Electrical calculator
Voltage Drop Calculator
Calculate voltage drop in a wire run based on length, gauge, and load.
About the voltage drop calculator
The voltage drop calculator calculates how much voltage is lost along a wire run, helping you ensure adequate voltage at the load.
How to use this calculator
- 1
Enter system voltage
Enter the nominal system voltage (e.g., 120V or 240V).
- 2
Enter current and length
Enter the circuit current in amps and wire run length in feet.
- 3
Choose wire gauge and material
Select AWG size and copper or aluminum.
How the calculation works
Voltage drop = 2 × length × current × resistance per ft. NEC recommends ≤3% for branch circuits.
Example: 120V, 15A, 100 ft, 12 AWG copper: ~3.9V drop (3.2%).
Safety first
These calculators provide estimates based on standard formulas and NEC guidelines. Always verify results with a licensed electrician before making installations. Electrical work can be dangerous — never work on live circuits, and always follow local building codes and regulations.
Understanding electrical units
Voltage (V) is electrical pressure, current (A) is flow rate, resistance (Ω) opposes flow, and power (W) is the rate of energy use. Ohm's law (V = I × R) relates the first three, while power formulas (P = VI, P = I²R) connect all four. Understanding these relationships is key to safe electrical design.
Important limitations
This tool provides a mathematical estimate based only on the values entered. It does not account for regulations, taxes, lender fees, market conditions, or individual circumstances. Results are for planning purposes only — consult a qualified professional for regulated, medical, financial, or construction decisions.
Frequently asked questions about voltage drop calculator
How accurate is the voltage drop calculator?+
The calculation follows the displayed formula and retains full numeric precision before formatting the result.
Can I use the result professionally?+
Use it as a planning estimate. Confirm regulated, medical, financial, or construction decisions with a qualified professional.