Volt Drop Calculator

If you’re planning an electrical installation, one of the most critical calculations is voltage drop—the amount of voltage lost as electric current travels through a conductor. Whether you’re working on residential wiring, industrial equipment, or solar power systems, knowing how much voltage will be lost can prevent inefficient performance and safety issues.

To simplify this task, we’ve created a Voltage Drop Calculator—a free, intuitive tool that helps electricians, engineers, and DIY enthusiasts calculate voltage loss quickly and accurately.

What Is Voltage Drop and Why Does It Matter?

Voltage drop occurs when electrical current flows through a conductor (like copper or aluminum wire) and meets resistance. This resistance causes some of the energy to dissipate as heat, reducing the voltage that ultimately reaches the load.

Too much voltage drop can lead to:

  • Dimmed lights
  • Overheated wiring
  • Equipment malfunction or inefficiency
  • Code violations (NEC recommends a maximum 3–5% drop)

This is why planning for proper wire sizing and circuit design is crucial—and where this calculator becomes invaluable.

How to Use the Voltage Drop Calculator (Step-by-Step)

Using the calculator is fast and easy. Just follow these steps:

1. Enter Voltage (V)

Input the system voltage (e.g., 120V, 240V, or 480V). This is the source voltage before any drop occurs.

2. Enter Current (A)

Provide the current load in amperes (A). You can find this from your equipment specifications or load calculations.

3. Enter Resistance (Ω)

Input the resistance per meter of the conductor. This depends on the wire type (copper or aluminum), size (AWG), and temperature. Typical values can be found in electrical reference tables.

4. Enter Conductor Length (m)

This is the one-way distance from the power source to the load. The calculator will automatically double it for single-phase or apply √3 for three-phase systems.

5. Select Phase Type

Choose between:

  • Single Phase (uses 2× length multiplier)
  • Three Phase (uses √3 multiplier)

6. Click Calculate

The calculator will display:

  • Voltage Drop (V) – the amount of voltage lost.
  • Voltage Drop % – how much of the original voltage is lost, shown as a percentage.

7. Click Reset (if needed)

Start a new calculation by hitting the reset button.

Practical Example: Voltage Drop Calculation in Real Life

Scenario:
You’re wiring a water pump 100 meters away from the panel using a single-phase 240V system. The pump draws 20 amps, and you’re using a copper wire with a resistance of 0.0002 ohms/meter.

Step-by-Step Input:

  • Voltage: 240V
  • Current: 20A
  • Resistance: 0.0002 Ω/m
  • Length: 100 meters
  • Phase: Single Phase

Calculation:

Voltage Drop = 20 × 0.0002 × 100 × 2 = 0.8V
Voltage Drop % = (0.8 / 240) × 100 = 0.33%

Conclusion: You’re well below the 3% recommended limit—safe and efficient.

Why Use a Voltage Drop Calculator?

  • ✅ Save Time: No manual formulas needed.
  • ✅ Improve Safety: Prevent overheated wires and system failure.
  • ✅ Stay Code-Compliant: Meet NEC guidelines (3% max recommended for branch circuits).
  • ✅ Optimize Design: Select proper wire sizes and avoid overspending on oversized cables.
  • ✅ Universal Use: Supports both single-phase and three-phase systems.

Common Use Cases for Voltage Drop Calculations

  • Residential electrical installations (e.g., long driveway lighting)
  • Solar panel systems with long wire runs
  • EV charger installations
  • Industrial motor wiring
  • Agricultural pump systems
  • Off-grid setups and inverters
  • Marine or RV electrical systems

17 Most Frequently Asked Questions (FAQs)

1. What is voltage drop?

Voltage drop is the reduction in voltage that occurs when electric current flows through a conductor with resistance.

2. Why is voltage drop important?

It can affect device performance, cause heat buildup, and potentially damage equipment or create safety hazards.

3. What is an acceptable voltage drop?

The NEC recommends a maximum of 3% for branch circuits and 5% total from the main panel to the furthest device.

4. Does wire length affect voltage drop?

Yes, the longer the conductor, the higher the resistance and the greater the voltage drop.

5. What does resistance mean in this context?

It’s the opposition to current flow in the wire, typically measured in ohms per meter (Ω/m). It varies by wire material and size.

6. How do I find the resistance of a wire?

Check manufacturer specs or use NEC tables that provide resistance values for copper or aluminum wires by size and temperature.

7. Does voltage drop apply to DC and AC circuits?

Yes, though the calculation differs slightly for DC. This calculator is tailored for AC circuits, both single and three-phase.

8. What’s the difference between single and three-phase voltage drop?

Three-phase systems use a multiplier of √3 in the calculation, while single-phase uses a multiplier of 2.

9. Can I use this calculator for aluminum wire?

Yes, just make sure to input the correct resistance value for aluminum wire, which is higher than copper.

10. Does wire gauge (AWG) matter?

Absolutely. Thicker wires (lower AWG numbers) have lower resistance and result in less voltage drop.

11. How accurate is this calculator?

It’s very accurate for typical use cases, assuming you provide correct values for resistance and length.

12. Can this calculator help me choose the right wire size?

Indirectly, yes. By adjusting wire resistance in the calculator, you can compare drops for different wire sizes.

13. What happens if voltage drop is too high?

Your devices may underperform, motors can overheat, and breakers may trip due to imbalance.

14. Do all loads tolerate the same voltage drop?

No. Sensitive electronics may require tighter voltage tolerances than basic lighting or heating equipment.

15. Should I consider both hot and neutral wires in the length?

Yes. For single-phase circuits, the calculator multiplies the entered one-way length by 2 to account for round-trip resistance.

16. What if I’m not sure of the exact resistance?

Use standard resistance values from NEC tables based on wire size and type as a reference.

17. Is this calculator compliant with NEC standards?

Yes, it follows NEC guidelines for voltage drop estimation and is a useful tool during planning and compliance checks.

Final Thoughts

The Voltage Drop Calculator is a smart addition to your toolkit, whether you’re a professional electrician, an engineer, or a DIYer managing a home project. It provides fast, accurate, and easy-to-understand results so you can avoid pitfalls like excessive energy loss, safety hazards, and failed inspections.

Try the calculator today and make your next installation more efficient, reliable, and compliant.