Calculator
Example Data Table
| Supply (V) | Current (A) | Length | Gauge | Material | Phase | Target Drop |
|---|---|---|---|---|---|---|
| 120 | 8 | 40 ft | 14 AWG | Copper | Single | 3% |
| 120 | 12 | 75 ft | 12 AWG | Copper | Single | 3% |
| 230 | 10 | 30 m | 14 AWG | Copper | Single | 5% |
Use these as starting points, then refine with your tool nameplate current.
Formula Used
This calculator estimates wire resistance from a gauge table, then adjusts it for temperature. Resistance per foot is corrected using:
- R(T) = R(20°C) × (1 + α × (T − 20))
- α is the temperature coefficient for the selected material.
After resistance is known, voltage drop is computed using common wiring approximations:
- Single-phase: Vdrop = I × R × (2L)
- Three-phase: Vdrop ≈ √3 × I × R × L
Percent drop and delivered voltage are then:
- %drop = (Vdrop / Vsupply) × 100
- Vload = Vsupply − Vdrop
- Cord heating loss: Ploss = I² × Rtotal
How to Use This Calculator
- Enter your supply voltage and the tool current rating.
- Enter the cord length and pick feet or meters.
- Select gauge and conductor material from the list.
- Choose phase, then set temperature and target drop.
- Press Calculate to view results above the form.
- Use CSV or PDF buttons to save your report.
Why Voltage Drop Matters Outdoors
Garden equipment often runs on long, portable cords. When resistance in the cord causes a drop in voltage, motors draw higher current to maintain torque. That extra current increases heating in the cord and in the tool, reducing efficiency and shortening service life. Lights may dim and pumps can stall under load. Managing drop helps tools start reliably and operate near rated performance.
Inputs That Drive the Calculation
The calculator uses supply voltage, load current, cord length, and wire gauge to estimate resistance and voltage drop. Material selection matters because copper and aluminum differ in conductivity. Temperature is included because resistance rises as conductors warm. A target percent-drop helps you judge whether a setup suits a blower, trimmer, pond pump, or lighting run.
Interpreting the Results
Voltage drop is shown in volts and percent, along with estimated voltage available at the tool. Loss in watts represents heating in the cord, which can be significant on higher-current loads. The status indicator compares your drop to the chosen limit, and the “max length” estimate shows how far the same gauge can run before exceeding the target.
Choosing a Practical Cord Setup
Start with the nameplate current and the longest realistic run. If the drop is above your target, shorten the cord, move the power source, or select a thicker gauge. Keep connectors clean and fully seated, because poor connections add resistance and can create hotspots.
Good Field Practices
Use cords rated for outdoor use and keep them uncoiled when carrying heavy current to avoid heat buildup. Avoid daisy-chaining multiple cords, and protect cords from sharp edges and standing water. After long runs, feel the plug ends; warmth suggests excessive loss or a loose connection. Recheck settings when changing tools or cord type. Record your best combinations so seasonal setups stay consistent and efficient always.
FAQs
What percent drop should I aim for?
For most outdoor tools, 3% is a practical goal for a single cord. Up to 5% may be acceptable for short-duty loads, but sensitive motors and pumps benefit from lower drop, especially during starting.
Does cord length mean one-way or total run?
Enter the physical cord length. The calculator uses a round-trip path for single-phase loads, because current travels out and back. For three-phase, it uses the common √3 method with one-way length.
Why does temperature change the result?
Conductor resistance increases as temperature rises. A warmer cord produces more drop and more heating for the same current. The temperature input applies a standard linear correction so results better match hot-weather and high-load conditions.
Can I use aluminum extension cords?
Many portable cords are copper. If you use aluminum conductors, expect higher resistance and larger drop at the same gauge. Choose a thicker gauge, keep connections tight, and verify the cord is rated for your application.
What if my tool current varies?
Use the highest expected running current, and consider the starting surge for motor-driven equipment. If results are borderline, select a thicker gauge or shorter length. This provides margin for real-world factors like connector wear.
Are the resistance values exact?
They are typical reference values for common gauges at about 20°C. Manufacturing differences, strand count, and connectors can add variation. Treat results as planning guidance, and inspect cords for heat or damage during use.