the run
Quick answer
Voltage drop is 2 × length × current × resistance per foot for a DC or single-phase run; keep it under 3% of supply voltage.
Vdrop = 2 × L × I × R/ft
How to use the Voltage Drop Calculator
- 1Enter supply voltage and load current.
- 2Enter the one-way cable length.
- 3Pick the wire gauge and check the percent drop.
Worked example
Using the values the calculator loads with:
Inputs
- Supply voltage120 V
- Current15 A
- One-way length100 ft
- Wire size12 AWG
Results
- Voltage drop4.764 V
- Percent drop3.97%
- Voltage at the load115.24 V
- Within 3% recommendation?No — use a thicker wire or shorten the run
Frequently asked questions
Why 3%?
The NEC recommends a maximum 3% drop on branch circuits and 5% total, so equipment receives adequate voltage.
Does aluminium differ?
Yes — aluminium has about 60% more resistance, so you generally go up two wire sizes for the same run.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.