Voltage Drop Calculator
Check whether a cable run is too long for the load.
Enter your values
Enter the supply voltage in V.
Enter the current in A.
Enter the one-way length in ft.
Choose one of: 14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG, 4 AWG.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Check whether a cable run is too long for the load. |
|---|---|
| Category | Science |
| Inputs needed | Supply voltage, Current, One-way length, Wire size |
| Main output | Voltage drop |
| Formula | Vdrop = 2 × L × I × R/ft |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Supply voltage(V)
- Enter the supply voltage in V.
- Current(A)
- Enter the current in A.
- One-way length(ft)
- Enter the one-way length in ft.
- Wire size
- Choose one of: 14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG, 4 AWG.
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.
What do I need to enter into the Voltage Drop Calculator?
Just 4 values: supply voltage, current, one-way length, wire size. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Voltage Drop Calculator work out the answer?
Voltage drop is 2 × length × current × resistance per foot for a DC or single-phase run; keep it under 3% of supply voltage. It applies the formula Vdrop = 2 × L × I × R/ft and shows the working so you can check each step by hand.
Is the Voltage Drop Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.