Voltage Drop Calculator
Find how much voltage a run of wire loses at a given current, and whether that loss is acceptable.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
This simplified calculator estimates resistive drop in an AWG conductor pair using fixed copper/aluminium constants and a round-trip path. The displayed maximum length is the length at this model’s selected 3% reference; it is not a code-compliance check. Installation method, conductor temperature, load and local rules affect real sizing. Have a qualified electrician verify an actual circuit.
Worked example
calculated with the default values below
Supply voltage 120 V · Current 15 A · One-way wire length 100 ft · Wire gauge 12 AWG · Conductor material Copper Voltage drop 4.94 % · Voltage lost 5.93 V · Voltage at the load 114.07 V · Round-trip resistance 0.3951 Ω · Length at a 3% drop 61 ft Acceptable but tight
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
R = 2KL÷CM; ΔV = I×R; pct = ΔV÷V×100; L_max = 0.03V÷(I×R÷L)
- I
- Current (A)
- L
- One-way length (ft)
- K
- Resistivity constant
- CM
- Circular mils (gauge)
- ΔV
- Voltage drop (V)
- pct
- Drop (%)
Uses fixed K values (copper 12.9, aluminium 21.2) and listed AWG circular-mil areas; length is one-way and loop resistance doubles it. The maximum-length output uses a model-selected 3% reference only; it is not a code check. Actual design depends on installation and local rules.
Frequently asked questions
What does the 3% maximum-length result mean?
It is the length at which this simplified resistive model reaches its selected 3% reference using fixed material constants and AWG areas. It is not a code-compliance determination or universal acceptable limit. Have a qualified electrician check real circuit design against local requirements.