Voltage Drop Calculator
Estimate voltage drop, receiving voltage, conductor loss and annual loss cost from circuit length, current and conductor cross-sectional area.
Default project preview
3.33 %
Receiving voltage
116 V
Circuit resistance
0.1998 Ω
Conductor loss
79.9 W
Annual loss energy
233.4 kWh
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Resistance = conductor resistivity × one-way length × path factor ÷ cross-sectional area. Voltage drop = current × resistance. Single-phase uses a two-conductor path; three-phase uses √3.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Voltage Drop Calculator questions
Why does conductor temperature matter?+
Conductor resistance rises with temperature, increasing voltage drop and power loss.
Does low voltage drop mean the wire is safe?+
No. Conductor ampacity and protection depend on many code and installation conditions beyond voltage drop.
What is the material factor?+
Use 1 for the copper reference or about 1.64 for aluminium as a rough resistivity multiplier.
Project guide
Use this calculator with confidence
Last reviewed: July 19, 2026
The voltage-drop calculator estimates conductor voltage loss from current, one-way distance, system voltage, material and conductor size. It is a planning and comparison tool that helps show how longer runs and higher current increase loss. It does not select conductor ampacity, insulation type, temperature rating, conduit fill, fault protection or code compliance; those decisions require the applicable electrical rules and qualified review.
How to use it
- 1Enter the expected operating current rather than only breaker rating.
- 2Measure one-way circuit distance; the formula accounts for the return path.
- 3Select the correct conductor material and size.
- 4Compare voltage and percentage drop while separately checking ampacity and code.
Worked example
For a 120 V circuit carrying 15 A over a long copper run, increasing conductor cross-section lowers resistance and therefore voltage drop. The calculator exposes that relationship without treating a low drop result as installation approval.
What the defaults mean
Operating current
Should reflect the load being analyzed.
One-way length
The circuit formula handles outgoing and return conductors.
Material resistivity
A planning value that changes with temperature.
| Change | Effect on resistance/drop | Reason |
|---|---|---|
| Longer run | increases | more conductor length |
| Higher current | increases | greater I × R loss |
| Larger conductor | decreases | more cross-sectional area |
| Higher temperature | generally increases | higher conductor resistance |
Common measurement mistakes
- Entering round-trip distance when the tool asks for one-way distance.
- Using breaker size instead of actual load.
- Checking voltage drop but not conductor ampacity.
- Ignoring starting current or sensitive equipment requirements.
Limits and safety
- Not electrical design, code or safety approval.
- Does not calculate fault current, protection or conductor derating.
- Use qualified electrical guidance for final conductor selection.
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